Estimating HugePages Requirements?

From a3f9d75237c14326cc0f96506f26f887d142e633 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Thu, 10 Jun 2021 03:00:49 +0000
Subject: [PATCH v1 1/1] add pg_ctl option for retreiving shmem size

---
 src/backend/postmaster/postmaster.c | 31 ++++++++++++++++++++++++-------
 src/backend/storage/ipc/ipci.c      |  9 +++++++--
 src/backend/tcop/postgres.c         |  6 +++++-
 src/backend/utils/init/postinit.c   |  2 +-
 src/bin/pg_ctl/pg_ctl.c             | 23 ++++++++++++++++++++++-
 src/include/storage/ipc.h           |  2 +-
 6 files changed, 60 insertions(+), 13 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 5a050898fe..b2fc67f4a2 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -586,6 +586,7 @@ PostmasterMain(int argc, char *argv[])
 	bool		listen_addr_saved = false;
 	int			i;
 	char	   *output_config_variable = NULL;
+	bool		output_shmem = false;
 
 	InitProcessGlobals();
 
@@ -702,7 +703,7 @@ PostmasterMain(int argc, char *argv[])
 	 * tcop/postgres.c (the option sets should not conflict) and with the
 	 * common help() function in main/main.c.
 	 */
-	while ((opt = getopt(argc, argv, "B:bc:C:D:d:EeFf:h:ijk:lN:nOPp:r:S:sTt:W:-:")) != -1)
+	while ((opt = getopt(argc, argv, "B:bc:C:D:d:EeFf:h:ijk:lMN:nOPp:r:S:sTt:W:-:")) != -1)
 	{
 		switch (opt)
 		{
@@ -768,6 +769,10 @@ PostmasterMain(int argc, char *argv[])
 				SetConfigOption("ssl", "true", PGC_POSTMASTER, PGC_S_ARGV);
 				break;
 
+			case 'M':
+				output_shmem = true;
+				break;
+
 			case 'N':
 				SetConfigOption("max_connections", optarg, PGC_POSTMASTER, PGC_S_ARGV);
 				break;
@@ -1019,6 +1024,18 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	if (output_shmem)
+	{
+		char output[64];
+		Size size;
+
+		size = CreateSharedMemoryAndSemaphores(true);
+		sprintf(output, "%zu", size);
+
+		puts(output);
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
@@ -2660,7 +2677,7 @@ reset_shared(void)
 	 * (if using SysV shmem and/or semas).  This helps ensure that we will
 	 * clean up dead IPC objects if the postmaster crashes and is restarted.
 	 */
-	CreateSharedMemoryAndSemaphores();
+	(void) CreateSharedMemoryAndSemaphores(false);
 }
 
 
@@ -5014,7 +5031,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		/* And run the backend */
 		BackendRun(&port);		/* does not return */
@@ -5028,7 +5045,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitAuxiliaryProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		AuxiliaryProcessMain(argc - 2, argv + 2);	/* does not return */
 	}
@@ -5041,7 +5058,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		AutoVacLauncherMain(argc - 2, argv + 2);	/* does not return */
 	}
@@ -5054,7 +5071,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		AutoVacWorkerMain(argc - 2, argv + 2);	/* does not return */
 	}
@@ -5072,7 +5089,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		/* Fetch MyBgworkerEntry from shared memory */
 		shmem_slot = atoi(argv[1] + 15);
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..0202e59748 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -91,8 +91,8 @@ RequestAddinShmemSpace(Size size)
  * check IsUnderPostmaster, rather than EXEC_BACKEND, to detect this case.
  * This is a bit code-wasteful and could be cleaned up.)
  */
-void
-CreateSharedMemoryAndSemaphores(void)
+Size
+CreateSharedMemoryAndSemaphores(bool size_only)
 {
 	PGShmemHeader *shim = NULL;
 
@@ -161,6 +161,9 @@ CreateSharedMemoryAndSemaphores(void)
 		/* might as well round it off to a multiple of a typical page size */
 		size = add_size(size, 8192 - (size % 8192));
 
+		if (size_only)
+			return size;
+
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
@@ -288,4 +291,6 @@ CreateSharedMemoryAndSemaphores(void)
 	 */
 	if (shmem_startup_hook)
 		shmem_startup_hook();
+
+	return 0;
 }
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 8cea10c901..1ef3fdf8d1 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -3711,7 +3711,7 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx,
 	 * postmaster/postmaster.c (the option sets should not conflict) and with
 	 * the common help() function in main/main.c.
 	 */
-	while ((flag = getopt(argc, argv, "B:bc:C:D:d:EeFf:h:ijk:lN:nOPp:r:S:sTt:v:W:-:")) != -1)
+	while ((flag = getopt(argc, argv, "B:bc:C:D:d:EeFf:h:ijk:lMN:nOPp:r:S:sTt:v:W:-:")) != -1)
 	{
 		switch (flag)
 		{
@@ -3777,6 +3777,10 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx,
 				SetConfigOption("ssl", "true", ctx, gucsource);
 				break;
 
+			case 'M':
+				/* ignored for consistency with postmaster */
+				break;
+
 			case 'N':
 				SetConfigOption("max_connections", optarg, ctx, gucsource);
 				break;
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 51d1bbef30..7c329009d2 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -437,7 +437,7 @@ InitCommunication(void)
 		 * We're running a postgres bootstrap process or a standalone backend,
 		 * so we need to set up shmem.
 		 */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 	}
 }
 
diff --git a/src/bin/pg_ctl/pg_ctl.c b/src/bin/pg_ctl/pg_ctl.c
index 7985da0a94..9678185930 100644
--- a/src/bin/pg_ctl/pg_ctl.c
+++ b/src/bin/pg_ctl/pg_ctl.c
@@ -67,7 +67,8 @@ typedef enum
 	KILL_COMMAND,
 	REGISTER_COMMAND,
 	UNREGISTER_COMMAND,
-	RUN_AS_SERVICE_COMMAND
+	RUN_AS_SERVICE_COMMAND,
+	OUTPUT_SHARED_MEMORY_COMMAND
 } CtlCommand;
 
 #define DEFAULT_WAIT	60
@@ -898,6 +899,9 @@ do_start(void)
 
 	pm_pid = start_postmaster();
 
+	if (ctl_command == OUTPUT_SHARED_MEMORY_COMMAND)
+		return;
+
 	if (do_wait)
 	{
 		/*
@@ -2469,6 +2473,20 @@ main(int argc, char **argv)
 			else if (strcmp(argv[optind], "runservice") == 0)
 				ctl_command = RUN_AS_SERVICE_COMMAND;
 #endif
+			else if (strcmp(argv[optind], "output_shared_memory") == 0)
+			{
+				ctl_command = OUTPUT_SHARED_MEMORY_COMMAND;
+
+				if (!post_opts)
+					post_opts = pstrdup("-M");
+				else
+				{
+					char *old_post_opts = post_opts;
+
+					post_opts = psprintf("%s %s", old_post_opts, "-M");
+					free(old_post_opts);
+				}
+			}
 			else
 			{
 				write_stderr(_("%s: unrecognized operation mode \"%s\"\n"), progname, argv[optind]);
@@ -2572,6 +2590,9 @@ main(int argc, char **argv)
 			pgwin32_doRunAsService();
 			break;
 #endif
+		case OUTPUT_SHARED_MEMORY_COMMAND:
+			do_start();
+			break;
 		default:
 			break;
 	}
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..cdb1dd266d 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,6 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
-extern void CreateSharedMemoryAndSemaphores(void);
+extern Size CreateSharedMemoryAndSemaphores(bool size_only);
 
 #endif							/* IPC_H */
-- 
2.16.6

From f17f5862c2dd5c01a41143eb4d9d7eafae83618f Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Mon, 9 Aug 2021 22:48:48 +0000
Subject: [PATCH v2 1/1] add pg_ctl option for retreiving shmem size

---
 src/backend/bootstrap/bootstrap.c   |  2 +-
 src/backend/postmaster/postmaster.c | 31 ++++++++++++++++++++++++-------
 src/backend/storage/ipc/ipci.c      |  9 +++++++--
 src/backend/tcop/postgres.c         |  8 ++++++--
 src/bin/pg_ctl/pg_ctl.c             | 23 ++++++++++++++++++++++-
 src/include/storage/ipc.h           |  2 +-
 6 files changed, 61 insertions(+), 14 deletions(-)

diff --git a/src/backend/bootstrap/bootstrap.c b/src/backend/bootstrap/bootstrap.c
index 48615c0ebc..9e57591add 100644
--- a/src/backend/bootstrap/bootstrap.c
+++ b/src/backend/bootstrap/bootstrap.c
@@ -324,7 +324,7 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 
 	InitializeMaxBackends();
 
-	CreateSharedMemoryAndSemaphores();
+	(void) CreateSharedMemoryAndSemaphores(false);
 
 	/*
 	 * XXX: It might make sense to move this into its own function at some
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index fc0bc8d99e..5227ce372f 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -586,6 +586,7 @@ PostmasterMain(int argc, char *argv[])
 	bool		listen_addr_saved = false;
 	int			i;
 	char	   *output_config_variable = NULL;
+	bool		output_shmem = false;
 
 	InitProcessGlobals();
 
@@ -709,7 +710,7 @@ PostmasterMain(int argc, char *argv[])
 	 * tcop/postgres.c (the option sets should not conflict) and with the
 	 * common help() function in main/main.c.
 	 */
-	while ((opt = getopt(argc, argv, "B:bc:C:D:d:EeFf:h:ijk:lN:nOPp:r:S:sTt:W:-:")) != -1)
+	while ((opt = getopt(argc, argv, "B:bc:C:D:d:EeFf:h:ijk:lMN:nOPp:r:S:sTt:W:-:")) != -1)
 	{
 		switch (opt)
 		{
@@ -775,6 +776,10 @@ PostmasterMain(int argc, char *argv[])
 				SetConfigOption("ssl", "true", PGC_POSTMASTER, PGC_S_ARGV);
 				break;
 
+			case 'M':
+				output_shmem = true;
+				break;
+
 			case 'N':
 				SetConfigOption("max_connections", optarg, PGC_POSTMASTER, PGC_S_ARGV);
 				break;
@@ -1026,6 +1031,18 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	if (output_shmem)
+	{
+		char output[64];
+		Size size;
+
+		size = CreateSharedMemoryAndSemaphores(true);
+		sprintf(output, "%zu", size);
+
+		puts(output);
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
@@ -2673,7 +2690,7 @@ reset_shared(void)
 	 * (if using SysV shmem and/or semas).  This helps ensure that we will
 	 * clean up dead IPC objects if the postmaster crashes and is restarted.
 	 */
-	CreateSharedMemoryAndSemaphores();
+	(void) CreateSharedMemoryAndSemaphores(false);
 }
 
 
@@ -5017,7 +5034,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		/* And run the backend */
 		BackendRun(&port);		/* does not return */
@@ -5035,7 +5052,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitAuxiliaryProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		auxtype = atoi(argv[3]);
 		AuxiliaryProcessMain(auxtype);	/* does not return */
@@ -5049,7 +5066,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		AutoVacLauncherMain(argc - 2, argv + 2);	/* does not return */
 	}
@@ -5062,7 +5079,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		AutoVacWorkerMain(argc - 2, argv + 2);	/* does not return */
 	}
@@ -5080,7 +5097,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 
 		/* Fetch MyBgworkerEntry from shared memory */
 		shmem_slot = atoi(argv[1] + 15);
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..0202e59748 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -91,8 +91,8 @@ RequestAddinShmemSpace(Size size)
  * check IsUnderPostmaster, rather than EXEC_BACKEND, to detect this case.
  * This is a bit code-wasteful and could be cleaned up.)
  */
-void
-CreateSharedMemoryAndSemaphores(void)
+Size
+CreateSharedMemoryAndSemaphores(bool size_only)
 {
 	PGShmemHeader *shim = NULL;
 
@@ -161,6 +161,9 @@ CreateSharedMemoryAndSemaphores(void)
 		/* might as well round it off to a multiple of a typical page size */
 		size = add_size(size, 8192 - (size % 8192));
 
+		if (size_only)
+			return size;
+
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
@@ -288,4 +291,6 @@ CreateSharedMemoryAndSemaphores(void)
 	 */
 	if (shmem_startup_hook)
 		shmem_startup_hook();
+
+	return 0;
 }
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 58b5960e27..eee4307fec 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -3711,7 +3711,7 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx,
 	 * postmaster/postmaster.c (the option sets should not conflict) and with
 	 * the common help() function in main/main.c.
 	 */
-	while ((flag = getopt(argc, argv, "B:bc:C:D:d:EeFf:h:ijk:lN:nOPp:r:S:sTt:v:W:-:")) != -1)
+	while ((flag = getopt(argc, argv, "B:bc:C:D:d:EeFf:h:ijk:lMN:nOPp:r:S:sTt:v:W:-:")) != -1)
 	{
 		switch (flag)
 		{
@@ -3777,6 +3777,10 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx,
 				SetConfigOption("ssl", "true", ctx, gucsource);
 				break;
 
+			case 'M':
+				/* ignored for consistency with postmaster */
+				break;
+
 			case 'N':
 				SetConfigOption("max_connections", optarg, ctx, gucsource);
 				break;
@@ -4051,7 +4055,7 @@ PostgresMain(int argc, char *argv[],
 		/* Initialize MaxBackends (if under postmaster, was done already) */
 		InitializeMaxBackends();
 
-		CreateSharedMemoryAndSemaphores();
+		(void) CreateSharedMemoryAndSemaphores(false);
 	}
 
 	/*
diff --git a/src/bin/pg_ctl/pg_ctl.c b/src/bin/pg_ctl/pg_ctl.c
index 7985da0a94..9678185930 100644
--- a/src/bin/pg_ctl/pg_ctl.c
+++ b/src/bin/pg_ctl/pg_ctl.c
@@ -67,7 +67,8 @@ typedef enum
 	KILL_COMMAND,
 	REGISTER_COMMAND,
 	UNREGISTER_COMMAND,
-	RUN_AS_SERVICE_COMMAND
+	RUN_AS_SERVICE_COMMAND,
+	OUTPUT_SHARED_MEMORY_COMMAND
 } CtlCommand;
 
 #define DEFAULT_WAIT	60
@@ -898,6 +899,9 @@ do_start(void)
 
 	pm_pid = start_postmaster();
 
+	if (ctl_command == OUTPUT_SHARED_MEMORY_COMMAND)
+		return;
+
 	if (do_wait)
 	{
 		/*
@@ -2469,6 +2473,20 @@ main(int argc, char **argv)
 			else if (strcmp(argv[optind], "runservice") == 0)
 				ctl_command = RUN_AS_SERVICE_COMMAND;
 #endif
+			else if (strcmp(argv[optind], "output_shared_memory") == 0)
+			{
+				ctl_command = OUTPUT_SHARED_MEMORY_COMMAND;
+
+				if (!post_opts)
+					post_opts = pstrdup("-M");
+				else
+				{
+					char *old_post_opts = post_opts;
+
+					post_opts = psprintf("%s %s", old_post_opts, "-M");
+					free(old_post_opts);
+				}
+			}
 			else
 			{
 				write_stderr(_("%s: unrecognized operation mode \"%s\"\n"), progname, argv[optind]);
@@ -2572,6 +2590,9 @@ main(int argc, char **argv)
 			pgwin32_doRunAsService();
 			break;
 #endif
+		case OUTPUT_SHARED_MEMORY_COMMAND:
+			do_start();
+			break;
 		default:
 			break;
 	}
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..cdb1dd266d 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,6 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
-extern void CreateSharedMemoryAndSemaphores(void);
+extern Size CreateSharedMemoryAndSemaphores(bool size_only);
 
 #endif							/* IPC_H */
-- 
2.16.6

From 6ff02fbc9f4b47456ccb367b21d2b489845620ce Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Wed, 11 Aug 2021 22:43:03 +0000
Subject: [PATCH v3 1/1] introduce option for retreiving shmem size

---
 src/backend/bootstrap/bootstrap.c |  18 +++--
 src/backend/main/main.c           |   6 +-
 src/backend/storage/ipc/ipci.c    | 142 ++++++++++++++++++++++----------------
 src/include/bootstrap/bootstrap.h |   2 +-
 src/include/storage/ipc.h         |   1 +
 5 files changed, 102 insertions(+), 67 deletions(-)

diff --git a/src/backend/bootstrap/bootstrap.c b/src/backend/bootstrap/bootstrap.c
index 48615c0ebc..c9b2284ad2 100644
--- a/src/backend/bootstrap/bootstrap.c
+++ b/src/backend/bootstrap/bootstrap.c
@@ -198,9 +198,13 @@ CheckerModeMain(void)
  *	 the current configuration, particularly the passed in options pertaining
  *	 to shared memory sizing, options work (or at least do not cause an error
  *	 up to shared memory creation).
+ *
+ *	 When output_shmem is true, startup is done only far enough to calculate the
+ *	 amount of shared memory required for the current configuration.  The result
+ *	 of this calculation is printed.
  */
 void
-BootstrapModeMain(int argc, char *argv[], bool check_only)
+BootstrapModeMain(int argc, char *argv[], bool check_only, bool output_shmem)
 {
 	int			i;
 	char	   *progname = argv[0];
@@ -214,10 +218,11 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 	/* Set defaults, to be overridden by explicit options below */
 	InitializeGUCOptions();
 
-	/* an initial --boot or --check should be present */
+	/* an initial --boot, --check, or --output-shmem should be present */
 	Assert(argc > 1
 		   && (strcmp(argv[1], "--boot") == 0
-			   || strcmp(argv[1], "--check") == 0));
+			   || strcmp(argv[1], "--check") == 0
+			   || strcmp(argv[1], "--output-shmem") == 0));
 	argv++;
 	argc--;
 
@@ -324,14 +329,17 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 
 	InitializeMaxBackends();
 
-	CreateSharedMemoryAndSemaphores();
+	if (output_shmem)
+		printf("%zu bytes\n", CalculateShmemSize(NULL));
+	else
+		CreateSharedMemoryAndSemaphores();
 
 	/*
 	 * XXX: It might make sense to move this into its own function at some
 	 * point. Right now it seems like it'd cause more code duplication than
 	 * it's worth.
 	 */
-	if (check_only)
+	if (check_only || output_shmem)
 	{
 		SetProcessingMode(NormalProcessing);
 		CheckerModeMain();
diff --git a/src/backend/main/main.c b/src/backend/main/main.c
index 3a2a0d598c..c141ae3d1c 100644
--- a/src/backend/main/main.c
+++ b/src/backend/main/main.c
@@ -182,9 +182,11 @@ main(int argc, char *argv[])
 	 */
 
 	if (argc > 1 && strcmp(argv[1], "--check") == 0)
-		BootstrapModeMain(argc, argv, true);
+		BootstrapModeMain(argc, argv, true, false);
+	else if (argc > 1 && strcmp(argv[1], "--output-shmem") == 0)
+		BootstrapModeMain(argc, argv, false, true);
 	else if (argc > 1 && strcmp(argv[1], "--boot") == 0)
-		BootstrapModeMain(argc, argv, false);
+		BootstrapModeMain(argc, argv, false, false);
 #ifdef EXEC_BACKEND
 	else if (argc > 1 && strncmp(argv[1], "--fork", 6) == 0)
 		SubPostmasterMain(argc, argv);
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..b225b1ee70 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -75,6 +75,87 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
+/*
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
+ *
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
+ */
+Size
+CalculateShmemSize(int *num_semaphores)
+{
+	Size size;
+	int numSemas;
+
+	/* Compute number of semaphores we'll need */
+	numSemas = ProcGlobalSemas();
+	numSemas += SpinlockSemas();
+
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
+	/*
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
+	 *
+	 * We take some care to ensure that the total size request doesn't overflow
+	 * size_t.  If this gets through, we don't need to be so careful during the
+	 * actual allocation phase.
+	 */
+	size = 100000;
+	size = add_size(size, PGSemaphoreShmemSize(numSemas));
+	size = add_size(size, SpinlockSemaSize());
+	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
+											 sizeof(ShmemIndexEnt)));
+	size = add_size(size, dsm_estimate_size());
+	size = add_size(size, BufferShmemSize());
+	size = add_size(size, LockShmemSize());
+	size = add_size(size, PredicateLockShmemSize());
+	size = add_size(size, ProcGlobalShmemSize());
+	size = add_size(size, XLOGShmemSize());
+	size = add_size(size, CLOGShmemSize());
+	size = add_size(size, CommitTsShmemSize());
+	size = add_size(size, SUBTRANSShmemSize());
+	size = add_size(size, TwoPhaseShmemSize());
+	size = add_size(size, BackgroundWorkerShmemSize());
+	size = add_size(size, MultiXactShmemSize());
+	size = add_size(size, LWLockShmemSize());
+	size = add_size(size, ProcArrayShmemSize());
+	size = add_size(size, BackendStatusShmemSize());
+	size = add_size(size, SInvalShmemSize());
+	size = add_size(size, PMSignalShmemSize());
+	size = add_size(size, ProcSignalShmemSize());
+	size = add_size(size, CheckpointerShmemSize());
+	size = add_size(size, AutoVacuumShmemSize());
+	size = add_size(size, ReplicationSlotsShmemSize());
+	size = add_size(size, ReplicationOriginShmemSize());
+	size = add_size(size, WalSndShmemSize());
+	size = add_size(size, WalRcvShmemSize());
+	size = add_size(size, PgArchShmemSize());
+	size = add_size(size, ApplyLauncherShmemSize());
+	size = add_size(size, SnapMgrShmemSize());
+	size = add_size(size, BTreeShmemSize());
+	size = add_size(size, SyncScanShmemSize());
+	size = add_size(size, AsyncShmemSize());
+#ifdef EXEC_BACKEND
+	size = add_size(size, ShmemBackendArraySize());
+#endif
+
+	/* freeze the addin request size and include it */
+	addin_request_allowed = false;
+	size = add_size(size, total_addin_request);
+
+	/* might as well round it off to a multiple of a typical page size */
+	size = add_size(size, 8192 - (size % 8192));
+
+	return size;
+}
 
 /*
  * CreateSharedMemoryAndSemaphores
@@ -102,65 +183,8 @@ CreateSharedMemoryAndSemaphores(void)
 		Size		size;
 		int			numSemas;
 
-		/* Compute number of semaphores we'll need */
-		numSemas = ProcGlobalSemas();
-		numSemas += SpinlockSemas();
-
-		/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
-		 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
-		 */
-		size = 100000;
-		size = add_size(size, PGSemaphoreShmemSize(numSemas));
-		size = add_size(size, SpinlockSemaSize());
-		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
-												 sizeof(ShmemIndexEnt)));
-		size = add_size(size, dsm_estimate_size());
-		size = add_size(size, BufferShmemSize());
-		size = add_size(size, LockShmemSize());
-		size = add_size(size, PredicateLockShmemSize());
-		size = add_size(size, ProcGlobalShmemSize());
-		size = add_size(size, XLOGShmemSize());
-		size = add_size(size, CLOGShmemSize());
-		size = add_size(size, CommitTsShmemSize());
-		size = add_size(size, SUBTRANSShmemSize());
-		size = add_size(size, TwoPhaseShmemSize());
-		size = add_size(size, BackgroundWorkerShmemSize());
-		size = add_size(size, MultiXactShmemSize());
-		size = add_size(size, LWLockShmemSize());
-		size = add_size(size, ProcArrayShmemSize());
-		size = add_size(size, BackendStatusShmemSize());
-		size = add_size(size, SInvalShmemSize());
-		size = add_size(size, PMSignalShmemSize());
-		size = add_size(size, ProcSignalShmemSize());
-		size = add_size(size, CheckpointerShmemSize());
-		size = add_size(size, AutoVacuumShmemSize());
-		size = add_size(size, ReplicationSlotsShmemSize());
-		size = add_size(size, ReplicationOriginShmemSize());
-		size = add_size(size, WalSndShmemSize());
-		size = add_size(size, WalRcvShmemSize());
-		size = add_size(size, PgArchShmemSize());
-		size = add_size(size, ApplyLauncherShmemSize());
-		size = add_size(size, SnapMgrShmemSize());
-		size = add_size(size, BTreeShmemSize());
-		size = add_size(size, SyncScanShmemSize());
-		size = add_size(size, AsyncShmemSize());
-#ifdef EXEC_BACKEND
-		size = add_size(size, ShmemBackendArraySize());
-#endif
-
-		/* freeze the addin request size and include it */
-		addin_request_allowed = false;
-		size = add_size(size, total_addin_request);
-
-		/* might as well round it off to a multiple of a typical page size */
-		size = add_size(size, 8192 - (size % 8192));
-
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
diff --git a/src/include/bootstrap/bootstrap.h b/src/include/bootstrap/bootstrap.h
index 7d3b78e374..8e420574f5 100644
--- a/src/include/bootstrap/bootstrap.h
+++ b/src/include/bootstrap/bootstrap.h
@@ -32,7 +32,7 @@ extern Form_pg_attribute attrtypes[MAXATTR];
 extern int	numattr;
 
 
-extern void BootstrapModeMain(int argc, char *argv[], bool check_only) pg_attribute_noreturn();
+extern void BootstrapModeMain(int argc, char *argv[], bool check_only, bool output_shmem) pg_attribute_noreturn();
 
 extern void closerel(char *name);
 extern void boot_openrel(char *name);
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..80e191d407 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From dff1d2b57a65ada412386b3dce2f1cf6d8409cac Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 27 Aug 2021 17:54:26 +0000
Subject: [PATCH v4 1/1] introduce option for retreiving shmem size

---
 doc/src/sgml/ref/postgres-ref.sgml |  17 +++++
 doc/src/sgml/runtime.sgml          |  17 ++---
 src/backend/bootstrap/bootstrap.c  |  25 +++++--
 src/backend/main/main.c            |   6 +-
 src/backend/storage/ipc/ipci.c     | 142 ++++++++++++++++++++++---------------
 src/include/bootstrap/bootstrap.h  |   2 +-
 src/include/storage/ipc.h          |   1 +
 7 files changed, 132 insertions(+), 78 deletions(-)

diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index 4aaa7abe1a..da20fb559a 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -280,6 +280,23 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--output-shmem</option></term>
+      <listitem>
+       <para>
+        Prints the amount of shared memory required for the current
+        configuration and exits.  This can be used on a running server.
+        This must be the first argument on the command line.
+       </para>
+
+       <para>
+        This option is useful for determining the number of huge pages needed
+        for the server.  For more information, see
+        <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-p <replaceable class="parameter">port</replaceable></option></term>
       <listitem>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..535eacadb2 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,17 +1442,14 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to determine the amount of shared memory
+    needed, and use the <filename>/proc</filename> and <filename>/sys</filename>
+    file systems to find the system's default and supported huge page sizes.
     This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
+$ <userinput>postgres --output-shmem -D $PGDATA</userinput>
+6646198272 bytes
 $ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
 Hugepagesize:       2048 kB
 $ <userinput>ls /sys/kernel/mm/hugepages</userinput>
@@ -1463,7 +1460,7 @@ hugepages-1048576kB  hugepages-2048kB
      either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
 
      Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
+     <literal>6646198272</literal> / <literal>2097152</literal> gives approximately
      <literal>3169.154</literal>, so in this example we need at
      least <literal>3170</literal> huge pages.  A larger setting would be
      appropriate if other programs on the machine also need huge pages.
diff --git a/src/backend/bootstrap/bootstrap.c b/src/backend/bootstrap/bootstrap.c
index 48615c0ebc..5bb491da8b 100644
--- a/src/backend/bootstrap/bootstrap.c
+++ b/src/backend/bootstrap/bootstrap.c
@@ -198,9 +198,13 @@ CheckerModeMain(void)
  *	 the current configuration, particularly the passed in options pertaining
  *	 to shared memory sizing, options work (or at least do not cause an error
  *	 up to shared memory creation).
+ *
+ *	 When output_shmem is true, startup is done only far enough to calculate the
+ *	 amount of shared memory required for the current configuration.  The result
+ *	 of this calculation is printed.
  */
 void
-BootstrapModeMain(int argc, char *argv[], bool check_only)
+BootstrapModeMain(int argc, char *argv[], bool check_only, bool output_shmem)
 {
 	int			i;
 	char	   *progname = argv[0];
@@ -214,10 +218,11 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 	/* Set defaults, to be overridden by explicit options below */
 	InitializeGUCOptions();
 
-	/* an initial --boot or --check should be present */
+	/* an initial --boot, --check, or --output-shmem should be present */
 	Assert(argc > 1
 		   && (strcmp(argv[1], "--boot") == 0
-			   || strcmp(argv[1], "--check") == 0));
+			   || strcmp(argv[1], "--check") == 0
+			   || strcmp(argv[1], "--output-shmem") == 0));
 	argv++;
 	argc--;
 
@@ -317,21 +322,29 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 	checkDataDir();
 	ChangeToDataDir();
 
-	CreateDataDirLockFile(false);
+	/*
+	 * We do not create the lock file when --output-shmem is specified so
+	 * that it can be used while the server is running.
+	 */
+	if (!output_shmem)
+		CreateDataDirLockFile(false);
 
 	SetProcessingMode(BootstrapProcessing);
 	IgnoreSystemIndexes = true;
 
 	InitializeMaxBackends();
 
-	CreateSharedMemoryAndSemaphores();
+	if (output_shmem)
+		printf("%zu bytes\n", CalculateShmemSize(NULL));
+	else
+		CreateSharedMemoryAndSemaphores();
 
 	/*
 	 * XXX: It might make sense to move this into its own function at some
 	 * point. Right now it seems like it'd cause more code duplication than
 	 * it's worth.
 	 */
-	if (check_only)
+	if (check_only || output_shmem)
 	{
 		SetProcessingMode(NormalProcessing);
 		CheckerModeMain();
diff --git a/src/backend/main/main.c b/src/backend/main/main.c
index 3a2a0d598c..c141ae3d1c 100644
--- a/src/backend/main/main.c
+++ b/src/backend/main/main.c
@@ -182,9 +182,11 @@ main(int argc, char *argv[])
 	 */
 
 	if (argc > 1 && strcmp(argv[1], "--check") == 0)
-		BootstrapModeMain(argc, argv, true);
+		BootstrapModeMain(argc, argv, true, false);
+	else if (argc > 1 && strcmp(argv[1], "--output-shmem") == 0)
+		BootstrapModeMain(argc, argv, false, true);
 	else if (argc > 1 && strcmp(argv[1], "--boot") == 0)
-		BootstrapModeMain(argc, argv, false);
+		BootstrapModeMain(argc, argv, false, false);
 #ifdef EXEC_BACKEND
 	else if (argc > 1 && strncmp(argv[1], "--fork", 6) == 0)
 		SubPostmasterMain(argc, argv);
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..b225b1ee70 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -75,6 +75,87 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
+/*
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
+ *
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
+ */
+Size
+CalculateShmemSize(int *num_semaphores)
+{
+	Size size;
+	int numSemas;
+
+	/* Compute number of semaphores we'll need */
+	numSemas = ProcGlobalSemas();
+	numSemas += SpinlockSemas();
+
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
+	/*
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
+	 *
+	 * We take some care to ensure that the total size request doesn't overflow
+	 * size_t.  If this gets through, we don't need to be so careful during the
+	 * actual allocation phase.
+	 */
+	size = 100000;
+	size = add_size(size, PGSemaphoreShmemSize(numSemas));
+	size = add_size(size, SpinlockSemaSize());
+	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
+											 sizeof(ShmemIndexEnt)));
+	size = add_size(size, dsm_estimate_size());
+	size = add_size(size, BufferShmemSize());
+	size = add_size(size, LockShmemSize());
+	size = add_size(size, PredicateLockShmemSize());
+	size = add_size(size, ProcGlobalShmemSize());
+	size = add_size(size, XLOGShmemSize());
+	size = add_size(size, CLOGShmemSize());
+	size = add_size(size, CommitTsShmemSize());
+	size = add_size(size, SUBTRANSShmemSize());
+	size = add_size(size, TwoPhaseShmemSize());
+	size = add_size(size, BackgroundWorkerShmemSize());
+	size = add_size(size, MultiXactShmemSize());
+	size = add_size(size, LWLockShmemSize());
+	size = add_size(size, ProcArrayShmemSize());
+	size = add_size(size, BackendStatusShmemSize());
+	size = add_size(size, SInvalShmemSize());
+	size = add_size(size, PMSignalShmemSize());
+	size = add_size(size, ProcSignalShmemSize());
+	size = add_size(size, CheckpointerShmemSize());
+	size = add_size(size, AutoVacuumShmemSize());
+	size = add_size(size, ReplicationSlotsShmemSize());
+	size = add_size(size, ReplicationOriginShmemSize());
+	size = add_size(size, WalSndShmemSize());
+	size = add_size(size, WalRcvShmemSize());
+	size = add_size(size, PgArchShmemSize());
+	size = add_size(size, ApplyLauncherShmemSize());
+	size = add_size(size, SnapMgrShmemSize());
+	size = add_size(size, BTreeShmemSize());
+	size = add_size(size, SyncScanShmemSize());
+	size = add_size(size, AsyncShmemSize());
+#ifdef EXEC_BACKEND
+	size = add_size(size, ShmemBackendArraySize());
+#endif
+
+	/* freeze the addin request size and include it */
+	addin_request_allowed = false;
+	size = add_size(size, total_addin_request);
+
+	/* might as well round it off to a multiple of a typical page size */
+	size = add_size(size, 8192 - (size % 8192));
+
+	return size;
+}
 
 /*
  * CreateSharedMemoryAndSemaphores
@@ -102,65 +183,8 @@ CreateSharedMemoryAndSemaphores(void)
 		Size		size;
 		int			numSemas;
 
-		/* Compute number of semaphores we'll need */
-		numSemas = ProcGlobalSemas();
-		numSemas += SpinlockSemas();
-
-		/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
-		 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
-		 */
-		size = 100000;
-		size = add_size(size, PGSemaphoreShmemSize(numSemas));
-		size = add_size(size, SpinlockSemaSize());
-		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
-												 sizeof(ShmemIndexEnt)));
-		size = add_size(size, dsm_estimate_size());
-		size = add_size(size, BufferShmemSize());
-		size = add_size(size, LockShmemSize());
-		size = add_size(size, PredicateLockShmemSize());
-		size = add_size(size, ProcGlobalShmemSize());
-		size = add_size(size, XLOGShmemSize());
-		size = add_size(size, CLOGShmemSize());
-		size = add_size(size, CommitTsShmemSize());
-		size = add_size(size, SUBTRANSShmemSize());
-		size = add_size(size, TwoPhaseShmemSize());
-		size = add_size(size, BackgroundWorkerShmemSize());
-		size = add_size(size, MultiXactShmemSize());
-		size = add_size(size, LWLockShmemSize());
-		size = add_size(size, ProcArrayShmemSize());
-		size = add_size(size, BackendStatusShmemSize());
-		size = add_size(size, SInvalShmemSize());
-		size = add_size(size, PMSignalShmemSize());
-		size = add_size(size, ProcSignalShmemSize());
-		size = add_size(size, CheckpointerShmemSize());
-		size = add_size(size, AutoVacuumShmemSize());
-		size = add_size(size, ReplicationSlotsShmemSize());
-		size = add_size(size, ReplicationOriginShmemSize());
-		size = add_size(size, WalSndShmemSize());
-		size = add_size(size, WalRcvShmemSize());
-		size = add_size(size, PgArchShmemSize());
-		size = add_size(size, ApplyLauncherShmemSize());
-		size = add_size(size, SnapMgrShmemSize());
-		size = add_size(size, BTreeShmemSize());
-		size = add_size(size, SyncScanShmemSize());
-		size = add_size(size, AsyncShmemSize());
-#ifdef EXEC_BACKEND
-		size = add_size(size, ShmemBackendArraySize());
-#endif
-
-		/* freeze the addin request size and include it */
-		addin_request_allowed = false;
-		size = add_size(size, total_addin_request);
-
-		/* might as well round it off to a multiple of a typical page size */
-		size = add_size(size, 8192 - (size % 8192));
-
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
diff --git a/src/include/bootstrap/bootstrap.h b/src/include/bootstrap/bootstrap.h
index 7d3b78e374..8e420574f5 100644
--- a/src/include/bootstrap/bootstrap.h
+++ b/src/include/bootstrap/bootstrap.h
@@ -32,7 +32,7 @@ extern Form_pg_attribute attrtypes[MAXATTR];
 extern int	numattr;
 
 
-extern void BootstrapModeMain(int argc, char *argv[], bool check_only) pg_attribute_noreturn();
+extern void BootstrapModeMain(int argc, char *argv[], bool check_only, bool output_shmem) pg_attribute_noreturn();
 
 extern void closerel(char *name);
 extern void boot_openrel(char *name);
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..80e191d407 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From 083cc81509c5d26b0dd431c58cf040bf311e9387 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 27 Aug 2021 19:24:38 +0000
Subject: [PATCH v5 1/1] introduce option for retreiving shmem size

---
 doc/src/sgml/ref/postgres-ref.sgml |  20 ++++++
 doc/src/sgml/runtime.sgml          |  17 ++---
 src/backend/bootstrap/bootstrap.c  |  25 +++++--
 src/backend/main/main.c            |   6 +-
 src/backend/storage/ipc/ipci.c     | 142 ++++++++++++++++++++++---------------
 src/include/bootstrap/bootstrap.h  |   2 +-
 src/include/storage/ipc.h          |   1 +
 7 files changed, 135 insertions(+), 78 deletions(-)

diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index 4aaa7abe1a..c28b6470c2 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -280,6 +280,26 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--output-shmem</option></term>
+      <listitem>
+       <para>
+        Prints the amount of shared memory required for the given
+        configuration and exits.  This can be used on a running server, but
+        the return value reflects the amount of shared memory needed based
+        on the current invocation.  It does not return the amount of shared
+        memory in use by the running server.  This must be the first
+        argument on the command line.
+       </para>
+
+       <para>
+        This option is useful for determining the number of huge pages
+        needed for the server.  For more information, see
+        <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-p <replaceable class="parameter">port</replaceable></option></term>
       <listitem>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..535eacadb2 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,17 +1442,14 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to determine the amount of shared memory
+    needed, and use the <filename>/proc</filename> and <filename>/sys</filename>
+    file systems to find the system's default and supported huge page sizes.
     This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
+$ <userinput>postgres --output-shmem -D $PGDATA</userinput>
+6646198272 bytes
 $ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
 Hugepagesize:       2048 kB
 $ <userinput>ls /sys/kernel/mm/hugepages</userinput>
@@ -1463,7 +1460,7 @@ hugepages-1048576kB  hugepages-2048kB
      either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
 
      Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
+     <literal>6646198272</literal> / <literal>2097152</literal> gives approximately
      <literal>3169.154</literal>, so in this example we need at
      least <literal>3170</literal> huge pages.  A larger setting would be
      appropriate if other programs on the machine also need huge pages.
diff --git a/src/backend/bootstrap/bootstrap.c b/src/backend/bootstrap/bootstrap.c
index 48615c0ebc..5bb491da8b 100644
--- a/src/backend/bootstrap/bootstrap.c
+++ b/src/backend/bootstrap/bootstrap.c
@@ -198,9 +198,13 @@ CheckerModeMain(void)
  *	 the current configuration, particularly the passed in options pertaining
  *	 to shared memory sizing, options work (or at least do not cause an error
  *	 up to shared memory creation).
+ *
+ *	 When output_shmem is true, startup is done only far enough to calculate the
+ *	 amount of shared memory required for the current configuration.  The result
+ *	 of this calculation is printed.
  */
 void
-BootstrapModeMain(int argc, char *argv[], bool check_only)
+BootstrapModeMain(int argc, char *argv[], bool check_only, bool output_shmem)
 {
 	int			i;
 	char	   *progname = argv[0];
@@ -214,10 +218,11 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 	/* Set defaults, to be overridden by explicit options below */
 	InitializeGUCOptions();
 
-	/* an initial --boot or --check should be present */
+	/* an initial --boot, --check, or --output-shmem should be present */
 	Assert(argc > 1
 		   && (strcmp(argv[1], "--boot") == 0
-			   || strcmp(argv[1], "--check") == 0));
+			   || strcmp(argv[1], "--check") == 0
+			   || strcmp(argv[1], "--output-shmem") == 0));
 	argv++;
 	argc--;
 
@@ -317,21 +322,29 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 	checkDataDir();
 	ChangeToDataDir();
 
-	CreateDataDirLockFile(false);
+	/*
+	 * We do not create the lock file when --output-shmem is specified so
+	 * that it can be used while the server is running.
+	 */
+	if (!output_shmem)
+		CreateDataDirLockFile(false);
 
 	SetProcessingMode(BootstrapProcessing);
 	IgnoreSystemIndexes = true;
 
 	InitializeMaxBackends();
 
-	CreateSharedMemoryAndSemaphores();
+	if (output_shmem)
+		printf("%zu bytes\n", CalculateShmemSize(NULL));
+	else
+		CreateSharedMemoryAndSemaphores();
 
 	/*
 	 * XXX: It might make sense to move this into its own function at some
 	 * point. Right now it seems like it'd cause more code duplication than
 	 * it's worth.
 	 */
-	if (check_only)
+	if (check_only || output_shmem)
 	{
 		SetProcessingMode(NormalProcessing);
 		CheckerModeMain();
diff --git a/src/backend/main/main.c b/src/backend/main/main.c
index 3a2a0d598c..c141ae3d1c 100644
--- a/src/backend/main/main.c
+++ b/src/backend/main/main.c
@@ -182,9 +182,11 @@ main(int argc, char *argv[])
 	 */
 
 	if (argc > 1 && strcmp(argv[1], "--check") == 0)
-		BootstrapModeMain(argc, argv, true);
+		BootstrapModeMain(argc, argv, true, false);
+	else if (argc > 1 && strcmp(argv[1], "--output-shmem") == 0)
+		BootstrapModeMain(argc, argv, false, true);
 	else if (argc > 1 && strcmp(argv[1], "--boot") == 0)
-		BootstrapModeMain(argc, argv, false);
+		BootstrapModeMain(argc, argv, false, false);
 #ifdef EXEC_BACKEND
 	else if (argc > 1 && strncmp(argv[1], "--fork", 6) == 0)
 		SubPostmasterMain(argc, argv);
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..b225b1ee70 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -75,6 +75,87 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
+/*
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
+ *
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
+ */
+Size
+CalculateShmemSize(int *num_semaphores)
+{
+	Size size;
+	int numSemas;
+
+	/* Compute number of semaphores we'll need */
+	numSemas = ProcGlobalSemas();
+	numSemas += SpinlockSemas();
+
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
+	/*
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
+	 *
+	 * We take some care to ensure that the total size request doesn't overflow
+	 * size_t.  If this gets through, we don't need to be so careful during the
+	 * actual allocation phase.
+	 */
+	size = 100000;
+	size = add_size(size, PGSemaphoreShmemSize(numSemas));
+	size = add_size(size, SpinlockSemaSize());
+	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
+											 sizeof(ShmemIndexEnt)));
+	size = add_size(size, dsm_estimate_size());
+	size = add_size(size, BufferShmemSize());
+	size = add_size(size, LockShmemSize());
+	size = add_size(size, PredicateLockShmemSize());
+	size = add_size(size, ProcGlobalShmemSize());
+	size = add_size(size, XLOGShmemSize());
+	size = add_size(size, CLOGShmemSize());
+	size = add_size(size, CommitTsShmemSize());
+	size = add_size(size, SUBTRANSShmemSize());
+	size = add_size(size, TwoPhaseShmemSize());
+	size = add_size(size, BackgroundWorkerShmemSize());
+	size = add_size(size, MultiXactShmemSize());
+	size = add_size(size, LWLockShmemSize());
+	size = add_size(size, ProcArrayShmemSize());
+	size = add_size(size, BackendStatusShmemSize());
+	size = add_size(size, SInvalShmemSize());
+	size = add_size(size, PMSignalShmemSize());
+	size = add_size(size, ProcSignalShmemSize());
+	size = add_size(size, CheckpointerShmemSize());
+	size = add_size(size, AutoVacuumShmemSize());
+	size = add_size(size, ReplicationSlotsShmemSize());
+	size = add_size(size, ReplicationOriginShmemSize());
+	size = add_size(size, WalSndShmemSize());
+	size = add_size(size, WalRcvShmemSize());
+	size = add_size(size, PgArchShmemSize());
+	size = add_size(size, ApplyLauncherShmemSize());
+	size = add_size(size, SnapMgrShmemSize());
+	size = add_size(size, BTreeShmemSize());
+	size = add_size(size, SyncScanShmemSize());
+	size = add_size(size, AsyncShmemSize());
+#ifdef EXEC_BACKEND
+	size = add_size(size, ShmemBackendArraySize());
+#endif
+
+	/* freeze the addin request size and include it */
+	addin_request_allowed = false;
+	size = add_size(size, total_addin_request);
+
+	/* might as well round it off to a multiple of a typical page size */
+	size = add_size(size, 8192 - (size % 8192));
+
+	return size;
+}
 
 /*
  * CreateSharedMemoryAndSemaphores
@@ -102,65 +183,8 @@ CreateSharedMemoryAndSemaphores(void)
 		Size		size;
 		int			numSemas;
 
-		/* Compute number of semaphores we'll need */
-		numSemas = ProcGlobalSemas();
-		numSemas += SpinlockSemas();
-
-		/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
-		 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
-		 */
-		size = 100000;
-		size = add_size(size, PGSemaphoreShmemSize(numSemas));
-		size = add_size(size, SpinlockSemaSize());
-		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
-												 sizeof(ShmemIndexEnt)));
-		size = add_size(size, dsm_estimate_size());
-		size = add_size(size, BufferShmemSize());
-		size = add_size(size, LockShmemSize());
-		size = add_size(size, PredicateLockShmemSize());
-		size = add_size(size, ProcGlobalShmemSize());
-		size = add_size(size, XLOGShmemSize());
-		size = add_size(size, CLOGShmemSize());
-		size = add_size(size, CommitTsShmemSize());
-		size = add_size(size, SUBTRANSShmemSize());
-		size = add_size(size, TwoPhaseShmemSize());
-		size = add_size(size, BackgroundWorkerShmemSize());
-		size = add_size(size, MultiXactShmemSize());
-		size = add_size(size, LWLockShmemSize());
-		size = add_size(size, ProcArrayShmemSize());
-		size = add_size(size, BackendStatusShmemSize());
-		size = add_size(size, SInvalShmemSize());
-		size = add_size(size, PMSignalShmemSize());
-		size = add_size(size, ProcSignalShmemSize());
-		size = add_size(size, CheckpointerShmemSize());
-		size = add_size(size, AutoVacuumShmemSize());
-		size = add_size(size, ReplicationSlotsShmemSize());
-		size = add_size(size, ReplicationOriginShmemSize());
-		size = add_size(size, WalSndShmemSize());
-		size = add_size(size, WalRcvShmemSize());
-		size = add_size(size, PgArchShmemSize());
-		size = add_size(size, ApplyLauncherShmemSize());
-		size = add_size(size, SnapMgrShmemSize());
-		size = add_size(size, BTreeShmemSize());
-		size = add_size(size, SyncScanShmemSize());
-		size = add_size(size, AsyncShmemSize());
-#ifdef EXEC_BACKEND
-		size = add_size(size, ShmemBackendArraySize());
-#endif
-
-		/* freeze the addin request size and include it */
-		addin_request_allowed = false;
-		size = add_size(size, total_addin_request);
-
-		/* might as well round it off to a multiple of a typical page size */
-		size = add_size(size, 8192 - (size % 8192));
-
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
diff --git a/src/include/bootstrap/bootstrap.h b/src/include/bootstrap/bootstrap.h
index 7d3b78e374..8e420574f5 100644
--- a/src/include/bootstrap/bootstrap.h
+++ b/src/include/bootstrap/bootstrap.h
@@ -32,7 +32,7 @@ extern Form_pg_attribute attrtypes[MAXATTR];
 extern int	numattr;
 
 
-extern void BootstrapModeMain(int argc, char *argv[], bool check_only) pg_attribute_noreturn();
+extern void BootstrapModeMain(int argc, char *argv[], bool check_only, bool output_shmem) pg_attribute_noreturn();
 
 extern void closerel(char *name);
 extern void boot_openrel(char *name);
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..80e191d407 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From 499e969ef82c5d93353be10610c801640099774f Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 27 Aug 2021 20:03:01 +0000
Subject: [PATCH 1/1] Move the shared memory size calculation to its own
 function.

This change refactors the shared memory size calculation in
CreateSharedMemoryAndSemaphores() to its own function.  This is
intended for use in a future change that will simplify the steps
for setting up huge pages.
---
 src/backend/storage/ipc/ipci.c | 142 ++++++++++++++++++++++++-----------------
 src/include/storage/ipc.h      |   1 +
 2 files changed, 84 insertions(+), 59 deletions(-)

diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..b225b1ee70 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -75,6 +75,87 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
+/*
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
+ *
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
+ */
+Size
+CalculateShmemSize(int *num_semaphores)
+{
+	Size size;
+	int numSemas;
+
+	/* Compute number of semaphores we'll need */
+	numSemas = ProcGlobalSemas();
+	numSemas += SpinlockSemas();
+
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
+	/*
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
+	 *
+	 * We take some care to ensure that the total size request doesn't overflow
+	 * size_t.  If this gets through, we don't need to be so careful during the
+	 * actual allocation phase.
+	 */
+	size = 100000;
+	size = add_size(size, PGSemaphoreShmemSize(numSemas));
+	size = add_size(size, SpinlockSemaSize());
+	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
+											 sizeof(ShmemIndexEnt)));
+	size = add_size(size, dsm_estimate_size());
+	size = add_size(size, BufferShmemSize());
+	size = add_size(size, LockShmemSize());
+	size = add_size(size, PredicateLockShmemSize());
+	size = add_size(size, ProcGlobalShmemSize());
+	size = add_size(size, XLOGShmemSize());
+	size = add_size(size, CLOGShmemSize());
+	size = add_size(size, CommitTsShmemSize());
+	size = add_size(size, SUBTRANSShmemSize());
+	size = add_size(size, TwoPhaseShmemSize());
+	size = add_size(size, BackgroundWorkerShmemSize());
+	size = add_size(size, MultiXactShmemSize());
+	size = add_size(size, LWLockShmemSize());
+	size = add_size(size, ProcArrayShmemSize());
+	size = add_size(size, BackendStatusShmemSize());
+	size = add_size(size, SInvalShmemSize());
+	size = add_size(size, PMSignalShmemSize());
+	size = add_size(size, ProcSignalShmemSize());
+	size = add_size(size, CheckpointerShmemSize());
+	size = add_size(size, AutoVacuumShmemSize());
+	size = add_size(size, ReplicationSlotsShmemSize());
+	size = add_size(size, ReplicationOriginShmemSize());
+	size = add_size(size, WalSndShmemSize());
+	size = add_size(size, WalRcvShmemSize());
+	size = add_size(size, PgArchShmemSize());
+	size = add_size(size, ApplyLauncherShmemSize());
+	size = add_size(size, SnapMgrShmemSize());
+	size = add_size(size, BTreeShmemSize());
+	size = add_size(size, SyncScanShmemSize());
+	size = add_size(size, AsyncShmemSize());
+#ifdef EXEC_BACKEND
+	size = add_size(size, ShmemBackendArraySize());
+#endif
+
+	/* freeze the addin request size and include it */
+	addin_request_allowed = false;
+	size = add_size(size, total_addin_request);
+
+	/* might as well round it off to a multiple of a typical page size */
+	size = add_size(size, 8192 - (size % 8192));
+
+	return size;
+}
 
 /*
  * CreateSharedMemoryAndSemaphores
@@ -102,65 +183,8 @@ CreateSharedMemoryAndSemaphores(void)
 		Size		size;
 		int			numSemas;
 
-		/* Compute number of semaphores we'll need */
-		numSemas = ProcGlobalSemas();
-		numSemas += SpinlockSemas();
-
-		/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
-		 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
-		 */
-		size = 100000;
-		size = add_size(size, PGSemaphoreShmemSize(numSemas));
-		size = add_size(size, SpinlockSemaSize());
-		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
-												 sizeof(ShmemIndexEnt)));
-		size = add_size(size, dsm_estimate_size());
-		size = add_size(size, BufferShmemSize());
-		size = add_size(size, LockShmemSize());
-		size = add_size(size, PredicateLockShmemSize());
-		size = add_size(size, ProcGlobalShmemSize());
-		size = add_size(size, XLOGShmemSize());
-		size = add_size(size, CLOGShmemSize());
-		size = add_size(size, CommitTsShmemSize());
-		size = add_size(size, SUBTRANSShmemSize());
-		size = add_size(size, TwoPhaseShmemSize());
-		size = add_size(size, BackgroundWorkerShmemSize());
-		size = add_size(size, MultiXactShmemSize());
-		size = add_size(size, LWLockShmemSize());
-		size = add_size(size, ProcArrayShmemSize());
-		size = add_size(size, BackendStatusShmemSize());
-		size = add_size(size, SInvalShmemSize());
-		size = add_size(size, PMSignalShmemSize());
-		size = add_size(size, ProcSignalShmemSize());
-		size = add_size(size, CheckpointerShmemSize());
-		size = add_size(size, AutoVacuumShmemSize());
-		size = add_size(size, ReplicationSlotsShmemSize());
-		size = add_size(size, ReplicationOriginShmemSize());
-		size = add_size(size, WalSndShmemSize());
-		size = add_size(size, WalRcvShmemSize());
-		size = add_size(size, PgArchShmemSize());
-		size = add_size(size, ApplyLauncherShmemSize());
-		size = add_size(size, SnapMgrShmemSize());
-		size = add_size(size, BTreeShmemSize());
-		size = add_size(size, SyncScanShmemSize());
-		size = add_size(size, AsyncShmemSize());
-#ifdef EXEC_BACKEND
-		size = add_size(size, ShmemBackendArraySize());
-#endif
-
-		/* freeze the addin request size and include it */
-		addin_request_allowed = false;
-		size = add_size(size, total_addin_request);
-
-		/* might as well round it off to a multiple of a typical page size */
-		size = add_size(size, 8192 - (size % 8192));
-
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..80e191d407 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From 2c6ed729d951621e34214abd1c532539530479ea Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Sat, 28 Aug 2021 05:02:55 +0000
Subject: [PATCH 1/1] add shared_memory_size GUC

---
 src/backend/postmaster/postmaster.c | 25 +++++++++++++++++++++++--
 src/backend/utils/misc/guc.c        | 13 +++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c2c98614a..38e28b3f8f 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -893,7 +893,8 @@ PostmasterMain(int argc, char *argv[])
 	if (!SelectConfigFiles(userDoption, progname))
 		ExitPostmaster(2);
 
-	if (output_config_variable != NULL)
+	if (output_config_variable != NULL &&
+		strcmp(output_config_variable, "shared_memory_size") != 0)
 	{
 		/*
 		 * "-C guc" was specified, so print GUC's value and exit.  No extra
@@ -983,7 +984,8 @@ PostmasterMain(int argc, char *argv[])
 	 * so it must happen before opening sockets so that at exit, the socket
 	 * lockfiles go away after CloseServerPorts runs.
 	 */
-	CreateDataDirLockFile(true);
+	if (output_config_variable == NULL)
+		CreateDataDirLockFile(true);
 
 	/*
 	 * Read the control file (for error checking and config info).
@@ -1026,6 +1028,25 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	{
+		char buf[64];
+		Size size;
+
+		size = CalculateShmemSize(NULL);
+		size += (1024 * 1024) - 1;
+		size /= (1024 * 1024);
+
+		sprintf(buf, "%lu MB", size);
+		SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
+
+	if (output_config_variable != NULL)
+	{
+		const char *val = GetConfigOption(output_config_variable, false, false);
+		puts(val);
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe..7c88dbeae9 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -620,6 +620,8 @@ char	   *pgstat_temp_directory;
 
 char	   *application_name;
 
+int			shmem_size_mb;
+
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
 int			tcp_keepalives_count;
@@ -2337,6 +2339,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the amount of shared memory allocated by the server (rounded up to the nearest MB)."),
+			NULL,
+			GUC_UNIT_MB
+		},
+		&shmem_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
-- 
2.16.6

From e2345206dacec8b4454c3b429d0cfac01d5a0a58 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 30 Aug 2021 16:24:59 +0900
Subject: [PATCH] Add shared_memory_size GUC

---
 src/include/storage/ipc.h           |   1 +
 src/backend/postmaster/postmaster.c |  60 +++++++-----
 src/backend/storage/ipc/ipci.c      | 142 ++++++++++++++++------------
 src/backend/utils/misc/guc.c        |  13 +++
 4 files changed, 135 insertions(+), 81 deletions(-)

diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..80e191d407 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c2c98614a..0d3b4b1be0 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -893,6 +893,44 @@ PostmasterMain(int argc, char *argv[])
 	if (!SelectConfigFiles(userDoption, progname))
 		ExitPostmaster(2);
 
+	/* Verify that DataDir looks reasonable */
+	checkDataDir();
+
+	/* Check that pg_control exists */
+	checkControlFile();
+
+	/* And switch working directory into it */
+	ChangeToDataDir();
+
+	/*
+	 * Register the apply launcher.  Since it registers a background worker,
+	 * it needs to be called before InitializeMaxBackends(), and it's probably
+	 * a good idea to call it before any modules had chance to take the
+	 * background worker slots.
+	 */
+	ApplyLauncherRegister();
+
+	/*
+	 * Process any libraries that should be preloaded at postmaster start.
+	 * Thie happens before running -C, so as it is possible to get an
+	 * estimation of the total shared memory size allocated to this system,
+	 * accounting for the portion from loaded libraries.
+	 */
+	process_shared_preload_libraries();
+
+	/* XXX: this should be moved around */
+	{
+		char buf[64];
+		Size size;
+
+		size = CalculateShmemSize(NULL);
+		size += (1024 * 1024) - 1;
+		size /= (1024 * 1024);
+
+		sprintf(buf, "%lu MB", size);
+		SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
+
 	if (output_config_variable != NULL)
 	{
 		/*
@@ -907,15 +945,6 @@ PostmasterMain(int argc, char *argv[])
 		ExitPostmaster(0);
 	}
 
-	/* Verify that DataDir looks reasonable */
-	checkDataDir();
-
-	/* Check that pg_control exists */
-	checkControlFile();
-
-	/* And switch working directory into it */
-	ChangeToDataDir();
-
 	/*
 	 * Check for invalid combinations of GUC settings.
 	 */
@@ -996,19 +1025,6 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	LocalProcessControlFile(false);
 
-	/*
-	 * Register the apply launcher.  Since it registers a background worker,
-	 * it needs to be called before InitializeMaxBackends(), and it's probably
-	 * a good idea to call it before any modules had chance to take the
-	 * background worker slots.
-	 */
-	ApplyLauncherRegister();
-
-	/*
-	 * process any libraries that should be preloaded at postmaster start
-	 */
-	process_shared_preload_libraries();
-
 	/*
 	 * Initialize SSL library, if specified.
 	 */
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..b225b1ee70 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -75,6 +75,87 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
+/*
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
+ *
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
+ */
+Size
+CalculateShmemSize(int *num_semaphores)
+{
+	Size size;
+	int numSemas;
+
+	/* Compute number of semaphores we'll need */
+	numSemas = ProcGlobalSemas();
+	numSemas += SpinlockSemas();
+
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
+	/*
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
+	 *
+	 * We take some care to ensure that the total size request doesn't overflow
+	 * size_t.  If this gets through, we don't need to be so careful during the
+	 * actual allocation phase.
+	 */
+	size = 100000;
+	size = add_size(size, PGSemaphoreShmemSize(numSemas));
+	size = add_size(size, SpinlockSemaSize());
+	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
+											 sizeof(ShmemIndexEnt)));
+	size = add_size(size, dsm_estimate_size());
+	size = add_size(size, BufferShmemSize());
+	size = add_size(size, LockShmemSize());
+	size = add_size(size, PredicateLockShmemSize());
+	size = add_size(size, ProcGlobalShmemSize());
+	size = add_size(size, XLOGShmemSize());
+	size = add_size(size, CLOGShmemSize());
+	size = add_size(size, CommitTsShmemSize());
+	size = add_size(size, SUBTRANSShmemSize());
+	size = add_size(size, TwoPhaseShmemSize());
+	size = add_size(size, BackgroundWorkerShmemSize());
+	size = add_size(size, MultiXactShmemSize());
+	size = add_size(size, LWLockShmemSize());
+	size = add_size(size, ProcArrayShmemSize());
+	size = add_size(size, BackendStatusShmemSize());
+	size = add_size(size, SInvalShmemSize());
+	size = add_size(size, PMSignalShmemSize());
+	size = add_size(size, ProcSignalShmemSize());
+	size = add_size(size, CheckpointerShmemSize());
+	size = add_size(size, AutoVacuumShmemSize());
+	size = add_size(size, ReplicationSlotsShmemSize());
+	size = add_size(size, ReplicationOriginShmemSize());
+	size = add_size(size, WalSndShmemSize());
+	size = add_size(size, WalRcvShmemSize());
+	size = add_size(size, PgArchShmemSize());
+	size = add_size(size, ApplyLauncherShmemSize());
+	size = add_size(size, SnapMgrShmemSize());
+	size = add_size(size, BTreeShmemSize());
+	size = add_size(size, SyncScanShmemSize());
+	size = add_size(size, AsyncShmemSize());
+#ifdef EXEC_BACKEND
+	size = add_size(size, ShmemBackendArraySize());
+#endif
+
+	/* freeze the addin request size and include it */
+	addin_request_allowed = false;
+	size = add_size(size, total_addin_request);
+
+	/* might as well round it off to a multiple of a typical page size */
+	size = add_size(size, 8192 - (size % 8192));
+
+	return size;
+}
 
 /*
  * CreateSharedMemoryAndSemaphores
@@ -102,65 +183,8 @@ CreateSharedMemoryAndSemaphores(void)
 		Size		size;
 		int			numSemas;
 
-		/* Compute number of semaphores we'll need */
-		numSemas = ProcGlobalSemas();
-		numSemas += SpinlockSemas();
-
-		/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
-		 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
-		 */
-		size = 100000;
-		size = add_size(size, PGSemaphoreShmemSize(numSemas));
-		size = add_size(size, SpinlockSemaSize());
-		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
-												 sizeof(ShmemIndexEnt)));
-		size = add_size(size, dsm_estimate_size());
-		size = add_size(size, BufferShmemSize());
-		size = add_size(size, LockShmemSize());
-		size = add_size(size, PredicateLockShmemSize());
-		size = add_size(size, ProcGlobalShmemSize());
-		size = add_size(size, XLOGShmemSize());
-		size = add_size(size, CLOGShmemSize());
-		size = add_size(size, CommitTsShmemSize());
-		size = add_size(size, SUBTRANSShmemSize());
-		size = add_size(size, TwoPhaseShmemSize());
-		size = add_size(size, BackgroundWorkerShmemSize());
-		size = add_size(size, MultiXactShmemSize());
-		size = add_size(size, LWLockShmemSize());
-		size = add_size(size, ProcArrayShmemSize());
-		size = add_size(size, BackendStatusShmemSize());
-		size = add_size(size, SInvalShmemSize());
-		size = add_size(size, PMSignalShmemSize());
-		size = add_size(size, ProcSignalShmemSize());
-		size = add_size(size, CheckpointerShmemSize());
-		size = add_size(size, AutoVacuumShmemSize());
-		size = add_size(size, ReplicationSlotsShmemSize());
-		size = add_size(size, ReplicationOriginShmemSize());
-		size = add_size(size, WalSndShmemSize());
-		size = add_size(size, WalRcvShmemSize());
-		size = add_size(size, PgArchShmemSize());
-		size = add_size(size, ApplyLauncherShmemSize());
-		size = add_size(size, SnapMgrShmemSize());
-		size = add_size(size, BTreeShmemSize());
-		size = add_size(size, SyncScanShmemSize());
-		size = add_size(size, AsyncShmemSize());
-#ifdef EXEC_BACKEND
-		size = add_size(size, ShmemBackendArraySize());
-#endif
-
-		/* freeze the addin request size and include it */
-		addin_request_allowed = false;
-		size = add_size(size, total_addin_request);
-
-		/* might as well round it off to a multiple of a typical page size */
-		size = add_size(size, 8192 - (size % 8192));
-
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe..872bdc1c8e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -620,6 +620,8 @@ char	   *pgstat_temp_directory;
 
 char	   *application_name;
 
+int			shmem_size_mb;
+
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
 int			tcp_keepalives_count;
@@ -2337,6 +2339,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the amount of shared memory allocated by the server (rounded up to the nearest MB)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+		},
+		&shmem_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
-- 
2.33.0

From 68a6974c73ef512ceb8e35649bf0add4b3547fa3 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 27 Aug 2021 20:03:01 +0000
Subject: [PATCH v3 1/2] Move the shared memory size calculation to its own
 function.

This change refactors the shared memory size calculation in
CreateSharedMemoryAndSemaphores() to its own function.  This is
intended for use in a future change that will simplify the steps
for setting up huge pages.
---
 src/backend/storage/ipc/ipci.c | 142 ++++++++++++++++++++++++-----------------
 src/include/storage/ipc.h      |   1 +
 2 files changed, 84 insertions(+), 59 deletions(-)

diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..b225b1ee70 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -75,6 +75,87 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
+/*
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
+ *
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
+ */
+Size
+CalculateShmemSize(int *num_semaphores)
+{
+	Size size;
+	int numSemas;
+
+	/* Compute number of semaphores we'll need */
+	numSemas = ProcGlobalSemas();
+	numSemas += SpinlockSemas();
+
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
+	/*
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
+	 *
+	 * We take some care to ensure that the total size request doesn't overflow
+	 * size_t.  If this gets through, we don't need to be so careful during the
+	 * actual allocation phase.
+	 */
+	size = 100000;
+	size = add_size(size, PGSemaphoreShmemSize(numSemas));
+	size = add_size(size, SpinlockSemaSize());
+	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
+											 sizeof(ShmemIndexEnt)));
+	size = add_size(size, dsm_estimate_size());
+	size = add_size(size, BufferShmemSize());
+	size = add_size(size, LockShmemSize());
+	size = add_size(size, PredicateLockShmemSize());
+	size = add_size(size, ProcGlobalShmemSize());
+	size = add_size(size, XLOGShmemSize());
+	size = add_size(size, CLOGShmemSize());
+	size = add_size(size, CommitTsShmemSize());
+	size = add_size(size, SUBTRANSShmemSize());
+	size = add_size(size, TwoPhaseShmemSize());
+	size = add_size(size, BackgroundWorkerShmemSize());
+	size = add_size(size, MultiXactShmemSize());
+	size = add_size(size, LWLockShmemSize());
+	size = add_size(size, ProcArrayShmemSize());
+	size = add_size(size, BackendStatusShmemSize());
+	size = add_size(size, SInvalShmemSize());
+	size = add_size(size, PMSignalShmemSize());
+	size = add_size(size, ProcSignalShmemSize());
+	size = add_size(size, CheckpointerShmemSize());
+	size = add_size(size, AutoVacuumShmemSize());
+	size = add_size(size, ReplicationSlotsShmemSize());
+	size = add_size(size, ReplicationOriginShmemSize());
+	size = add_size(size, WalSndShmemSize());
+	size = add_size(size, WalRcvShmemSize());
+	size = add_size(size, PgArchShmemSize());
+	size = add_size(size, ApplyLauncherShmemSize());
+	size = add_size(size, SnapMgrShmemSize());
+	size = add_size(size, BTreeShmemSize());
+	size = add_size(size, SyncScanShmemSize());
+	size = add_size(size, AsyncShmemSize());
+#ifdef EXEC_BACKEND
+	size = add_size(size, ShmemBackendArraySize());
+#endif
+
+	/* freeze the addin request size and include it */
+	addin_request_allowed = false;
+	size = add_size(size, total_addin_request);
+
+	/* might as well round it off to a multiple of a typical page size */
+	size = add_size(size, 8192 - (size % 8192));
+
+	return size;
+}
 
 /*
  * CreateSharedMemoryAndSemaphores
@@ -102,65 +183,8 @@ CreateSharedMemoryAndSemaphores(void)
 		Size		size;
 		int			numSemas;
 
-		/* Compute number of semaphores we'll need */
-		numSemas = ProcGlobalSemas();
-		numSemas += SpinlockSemas();
-
-		/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
-		 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
-		 */
-		size = 100000;
-		size = add_size(size, PGSemaphoreShmemSize(numSemas));
-		size = add_size(size, SpinlockSemaSize());
-		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
-												 sizeof(ShmemIndexEnt)));
-		size = add_size(size, dsm_estimate_size());
-		size = add_size(size, BufferShmemSize());
-		size = add_size(size, LockShmemSize());
-		size = add_size(size, PredicateLockShmemSize());
-		size = add_size(size, ProcGlobalShmemSize());
-		size = add_size(size, XLOGShmemSize());
-		size = add_size(size, CLOGShmemSize());
-		size = add_size(size, CommitTsShmemSize());
-		size = add_size(size, SUBTRANSShmemSize());
-		size = add_size(size, TwoPhaseShmemSize());
-		size = add_size(size, BackgroundWorkerShmemSize());
-		size = add_size(size, MultiXactShmemSize());
-		size = add_size(size, LWLockShmemSize());
-		size = add_size(size, ProcArrayShmemSize());
-		size = add_size(size, BackendStatusShmemSize());
-		size = add_size(size, SInvalShmemSize());
-		size = add_size(size, PMSignalShmemSize());
-		size = add_size(size, ProcSignalShmemSize());
-		size = add_size(size, CheckpointerShmemSize());
-		size = add_size(size, AutoVacuumShmemSize());
-		size = add_size(size, ReplicationSlotsShmemSize());
-		size = add_size(size, ReplicationOriginShmemSize());
-		size = add_size(size, WalSndShmemSize());
-		size = add_size(size, WalRcvShmemSize());
-		size = add_size(size, PgArchShmemSize());
-		size = add_size(size, ApplyLauncherShmemSize());
-		size = add_size(size, SnapMgrShmemSize());
-		size = add_size(size, BTreeShmemSize());
-		size = add_size(size, SyncScanShmemSize());
-		size = add_size(size, AsyncShmemSize());
-#ifdef EXEC_BACKEND
-		size = add_size(size, ShmemBackendArraySize());
-#endif
-
-		/* freeze the addin request size and include it */
-		addin_request_allowed = false;
-		size = add_size(size, total_addin_request);
-
-		/* might as well round it off to a multiple of a typical page size */
-		size = add_size(size, 8192 - (size % 8192));
-
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..80e191d407 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From c7e8d7937a940ecc5d22c4cc757d452bb536ac3e Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Tue, 31 Aug 2021 05:05:43 +0000
Subject: [PATCH v3 2/2] Introduce shared_memory_size and huge_pages_required
 GUCs.

These parameters are intended to simplify huge pages setup.
Instead of manually calculating the number of huge pages required
for the main shared memory segment, a command like the following
can be used to determine how many are needed:

	postgres -D $PGDATA -C huge_pages_required
---
 doc/src/sgml/config.sgml            | 30 ++++++++++++++++++++
 doc/src/sgml/runtime.sgml           | 33 +++++++---------------
 src/backend/port/sysv_shmem.c       |  2 +-
 src/backend/postmaster/postmaster.c | 55 ++++++++++++++++++++++---------------
 src/backend/storage/ipc/ipci.c      | 48 ++++++++++++++++++++++++++++++++
 src/backend/utils/misc/guc.c        | 25 +++++++++++++++++
 src/include/storage/ipc.h           |  1 +
 src/include/storage/pg_shmem.h      |  4 +++
 8 files changed, 152 insertions(+), 46 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2c31c35a6b..e586427640 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10101,6 +10101,22 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-required" xreflabel="huge_pages_required">
+      <term><varname>huge_pages_required</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>huge_pages_required</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are required for the main
+        shared memory segment based on the specified
+        <xref linkend="guc-huge-page-size"/>.  If the huge page size cannot
+        be determined, this will be <literal>-1</literal>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>
@@ -10275,6 +10291,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size" xreflabel="shared_memory_size">
+      <term><varname>shared_memory_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the size of the main shared memory segment, rounded up to
+        the nearest megabyte.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..28bc36283e 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,32 +1442,19 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
-    This might look like:
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-huge-pages-required"/>.  This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C huge_pages_required</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
-
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
-     We can set this with:
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.  While we need at least
+     <literal>3170</literal> huge pages in this example, a larger setting
+     would be appropriate if other programs on the machine also need huge
+     pages.  We can allocate the huge pages with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
 </programlisting>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..f42f1ac171 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -478,7 +478,7 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
  * and the hugepage-related mmap flags to use into *mmap_flags.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
 	Size		default_hugepagesize = 0;
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c2c98614a..c32c21d632 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -893,6 +893,39 @@ PostmasterMain(int argc, char *argv[])
 	if (!SelectConfigFiles(userDoption, progname))
 		ExitPostmaster(2);
 
+	/* Verify that DataDir looks reasonable */
+	checkDataDir();
+
+	/* Check that pg_control exists */
+	checkControlFile();
+
+	/* And switch working directory into it */
+	ChangeToDataDir();
+
+	/*
+	 * Register the apply launcher.  Since it registers a background worker,
+	 * it needs to be called before InitializeMaxBackends(), and it's probably
+	 * a good idea to call it before any modules had chance to take the
+	 * background worker slots.
+	 */
+	ApplyLauncherRegister();
+
+	/*
+	 * Process any libraries that should be preloaded at postmaster start.
+	 * Thie happens before running -C, so as it is possible to get an
+	 * estimation of the total shared memory size allocated to this system,
+	 * accounting for the portion from loaded libraries.
+	 */
+	process_shared_preload_libraries();
+
+	/*
+	 * Determine the value of any runtime-computed GUCs that depend on the
+	 * amount of shared memory required.  It is important to do this after
+	 * preloaded libraries have had a chance to request additional shared
+	 * memory.
+	 */
+	InitializeShmemGUCs();
+
 	if (output_config_variable != NULL)
 	{
 		/*
@@ -907,15 +940,6 @@ PostmasterMain(int argc, char *argv[])
 		ExitPostmaster(0);
 	}
 
-	/* Verify that DataDir looks reasonable */
-	checkDataDir();
-
-	/* Check that pg_control exists */
-	checkControlFile();
-
-	/* And switch working directory into it */
-	ChangeToDataDir();
-
 	/*
 	 * Check for invalid combinations of GUC settings.
 	 */
@@ -996,19 +1020,6 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	LocalProcessControlFile(false);
 
-	/*
-	 * Register the apply launcher.  Since it registers a background worker,
-	 * it needs to be called before InitializeMaxBackends(), and it's probably
-	 * a good idea to call it before any modules had chance to take the
-	 * background worker slots.
-	 */
-	ApplyLauncherRegister();
-
-	/*
-	 * process any libraries that should be preloaded at postmaster start
-	 */
-	process_shared_preload_libraries();
-
 	/*
 	 * Initialize SSL library, if specified.
 	 */
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index b225b1ee70..86061da1bc 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -14,6 +14,10 @@
  */
 #include "postgres.h"
 
+#ifndef WIN32
+#include <sys/mman.h>
+#endif
+
 #include "access/clog.h"
 #include "access/commit_ts.h"
 #include "access/heapam.h"
@@ -313,3 +317,47 @@ CreateSharedMemoryAndSemaphores(void)
 	if (shmem_startup_hook)
 		shmem_startup_hook();
 }
+
+/*
+ * InitializeShmemGUCs
+ *
+ * This function initializes runtime-computed GUCs related to the amount of
+ * shared memory required for the current configuration.
+ */
+void
+InitializeShmemGUCs(void)
+{
+	char	buf[64];
+	Size	size_b;
+	Size	size_mb;
+#if defined(MAP_HUGETLB) || defined(WIN32)
+	Size	hp_size;
+	Size	hp_required;
+#endif
+#ifdef MAP_HUGETLB
+	int		unused;
+#endif
+
+	/*
+	 * Calculate the shared memory size in bytes and in megabytes (rounded
+	 * up to the nearest megabyte).
+	 */
+	size_b = CalculateShmemSize(NULL);
+	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
+
+	sprintf(buf, "%lu MB", size_mb);
+	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+	/* Calculate the number of huge_pages required */
+#if defined(MAP_HUGETLB)
+	GetHugePageSize(&hp_size, &unused);
+#elif defined(WIN32)
+	hp_size = GetLargePageMinimum();
+#endif
+
+#if defined(MAP_HUGETLB) || defined(WIN32)
+	hp_required = (size_b / hp_size) + 1;
+	sprintf(buf, "%lu", hp_required);
+	SetConfigOption("huge_pages_required", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+#endif
+}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe..0d4dd27394 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -620,6 +620,9 @@ char	   *pgstat_temp_directory;
 
 char	   *application_name;
 
+int			shmem_size_mb;
+int			huge_pages_required;
+
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
 int			tcp_keepalives_count;
@@ -2223,6 +2226,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_required", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the huge page size could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_required,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* This is PGC_SUSET to prevent hiding from log_lock_waits. */
 		{"deadlock_timeout", PGC_SUSET, LOCK_MANAGEMENT,
@@ -2337,6 +2351,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the amount of shared memory allocated by the server (rounded up to the nearest MB)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+		},
+		&shmem_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 80e191d407..7a1ebc8559 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -79,5 +79,6 @@ extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
 extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
+extern void InitializeShmemGUCs(void);
 
 #endif							/* IPC_H */
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..c44403ed6a 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -88,4 +88,8 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
 
+#ifdef MAP_HUGETLB
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
+#endif
+
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From ca71493463921ccffa080f353da8c1d47442c644 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 27 Aug 2021 20:03:01 +0000
Subject: [PATCH v4 1/3] Move the shared memory size calculation to its own
 function.

This change refactors the shared memory size calculation in
CreateSharedMemoryAndSemaphores() to its own function.  This is
intended for use in a future change that will simplify the steps
for setting up huge pages.
---
 src/backend/storage/ipc/ipci.c | 142 ++++++++++++++++++++++++-----------------
 src/include/storage/ipc.h      |   1 +
 2 files changed, 84 insertions(+), 59 deletions(-)

diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..b225b1ee70 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -75,6 +75,87 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
+/*
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
+ *
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
+ */
+Size
+CalculateShmemSize(int *num_semaphores)
+{
+	Size size;
+	int numSemas;
+
+	/* Compute number of semaphores we'll need */
+	numSemas = ProcGlobalSemas();
+	numSemas += SpinlockSemas();
+
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
+	/*
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
+	 *
+	 * We take some care to ensure that the total size request doesn't overflow
+	 * size_t.  If this gets through, we don't need to be so careful during the
+	 * actual allocation phase.
+	 */
+	size = 100000;
+	size = add_size(size, PGSemaphoreShmemSize(numSemas));
+	size = add_size(size, SpinlockSemaSize());
+	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
+											 sizeof(ShmemIndexEnt)));
+	size = add_size(size, dsm_estimate_size());
+	size = add_size(size, BufferShmemSize());
+	size = add_size(size, LockShmemSize());
+	size = add_size(size, PredicateLockShmemSize());
+	size = add_size(size, ProcGlobalShmemSize());
+	size = add_size(size, XLOGShmemSize());
+	size = add_size(size, CLOGShmemSize());
+	size = add_size(size, CommitTsShmemSize());
+	size = add_size(size, SUBTRANSShmemSize());
+	size = add_size(size, TwoPhaseShmemSize());
+	size = add_size(size, BackgroundWorkerShmemSize());
+	size = add_size(size, MultiXactShmemSize());
+	size = add_size(size, LWLockShmemSize());
+	size = add_size(size, ProcArrayShmemSize());
+	size = add_size(size, BackendStatusShmemSize());
+	size = add_size(size, SInvalShmemSize());
+	size = add_size(size, PMSignalShmemSize());
+	size = add_size(size, ProcSignalShmemSize());
+	size = add_size(size, CheckpointerShmemSize());
+	size = add_size(size, AutoVacuumShmemSize());
+	size = add_size(size, ReplicationSlotsShmemSize());
+	size = add_size(size, ReplicationOriginShmemSize());
+	size = add_size(size, WalSndShmemSize());
+	size = add_size(size, WalRcvShmemSize());
+	size = add_size(size, PgArchShmemSize());
+	size = add_size(size, ApplyLauncherShmemSize());
+	size = add_size(size, SnapMgrShmemSize());
+	size = add_size(size, BTreeShmemSize());
+	size = add_size(size, SyncScanShmemSize());
+	size = add_size(size, AsyncShmemSize());
+#ifdef EXEC_BACKEND
+	size = add_size(size, ShmemBackendArraySize());
+#endif
+
+	/* freeze the addin request size and include it */
+	addin_request_allowed = false;
+	size = add_size(size, total_addin_request);
+
+	/* might as well round it off to a multiple of a typical page size */
+	size = add_size(size, 8192 - (size % 8192));
+
+	return size;
+}
 
 /*
  * CreateSharedMemoryAndSemaphores
@@ -102,65 +183,8 @@ CreateSharedMemoryAndSemaphores(void)
 		Size		size;
 		int			numSemas;
 
-		/* Compute number of semaphores we'll need */
-		numSemas = ProcGlobalSemas();
-		numSemas += SpinlockSemas();
-
-		/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
-		 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
-		 */
-		size = 100000;
-		size = add_size(size, PGSemaphoreShmemSize(numSemas));
-		size = add_size(size, SpinlockSemaSize());
-		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
-												 sizeof(ShmemIndexEnt)));
-		size = add_size(size, dsm_estimate_size());
-		size = add_size(size, BufferShmemSize());
-		size = add_size(size, LockShmemSize());
-		size = add_size(size, PredicateLockShmemSize());
-		size = add_size(size, ProcGlobalShmemSize());
-		size = add_size(size, XLOGShmemSize());
-		size = add_size(size, CLOGShmemSize());
-		size = add_size(size, CommitTsShmemSize());
-		size = add_size(size, SUBTRANSShmemSize());
-		size = add_size(size, TwoPhaseShmemSize());
-		size = add_size(size, BackgroundWorkerShmemSize());
-		size = add_size(size, MultiXactShmemSize());
-		size = add_size(size, LWLockShmemSize());
-		size = add_size(size, ProcArrayShmemSize());
-		size = add_size(size, BackendStatusShmemSize());
-		size = add_size(size, SInvalShmemSize());
-		size = add_size(size, PMSignalShmemSize());
-		size = add_size(size, ProcSignalShmemSize());
-		size = add_size(size, CheckpointerShmemSize());
-		size = add_size(size, AutoVacuumShmemSize());
-		size = add_size(size, ReplicationSlotsShmemSize());
-		size = add_size(size, ReplicationOriginShmemSize());
-		size = add_size(size, WalSndShmemSize());
-		size = add_size(size, WalRcvShmemSize());
-		size = add_size(size, PgArchShmemSize());
-		size = add_size(size, ApplyLauncherShmemSize());
-		size = add_size(size, SnapMgrShmemSize());
-		size = add_size(size, BTreeShmemSize());
-		size = add_size(size, SyncScanShmemSize());
-		size = add_size(size, AsyncShmemSize());
-#ifdef EXEC_BACKEND
-		size = add_size(size, ShmemBackendArraySize());
-#endif
-
-		/* freeze the addin request size and include it */
-		addin_request_allowed = false;
-		size = add_size(size, total_addin_request);
-
-		/* might as well round it off to a multiple of a typical page size */
-		size = add_size(size, 8192 - (size % 8192));
-
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..80e191d407 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From e51d8d40dd21697e5a4e2b79be8125bec3707b49 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Wed, 1 Sep 2021 18:12:28 +0000
Subject: [PATCH v4 3/3] Introduce huge_pages_required GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.  It can be viewed with
'postgres -C' so that the huge pages can be allocated prior to
startup.
---
 doc/src/sgml/config.sgml       | 16 ++++++++++++++++
 doc/src/sgml/runtime.sgml      | 27 +++++++++------------------
 src/backend/port/sysv_shmem.c  |  2 +-
 src/backend/storage/ipc/ipci.c | 26 ++++++++++++++++++++++++++
 src/backend/utils/misc/guc.c   | 12 ++++++++++++
 src/include/storage/pg_shmem.h |  4 ++++
 6 files changed, 68 insertions(+), 19 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..781147f719 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10101,6 +10101,22 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-required" xreflabel="huge_pages_required">
+      <term><varname>huge_pages_required</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>huge_pages_required</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are required for the main
+        shared memory area based on the specified
+        <xref linkend="guc-huge-page-size"/>.  If the huge page size cannot
+        be determined, this will be <literal>-1</literal>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 2144c0abad..d955639900 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1444,28 +1444,19 @@ export PG_OOM_ADJUST_VALUE=0
     the operating system to provide enough huge pages of the desired size.
     To estimate the number of huge pages needed, use the
     <command>postgres</command> command to see the value of
-    <xref linkend="guc-shared-memory-size"/>, and use the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems
-    to find the system's default and supported huge page sizes.  This might
-    look like:
+    <xref linkend="guc-huge-pages-required"/>.  This might look like:
 <programlisting>
-$ <userinput>postgres -D $PGDATA -C shared_memory_size</userinput>
-6339
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C huge_pages_required</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6339</literal> / <literal>2</literal> gives
-     <literal>3169.5</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
-     We can set this with:
+     While we need at least <literal>3170</literal> huge pages in this
+     example, a larger setting would be appropriate if other programs on
+     the machine also need huge pages.  We can allocate the huge pages
+     with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
 </programlisting>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..f42f1ac171 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -478,7 +478,7 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
  * and the hugepage-related mmap flags to use into *mmap_flags.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
 	Size		default_hugepagesize = 0;
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index f5736703a8..a625dd7701 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -14,6 +14,10 @@
  */
 #include "postgres.h"
 
+#ifndef WIN32
+#include <sys/mman.h>
+#endif
+
 #include "access/clog.h"
 #include "access/commit_ts.h"
 #include "access/heapam.h"
@@ -326,6 +330,13 @@ InitializeShmemGUCs(void)
 	char	buf[64];
 	Size	size_b;
 	Size	size_mb;
+#if defined(MAP_HUGETLB) || defined(WIN32)
+	Size	hp_size;
+	Size	hp_required;
+#endif
+#ifdef MAP_HUGETLB
+	int		unused;
+#endif
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest
@@ -336,4 +347,19 @@ InitializeShmemGUCs(void)
 
 	sprintf(buf, "%lu MB", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+	/* Calculate the number of huge pages required */
+#if defined(MAP_HUGETLB)
+	GetHugePageSize(&hp_size, &unused);
+#elif defined(WIN32)
+	hp_size = GetLargePageMinimum();
+#endif
+
+#if defined(MAP_HUGETLB) || defined(WIN32)
+	if (hp_size == 0)
+		return;
+	hp_required = (size_b / hp_size) + 1;
+	sprintf(buf, "%lu", hp_required);
+	SetConfigOption("huge_pages_required", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+#endif
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a11387c5ce..3e9a01746e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -621,6 +621,7 @@ char	   *pgstat_temp_directory;
 char	   *application_name;
 
 int			shmem_size_mb;
+int			huge_pages_required;
 
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
@@ -2225,6 +2226,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_required", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the huge page size could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_required,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* This is PGC_SUSET to prevent hiding from log_lock_waits. */
 		{"deadlock_timeout", PGC_SUSET, LOCK_MANAGEMENT,
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..c44403ed6a 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -88,4 +88,8 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
 
+#ifdef MAP_HUGETLB
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
+#endif
+
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From bda20ec9080b739ee6698b2b5f34d04fbe1b86f6 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Wed, 1 Sep 2021 18:22:11 +0000
Subject: [PATCH v4 2/3] Introduce shared_memory_size GUC.

This runtime-computed GUC shows the size of the server's main
shared memory area.  It can also be viewed with 'postgres -C',
which is useful for calculating the number of huge pages required
prior to startup.
---
 doc/src/sgml/config.sgml            | 14 ++++++++++
 doc/src/sgml/runtime.sgml           | 22 +++++++--------
 src/backend/postmaster/postmaster.c | 55 ++++++++++++++++++++++---------------
 src/backend/storage/ipc/ipci.c      | 24 ++++++++++++++++
 src/backend/utils/misc/guc.c        | 13 +++++++++
 src/include/storage/ipc.h           |  1 +
 6 files changed, 95 insertions(+), 34 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2c31c35a6b..ef0e2a7746 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10275,6 +10275,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size" xreflabel="shared_memory_size">
+      <term><varname>shared_memory_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the size of the main shared memory area, rounded up to the
+        nearest megabyte.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..2144c0abad 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,17 +1442,15 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
-    This might look like:
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-shared-memory-size"/>, and use the
+    <filename>/proc</filename> and <filename>/sys</filename> file systems
+    to find the system's default and supported huge page sizes.  This might
+    look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
+$ <userinput>postgres -D $PGDATA -C shared_memory_size</userinput>
+6339
 $ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
 Hugepagesize:       2048 kB
 $ <userinput>ls /sys/kernel/mm/hugepages</userinput>
@@ -1463,8 +1461,8 @@ hugepages-1048576kB  hugepages-2048kB
      either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
 
      Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
+     <literal>6339</literal> / <literal>2</literal> gives
+     <literal>3169.5</literal>, so in this example we need at
      least <literal>3170</literal> huge pages.  A larger setting would be
      appropriate if other programs on the machine also need huge pages.
      We can set this with:
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c2c98614a..c32c21d632 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -893,6 +893,39 @@ PostmasterMain(int argc, char *argv[])
 	if (!SelectConfigFiles(userDoption, progname))
 		ExitPostmaster(2);
 
+	/* Verify that DataDir looks reasonable */
+	checkDataDir();
+
+	/* Check that pg_control exists */
+	checkControlFile();
+
+	/* And switch working directory into it */
+	ChangeToDataDir();
+
+	/*
+	 * Register the apply launcher.  Since it registers a background worker,
+	 * it needs to be called before InitializeMaxBackends(), and it's probably
+	 * a good idea to call it before any modules had chance to take the
+	 * background worker slots.
+	 */
+	ApplyLauncherRegister();
+
+	/*
+	 * Process any libraries that should be preloaded at postmaster start.
+	 * Thie happens before running -C, so as it is possible to get an
+	 * estimation of the total shared memory size allocated to this system,
+	 * accounting for the portion from loaded libraries.
+	 */
+	process_shared_preload_libraries();
+
+	/*
+	 * Determine the value of any runtime-computed GUCs that depend on the
+	 * amount of shared memory required.  It is important to do this after
+	 * preloaded libraries have had a chance to request additional shared
+	 * memory.
+	 */
+	InitializeShmemGUCs();
+
 	if (output_config_variable != NULL)
 	{
 		/*
@@ -907,15 +940,6 @@ PostmasterMain(int argc, char *argv[])
 		ExitPostmaster(0);
 	}
 
-	/* Verify that DataDir looks reasonable */
-	checkDataDir();
-
-	/* Check that pg_control exists */
-	checkControlFile();
-
-	/* And switch working directory into it */
-	ChangeToDataDir();
-
 	/*
 	 * Check for invalid combinations of GUC settings.
 	 */
@@ -996,19 +1020,6 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	LocalProcessControlFile(false);
 
-	/*
-	 * Register the apply launcher.  Since it registers a background worker,
-	 * it needs to be called before InitializeMaxBackends(), and it's probably
-	 * a good idea to call it before any modules had chance to take the
-	 * background worker slots.
-	 */
-	ApplyLauncherRegister();
-
-	/*
-	 * process any libraries that should be preloaded at postmaster start
-	 */
-	process_shared_preload_libraries();
-
 	/*
 	 * Initialize SSL library, if specified.
 	 */
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index b225b1ee70..f5736703a8 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -313,3 +313,27 @@ CreateSharedMemoryAndSemaphores(void)
 	if (shmem_startup_hook)
 		shmem_startup_hook();
 }
+
+/*
+ * InitializeShmemGUCs
+ *
+ * This function initializes runtime-computed GUCs related to the amount of
+ * shared memory required for the current configuration.
+ */
+void
+InitializeShmemGUCs(void)
+{
+	char	buf[64];
+	Size	size_b;
+	Size	size_mb;
+
+	/*
+	 * Calculate the shared memory size and round up to the nearest
+	 * megabyte.
+	 */
+	size_b = CalculateShmemSize(NULL);
+	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
+
+	sprintf(buf, "%lu MB", size_mb);
+	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe..a11387c5ce 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -620,6 +620,8 @@ char	   *pgstat_temp_directory;
 
 char	   *application_name;
 
+int			shmem_size_mb;
+
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
 int			tcp_keepalives_count;
@@ -2337,6 +2339,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+		},
+		&shmem_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 80e191d407..7a1ebc8559 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -79,5 +79,6 @@ extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
 extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
+extern void InitializeShmemGUCs(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From 7939bf84c276cc507ff14d0284dfb3fb7db705d6 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 27 Aug 2021 20:03:01 +0000
Subject: [PATCH v5 1/4] Move the shared memory size calculation to its own
 function.

This change refactors the shared memory size calculation in
CreateSharedMemoryAndSemaphores() to its own function.  This is
intended for use in a future change that will simplify the steps
for setting up huge pages.
---
 src/backend/storage/ipc/ipci.c | 142 ++++++++++++++++++++++++-----------------
 src/include/storage/ipc.h      |   1 +
 2 files changed, 84 insertions(+), 59 deletions(-)

diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3e4ec53a97..b225b1ee70 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -75,6 +75,87 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
+/*
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
+ *
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
+ */
+Size
+CalculateShmemSize(int *num_semaphores)
+{
+	Size size;
+	int numSemas;
+
+	/* Compute number of semaphores we'll need */
+	numSemas = ProcGlobalSemas();
+	numSemas += SpinlockSemas();
+
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
+	/*
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
+	 *
+	 * We take some care to ensure that the total size request doesn't overflow
+	 * size_t.  If this gets through, we don't need to be so careful during the
+	 * actual allocation phase.
+	 */
+	size = 100000;
+	size = add_size(size, PGSemaphoreShmemSize(numSemas));
+	size = add_size(size, SpinlockSemaSize());
+	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
+											 sizeof(ShmemIndexEnt)));
+	size = add_size(size, dsm_estimate_size());
+	size = add_size(size, BufferShmemSize());
+	size = add_size(size, LockShmemSize());
+	size = add_size(size, PredicateLockShmemSize());
+	size = add_size(size, ProcGlobalShmemSize());
+	size = add_size(size, XLOGShmemSize());
+	size = add_size(size, CLOGShmemSize());
+	size = add_size(size, CommitTsShmemSize());
+	size = add_size(size, SUBTRANSShmemSize());
+	size = add_size(size, TwoPhaseShmemSize());
+	size = add_size(size, BackgroundWorkerShmemSize());
+	size = add_size(size, MultiXactShmemSize());
+	size = add_size(size, LWLockShmemSize());
+	size = add_size(size, ProcArrayShmemSize());
+	size = add_size(size, BackendStatusShmemSize());
+	size = add_size(size, SInvalShmemSize());
+	size = add_size(size, PMSignalShmemSize());
+	size = add_size(size, ProcSignalShmemSize());
+	size = add_size(size, CheckpointerShmemSize());
+	size = add_size(size, AutoVacuumShmemSize());
+	size = add_size(size, ReplicationSlotsShmemSize());
+	size = add_size(size, ReplicationOriginShmemSize());
+	size = add_size(size, WalSndShmemSize());
+	size = add_size(size, WalRcvShmemSize());
+	size = add_size(size, PgArchShmemSize());
+	size = add_size(size, ApplyLauncherShmemSize());
+	size = add_size(size, SnapMgrShmemSize());
+	size = add_size(size, BTreeShmemSize());
+	size = add_size(size, SyncScanShmemSize());
+	size = add_size(size, AsyncShmemSize());
+#ifdef EXEC_BACKEND
+	size = add_size(size, ShmemBackendArraySize());
+#endif
+
+	/* freeze the addin request size and include it */
+	addin_request_allowed = false;
+	size = add_size(size, total_addin_request);
+
+	/* might as well round it off to a multiple of a typical page size */
+	size = add_size(size, 8192 - (size % 8192));
+
+	return size;
+}
 
 /*
  * CreateSharedMemoryAndSemaphores
@@ -102,65 +183,8 @@ CreateSharedMemoryAndSemaphores(void)
 		Size		size;
 		int			numSemas;
 
-		/* Compute number of semaphores we'll need */
-		numSemas = ProcGlobalSemas();
-		numSemas += SpinlockSemas();
-
-		/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
-		 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
-		 */
-		size = 100000;
-		size = add_size(size, PGSemaphoreShmemSize(numSemas));
-		size = add_size(size, SpinlockSemaSize());
-		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
-												 sizeof(ShmemIndexEnt)));
-		size = add_size(size, dsm_estimate_size());
-		size = add_size(size, BufferShmemSize());
-		size = add_size(size, LockShmemSize());
-		size = add_size(size, PredicateLockShmemSize());
-		size = add_size(size, ProcGlobalShmemSize());
-		size = add_size(size, XLOGShmemSize());
-		size = add_size(size, CLOGShmemSize());
-		size = add_size(size, CommitTsShmemSize());
-		size = add_size(size, SUBTRANSShmemSize());
-		size = add_size(size, TwoPhaseShmemSize());
-		size = add_size(size, BackgroundWorkerShmemSize());
-		size = add_size(size, MultiXactShmemSize());
-		size = add_size(size, LWLockShmemSize());
-		size = add_size(size, ProcArrayShmemSize());
-		size = add_size(size, BackendStatusShmemSize());
-		size = add_size(size, SInvalShmemSize());
-		size = add_size(size, PMSignalShmemSize());
-		size = add_size(size, ProcSignalShmemSize());
-		size = add_size(size, CheckpointerShmemSize());
-		size = add_size(size, AutoVacuumShmemSize());
-		size = add_size(size, ReplicationSlotsShmemSize());
-		size = add_size(size, ReplicationOriginShmemSize());
-		size = add_size(size, WalSndShmemSize());
-		size = add_size(size, WalRcvShmemSize());
-		size = add_size(size, PgArchShmemSize());
-		size = add_size(size, ApplyLauncherShmemSize());
-		size = add_size(size, SnapMgrShmemSize());
-		size = add_size(size, BTreeShmemSize());
-		size = add_size(size, SyncScanShmemSize());
-		size = add_size(size, AsyncShmemSize());
-#ifdef EXEC_BACKEND
-		size = add_size(size, ShmemBackendArraySize());
-#endif
-
-		/* freeze the addin request size and include it */
-		addin_request_allowed = false;
-		size = add_size(size, total_addin_request);
-
-		/* might as well round it off to a multiple of a typical page size */
-		size = add_size(size, 8192 - (size % 8192));
-
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 753a6dd4d7..80e191d407 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From e65d7c001b6fb6ae6e903eaffa155935459bb2d8 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 3 Sep 2021 17:11:51 +0000
Subject: [PATCH v5 4/4] Introduce huge_pages_required GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.  It can be viewed with
'postgres -C' so that the huge pages can be allocated prior to
startup.
---
 doc/src/sgml/config.sgml       | 21 +++++++++++++++++++++
 doc/src/sgml/runtime.sgml      | 27 +++++++++------------------
 src/backend/port/sysv_shmem.c  |  2 +-
 src/backend/storage/ipc/ipci.c | 20 ++++++++++++++++++++
 src/backend/utils/misc/guc.c   | 12 ++++++++++++
 src/include/storage/pg_shmem.h |  4 ++++
 6 files changed, 67 insertions(+), 19 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..b27d8aff15 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10101,6 +10101,27 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-required" xreflabel="huge_pages_required">
+      <term><varname>huge_pages_required</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>huge_pages_required</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are required for the main
+        shared memory area based on the specified
+        <xref linkend="guc-huge-page-size"/>.  If huge pages are not supported,
+        this will be <literal>-1</literal>.
+       </para>
+       <para>
+        This setting is supported only on Linux.  It is always set to
+        <literal>-1</literal> on other platforms.  For more details about using
+        huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 2144c0abad..d955639900 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1444,28 +1444,19 @@ export PG_OOM_ADJUST_VALUE=0
     the operating system to provide enough huge pages of the desired size.
     To estimate the number of huge pages needed, use the
     <command>postgres</command> command to see the value of
-    <xref linkend="guc-shared-memory-size"/>, and use the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems
-    to find the system's default and supported huge page sizes.  This might
-    look like:
+    <xref linkend="guc-huge-pages-required"/>.  This might look like:
 <programlisting>
-$ <userinput>postgres -D $PGDATA -C shared_memory_size</userinput>
-6339
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C huge_pages_required</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6339</literal> / <literal>2</literal> gives
-     <literal>3169.5</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
-     We can set this with:
+     While we need at least <literal>3170</literal> huge pages in this
+     example, a larger setting would be appropriate if other programs on
+     the machine also need huge pages.  We can allocate the huge pages
+     with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
 </programlisting>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..f42f1ac171 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -478,7 +478,7 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
  * and the hugepage-related mmap flags to use into *mmap_flags.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
 	Size		default_hugepagesize = 0;
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index f5736703a8..19ff9dff26 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -14,6 +14,10 @@
  */
 #include "postgres.h"
 
+#ifndef WIN32
+#include <sys/mman.h>
+#endif
+
 #include "access/clog.h"
 #include "access/commit_ts.h"
 #include "access/heapam.h"
@@ -326,6 +330,11 @@ InitializeShmemGUCs(void)
 	char	buf[64];
 	Size	size_b;
 	Size	size_mb;
+#if defined(MAP_HUGETLB)
+	Size	hp_size;
+	Size	hp_required;
+	int		unused;
+#endif
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest
@@ -336,4 +345,15 @@ InitializeShmemGUCs(void)
 
 	sprintf(buf, "%lu MB", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+#if defined(MAP_HUGETLB)
+
+	/* Calculate the number of huge pages required */
+	GetHugePageSize(&hp_size, &unused);
+	hp_required = (size_b / hp_size) + 1;
+
+	sprintf(buf, "%lu", hp_required);
+	SetConfigOption("huge_pages_required", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+#endif
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a11387c5ce..1a30d635fc 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -621,6 +621,7 @@ char	   *pgstat_temp_directory;
 char	   *application_name;
 
 int			shmem_size_mb;
+int			huge_pages_required;
 
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
@@ -2225,6 +2226,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_required", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the value could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_required,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* This is PGC_SUSET to prevent hiding from log_lock_waits. */
 		{"deadlock_timeout", PGC_SUSET, LOCK_MANAGEMENT,
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..c44403ed6a 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -88,4 +88,8 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
 
+#ifdef MAP_HUGETLB
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
+#endif
+
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From 2e2638b2c5ad167620dd5b41eec10aaa790b8c5a Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 3 Sep 2021 16:49:45 +0000
Subject: [PATCH v5 3/4] Introduce shared_memory_size GUC.

This runtime-computed GUC shows the size of the server's main
shared memory area.  It can also be viewed with 'postgres -C',
which is useful for calculating the number of huge pages required
prior to startup.
---
 doc/src/sgml/config.sgml            | 14 ++++++++++++++
 doc/src/sgml/runtime.sgml           | 22 ++++++++++------------
 src/backend/postmaster/postmaster.c |  7 +++++++
 src/backend/storage/ipc/ipci.c      | 24 ++++++++++++++++++++++++
 src/backend/utils/misc/guc.c        | 13 +++++++++++++
 src/include/storage/ipc.h           |  1 +
 6 files changed, 69 insertions(+), 12 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2c31c35a6b..ef0e2a7746 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10275,6 +10275,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size" xreflabel="shared_memory_size">
+      <term><varname>shared_memory_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the size of the main shared memory area, rounded up to the
+        nearest megabyte.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..2144c0abad 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,17 +1442,15 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
-    This might look like:
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-shared-memory-size"/>, and use the
+    <filename>/proc</filename> and <filename>/sys</filename> file systems
+    to find the system's default and supported huge page sizes.  This might
+    look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
+$ <userinput>postgres -D $PGDATA -C shared_memory_size</userinput>
+6339
 $ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
 Hugepagesize:       2048 kB
 $ <userinput>ls /sys/kernel/mm/hugepages</userinput>
@@ -1463,8 +1461,8 @@ hugepages-1048576kB  hugepages-2048kB
      either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
 
      Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
+     <literal>6339</literal> / <literal>2</literal> gives
+     <literal>3169.5</literal>, so in this example we need at
      least <literal>3170</literal> huge pages.  A larger setting would be
      appropriate if other programs on the machine also need huge pages.
      We can set this with:
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 0fa2f41ddc..c0fbc83539 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1016,6 +1016,13 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	/*
+	 * Now that loadable modules have had their chance to request additional
+	 * shared memory, determine the value of any runtime-computed GUCs that
+	 * depend on the amount of shared memory required.
+	 */
+	InitializeShmemGUCs();
+
 	/*
 	 * If user requested that we print a GUC's value and exit (via "-C guc"),
 	 * do that now since we've loaded all configurations.
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index b225b1ee70..f5736703a8 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -313,3 +313,27 @@ CreateSharedMemoryAndSemaphores(void)
 	if (shmem_startup_hook)
 		shmem_startup_hook();
 }
+
+/*
+ * InitializeShmemGUCs
+ *
+ * This function initializes runtime-computed GUCs related to the amount of
+ * shared memory required for the current configuration.
+ */
+void
+InitializeShmemGUCs(void)
+{
+	char	buf[64];
+	Size	size_b;
+	Size	size_mb;
+
+	/*
+	 * Calculate the shared memory size and round up to the nearest
+	 * megabyte.
+	 */
+	size_b = CalculateShmemSize(NULL);
+	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
+
+	sprintf(buf, "%lu MB", size_mb);
+	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe..a11387c5ce 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -620,6 +620,8 @@ char	   *pgstat_temp_directory;
 
 char	   *application_name;
 
+int			shmem_size_mb;
+
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
 int			tcp_keepalives_count;
@@ -2337,6 +2339,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+		},
+		&shmem_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 80e191d407..7a1ebc8559 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -79,5 +79,6 @@ extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
 extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
+extern void InitializeShmemGUCs(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From ba43a45c5ace5573589735514284d2dd1a6aec42 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 3 Sep 2021 16:41:31 +0000
Subject: [PATCH v5 2/4] Move postmaster "-C" logic to after processing preload
 libraries.

This change allows us to return correct values for runtime-computed
GUCs such as data_checksums and for future GUCs related to the
amount of shared memory required for the server.

One notable behavior change is that "-C" will now run all preload
libraries' _PG_init() functions without locking the data directory.
If a library's _PG_init() function might do something that would
break a running server, it will need to be adjusted to be safe.
---
 src/backend/postmaster/postmaster.c | 33 ++++++++++++++++++---------------
 1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c2c98614a..0fa2f41ddc 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -893,20 +893,6 @@ PostmasterMain(int argc, char *argv[])
 	if (!SelectConfigFiles(userDoption, progname))
 		ExitPostmaster(2);
 
-	if (output_config_variable != NULL)
-	{
-		/*
-		 * "-C guc" was specified, so print GUC's value and exit.  No extra
-		 * permission check is needed because the user is reading inside the
-		 * data dir.
-		 */
-		const char *config_val = GetConfigOption(output_config_variable,
-												 false, false);
-
-		puts(config_val ? config_val : "");
-		ExitPostmaster(0);
-	}
-
 	/* Verify that DataDir looks reasonable */
 	checkDataDir();
 
@@ -982,8 +968,12 @@ PostmasterMain(int argc, char *argv[])
 	 * is responsible for removing both data directory and socket lockfiles;
 	 * so it must happen before opening sockets so that at exit, the socket
 	 * lockfiles go away after CloseServerPorts runs.
+	 *
+	 * We skip this step if we are just going to print a GUC's value and exit
+	 * a few steps down.
 	 */
-	CreateDataDirLockFile(true);
+	if (output_config_variable == NULL)
+		CreateDataDirLockFile(true);
 
 	/*
 	 * Read the control file (for error checking and config info).
@@ -1026,6 +1016,19 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	/*
+	 * If user requested that we print a GUC's value and exit (via "-C guc"),
+	 * do that now since we've loaded all configurations.
+	 */
+	if (output_config_variable != NULL)
+	{
+		const char *config_val = GetConfigOption(output_config_variable,
+												 false, false);
+
+		puts(config_val ? config_val : "");
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
-- 
2.16.6

From bce822ac162a63be8ece7ee035539d7951170b3e Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Mon, 6 Sep 2021 23:31:41 +0000
Subject: [PATCH v6 3/3] Provide useful values for 'postgres -C' with
 runtime-computed GUCs.

The -C option is handled before a small subset of GUCs that are
computed at runtime are initialized.  Unfortunately, we cannot move
this handling to after they are initialized without disallowing
'postgres -C' on a running server.  One notable reason for this is
that loadable libraries' _PG_init() functions are called before all
runtime-computed GUCs are initialized, and this is not guaranteed
to be safe to do on running servers.

In order to provide useful values for 'postgres -C' for runtime-
computed GUCs, this change adds a new section for handling just
these GUCs just before shared memory is initialized.  While users
won't be able to use -C for runtime-computed GUCs on running
servers, providing a useful value with this restriction seems
better than not providing a useful value at all.
---
 doc/src/sgml/ref/postgres-ref.sgml  | 10 ++++++--
 doc/src/sgml/runtime.sgml           | 33 ++++++++----------------
 src/backend/postmaster/postmaster.c | 50 +++++++++++++++++++++++++++++++------
 src/backend/utils/misc/guc.c        | 10 ++++----
 src/include/utils/guc.h             |  6 +++++
 5 files changed, 73 insertions(+), 36 deletions(-)

diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index 4aaa7abe1a..f6f2246bff 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -133,13 +133,19 @@ PostgreSQL documentation
       <listitem>
        <para>
         Prints the value of the named run-time parameter, and exits.
-        (See the <option>-c</option> option above for details.)  This can
-        be used on a running server, and returns values from
+        (See the <option>-c</option> option above for details.)  This
+        returns values from
         <filename>postgresql.conf</filename>, modified by any parameters
         supplied in this invocation.  It does not reflect parameters
         supplied when the cluster was started.
        </para>
 
+       <para>
+        This can be used on a running server for most parameters.  However,
+        the server must be shut down for some runtime-computed parameters
+        (e.g., <xref linkend="guc-huge-pages-required"/>).
+       </para>
+
        <para>
         This option is meant for other programs that interact with a server
         instance, such as <xref linkend="app-pg-ctl"/>, to query configuration
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..d955639900 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,32 +1442,21 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
-    This might look like:
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-huge-pages-required"/>.  This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C huge_pages_required</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
-     We can set this with:
+     While we need at least <literal>3170</literal> huge pages in this
+     example, a larger setting would be appropriate if other programs on
+     the machine also need huge pages.  We can allocate the huge pages
+     with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
 </programlisting>
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 963c03fa93..d3cbb14d33 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -896,15 +896,31 @@ PostmasterMain(int argc, char *argv[])
 	if (output_config_variable != NULL)
 	{
 		/*
-		 * "-C guc" was specified, so print GUC's value and exit.  No extra
-		 * permission check is needed because the user is reading inside the
-		 * data dir.
+		 * If this is a runtime-computed GUC, it hasn't yet been initialized,
+		 * and the present value is not useful.  However, this is a convenient
+		 * place to print the value for most GUCs because it is safe to run
+		 * postmaster startup to this point even if the server is already
+		 * running.  For the handful of runtime-computed GUCs that we can't
+		 * provide meaningful values for yet, we wait until later in postmaster
+		 * startup to print the value.  We won't be able to use -C on running
+		 * servers for those GUCs, but otherwise this option is unusable for
+		 * them.
 		 */
-		const char *config_val = GetConfigOption(output_config_variable,
-												 false, false);
+		int flags = GetConfigOptionFlags(output_config_variable, true);
 
-		puts(config_val ? config_val : "");
-		ExitPostmaster(0);
+		if ((flags & GUC_RUNTIME_COMPUTED) == 0)
+		{
+			/*
+			 * "-C guc" was specified, so print GUC's value and exit.  No extra
+			 * permission check is needed because the user is reading inside
+			 * the data dir.
+			 */
+			const char *config_val = GetConfigOption(output_config_variable,
+													 false, false);
+
+			puts(config_val ? config_val : "");
+			ExitPostmaster(0);
+		}
 	}
 
 	/* Verify that DataDir looks reasonable */
@@ -1033,6 +1049,26 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeShmemGUCs();
 
+	/*
+	 * If -C was specified with a runtime-computed GUC, we held off printing
+	 * the value earlier, as the GUC was not yet initialized.  We handle -C for
+	 * most GUCs before we lock the data directory so that the option may be
+	 * used on a running server.  However, a handful of GUCs are runtime-
+	 * computed and do not have meaningful values until after locking the data
+	 * directory, and we cannot safely calculate their values earlier on a
+	 * running server.  At this point, such GUCs should be properly
+	 * initialized, and we haven't yet set up shared memory, so this is a good
+	 * time to handle the -C option for these special GUCs.
+	 */
+	if (output_config_variable != NULL)
+	{
+		const char *config_val = GetConfigOption(output_config_variable,
+												 false, false);
+
+		puts(config_val ? config_val : "");
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 1a30d635fc..cabbd083d9 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1985,7 +1985,7 @@ static struct config_bool ConfigureNamesBool[] =
 		{"data_checksums", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows whether data checksums are turned on for this cluster."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_checksums,
 		false,
@@ -2230,7 +2230,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"huge_pages_required", PGC_INTERNAL, RESOURCES_MEM,
 			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
 			gettext_noop("-1 indicates that the value could not be determined."),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&huge_pages_required,
 		-1, -1, INT_MAX,
@@ -2355,7 +2355,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
 			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB | GUC_RUNTIME_COMPUTED
 		},
 		&shmem_size_mb,
 		0, 0, INT_MAX,
@@ -2420,7 +2420,7 @@ static struct config_int ConfigureNamesInt[] =
 						 "in the form accepted by the chmod and umask system "
 						 "calls. (To use the customary octal format the number "
 						 "must start with a 0 (zero).)"),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_directory_mode,
 		0700, 0000, 0777,
@@ -3244,7 +3244,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"wal_segment_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of write ahead log segments."),
 			NULL,
-			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&wal_segment_size,
 		DEFAULT_XLOG_SEG_SIZE,
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index a7c3a4958e..aa18d304ac 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -229,6 +229,12 @@ typedef enum
 
 #define GUC_EXPLAIN			  0x100000	/* include in explain */
 
+/*
+ * GUC_RUNTIME_COMPUTED is intended for runtime-computed GUCs that are only
+ * available via 'postgres -C' if the server is not running.
+ */
+#define GUC_RUNTIME_COMPUTED  0x200000
+
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
 
-- 
2.16.6

From 6010e5bc8c1574a982d4cec2cc323f972f553071 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Mon, 6 Sep 2021 18:04:59 +0000
Subject: [PATCH v6 2/3] Introduce huge_pages_required GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.  Like shared_memory_size,
it cannot be viewed with 'postgres -C' yet.
---
 doc/src/sgml/config.sgml       | 21 +++++++++++++++++++++
 src/backend/port/sysv_shmem.c  |  2 +-
 src/backend/storage/ipc/ipci.c | 20 ++++++++++++++++++++
 src/backend/utils/misc/guc.c   | 12 ++++++++++++
 src/include/storage/pg_shmem.h |  4 ++++
 5 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..b27d8aff15 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10101,6 +10101,27 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-required" xreflabel="huge_pages_required">
+      <term><varname>huge_pages_required</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>huge_pages_required</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are required for the main
+        shared memory area based on the specified
+        <xref linkend="guc-huge-page-size"/>.  If huge pages are not supported,
+        this will be <literal>-1</literal>.
+       </para>
+       <para>
+        This setting is supported only on Linux.  It is always set to
+        <literal>-1</literal> on other platforms.  For more details about using
+        huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..f42f1ac171 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -478,7 +478,7 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
  * and the hugepage-related mmap flags to use into *mmap_flags.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
 	Size		default_hugepagesize = 0;
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 614b8e92c4..e5f94f9e69 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -14,6 +14,10 @@
  */
 #include "postgres.h"
 
+#ifndef WIN32
+#include <sys/mman.h>
+#endif
+
 #include "access/clog.h"
 #include "access/commit_ts.h"
 #include "access/heapam.h"
@@ -326,6 +330,11 @@ InitializeShmemGUCs(void)
 	char	buf[64];
 	Size	size_b;
 	Size	size_mb;
+#if defined(MAP_HUGETLB)
+	Size	hp_size;
+	Size	hp_required;
+	int		unused;
+#endif
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest
@@ -336,4 +345,15 @@ InitializeShmemGUCs(void)
 
 	sprintf(buf, "%lu MB", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+#if defined(MAP_HUGETLB)
+
+	/* Calculate the number of huge pages required */
+	GetHugePageSize(&hp_size, &unused);
+	hp_required = (size_b / hp_size) + 1;
+
+	sprintf(buf, "%lu", hp_required);
+	SetConfigOption("huge_pages_required", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+#endif
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a11387c5ce..1a30d635fc 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -621,6 +621,7 @@ char	   *pgstat_temp_directory;
 char	   *application_name;
 
 int			shmem_size_mb;
+int			huge_pages_required;
 
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
@@ -2225,6 +2226,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_required", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the value could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_required,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* This is PGC_SUSET to prevent hiding from log_lock_waits. */
 		{"deadlock_timeout", PGC_SUSET, LOCK_MANAGEMENT,
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..c44403ed6a 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -88,4 +88,8 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
 
+#ifdef MAP_HUGETLB
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
+#endif
+
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From fedabb6b8fec47ba783812b3408f3e6eeb122f74 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Mon, 6 Sep 2021 18:00:02 +0000
Subject: [PATCH v6 1/3] Introduce shared_memory_size GUC.

This runtime-computed GUC shows the size of the server's main
shared memory area.  While this is intended to be useful for
calculating the number of huge pages required prior to startup, it
cannot be viewed via 'postgres -C' yet.  This may be addressed in a
future change.
---
 doc/src/sgml/config.sgml            | 14 ++++++++++++++
 src/backend/postmaster/postmaster.c |  7 +++++++
 src/backend/storage/ipc/ipci.c      | 24 ++++++++++++++++++++++++
 src/backend/utils/misc/guc.c        | 13 +++++++++++++
 src/include/storage/ipc.h           |  1 +
 5 files changed, 59 insertions(+)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2c31c35a6b..ef0e2a7746 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10275,6 +10275,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size" xreflabel="shared_memory_size">
+      <term><varname>shared_memory_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the size of the main shared memory area, rounded up to the
+        nearest megabyte.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c2c98614a..963c03fa93 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1026,6 +1026,13 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	/*
+	 * Now that loadable modules have had their chance to request additional
+	 * shared memory, determine the value of any runtime-computed GUCs that
+	 * depend on the amount of shared memory required.
+	 */
+	InitializeShmemGUCs();
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 64bc16fa84..614b8e92c4 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -313,3 +313,27 @@ CreateSharedMemoryAndSemaphores(void)
 	if (shmem_startup_hook)
 		shmem_startup_hook();
 }
+
+/*
+ * InitializeShmemGUCs
+ *
+ * This function initializes runtime-computed GUCs related to the amount of
+ * shared memory required for the current configuration.
+ */
+void
+InitializeShmemGUCs(void)
+{
+	char	buf[64];
+	Size	size_b;
+	Size	size_mb;
+
+	/*
+	 * Calculate the shared memory size and round up to the nearest
+	 * megabyte.
+	 */
+	size_b = CalculateShmemSize(NULL);
+	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
+
+	sprintf(buf, "%lu MB", size_mb);
+	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe..a11387c5ce 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -620,6 +620,8 @@ char	   *pgstat_temp_directory;
 
 char	   *application_name;
 
+int			shmem_size_mb;
+
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
 int			tcp_keepalives_count;
@@ -2337,6 +2339,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+		},
+		&shmem_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 80e191d407..7a1ebc8559 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -79,5 +79,6 @@ extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
 extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
+extern void InitializeShmemGUCs(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From 535a46135b2e836882197cb5e4ee4eadd1a945a0 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Mon, 6 Sep 2021 23:31:41 +0000
Subject: [PATCH v7 3/3] Provide useful values for 'postgres -C' with
 runtime-computed GUCs.

The -C option is handled before a small subset of GUCs that are
computed at runtime are initialized.  Unfortunately, we cannot move
this handling to after they are initialized without disallowing
'postgres -C' on a running server.  One notable reason for this is
that loadable libraries' _PG_init() functions are called before all
runtime-computed GUCs are initialized, and this is not guaranteed
to be safe to do on running servers.

In order to provide useful values for 'postgres -C' for runtime-
computed GUCs, this change adds a new section for handling just
these GUCs just before shared memory is initialized.  While users
won't be able to use -C for runtime-computed GUCs on running
servers, providing a useful value with this restriction seems
better than not providing a useful value at all.
---
 doc/src/sgml/ref/postgres-ref.sgml  | 10 ++++++--
 doc/src/sgml/runtime.sgml           | 33 ++++++++----------------
 src/backend/postmaster/postmaster.c | 50 +++++++++++++++++++++++++++++++------
 src/backend/utils/misc/guc.c        | 10 ++++----
 src/include/utils/guc.h             |  6 +++++
 5 files changed, 73 insertions(+), 36 deletions(-)

diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index 4aaa7abe1a..f6f2246bff 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -133,13 +133,19 @@ PostgreSQL documentation
       <listitem>
        <para>
         Prints the value of the named run-time parameter, and exits.
-        (See the <option>-c</option> option above for details.)  This can
-        be used on a running server, and returns values from
+        (See the <option>-c</option> option above for details.)  This
+        returns values from
         <filename>postgresql.conf</filename>, modified by any parameters
         supplied in this invocation.  It does not reflect parameters
         supplied when the cluster was started.
        </para>
 
+       <para>
+        This can be used on a running server for most parameters.  However,
+        the server must be shut down for some runtime-computed parameters
+        (e.g., <xref linkend="guc-huge-pages-required"/>).
+       </para>
+
        <para>
         This option is meant for other programs that interact with a server
         instance, such as <xref linkend="app-pg-ctl"/>, to query configuration
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..d955639900 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,32 +1442,21 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
-    This might look like:
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-huge-pages-required"/>.  This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C huge_pages_required</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
-     We can set this with:
+     While we need at least <literal>3170</literal> huge pages in this
+     example, a larger setting would be appropriate if other programs on
+     the machine also need huge pages.  We can allocate the huge pages
+     with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
 </programlisting>
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 963c03fa93..d3cbb14d33 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -896,15 +896,31 @@ PostmasterMain(int argc, char *argv[])
 	if (output_config_variable != NULL)
 	{
 		/*
-		 * "-C guc" was specified, so print GUC's value and exit.  No extra
-		 * permission check is needed because the user is reading inside the
-		 * data dir.
+		 * If this is a runtime-computed GUC, it hasn't yet been initialized,
+		 * and the present value is not useful.  However, this is a convenient
+		 * place to print the value for most GUCs because it is safe to run
+		 * postmaster startup to this point even if the server is already
+		 * running.  For the handful of runtime-computed GUCs that we can't
+		 * provide meaningful values for yet, we wait until later in postmaster
+		 * startup to print the value.  We won't be able to use -C on running
+		 * servers for those GUCs, but otherwise this option is unusable for
+		 * them.
 		 */
-		const char *config_val = GetConfigOption(output_config_variable,
-												 false, false);
+		int flags = GetConfigOptionFlags(output_config_variable, true);
 
-		puts(config_val ? config_val : "");
-		ExitPostmaster(0);
+		if ((flags & GUC_RUNTIME_COMPUTED) == 0)
+		{
+			/*
+			 * "-C guc" was specified, so print GUC's value and exit.  No extra
+			 * permission check is needed because the user is reading inside
+			 * the data dir.
+			 */
+			const char *config_val = GetConfigOption(output_config_variable,
+													 false, false);
+
+			puts(config_val ? config_val : "");
+			ExitPostmaster(0);
+		}
 	}
 
 	/* Verify that DataDir looks reasonable */
@@ -1033,6 +1049,26 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeShmemGUCs();
 
+	/*
+	 * If -C was specified with a runtime-computed GUC, we held off printing
+	 * the value earlier, as the GUC was not yet initialized.  We handle -C for
+	 * most GUCs before we lock the data directory so that the option may be
+	 * used on a running server.  However, a handful of GUCs are runtime-
+	 * computed and do not have meaningful values until after locking the data
+	 * directory, and we cannot safely calculate their values earlier on a
+	 * running server.  At this point, such GUCs should be properly
+	 * initialized, and we haven't yet set up shared memory, so this is a good
+	 * time to handle the -C option for these special GUCs.
+	 */
+	if (output_config_variable != NULL)
+	{
+		const char *config_val = GetConfigOption(output_config_variable,
+												 false, false);
+
+		puts(config_val ? config_val : "");
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 1a30d635fc..cabbd083d9 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1985,7 +1985,7 @@ static struct config_bool ConfigureNamesBool[] =
 		{"data_checksums", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows whether data checksums are turned on for this cluster."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_checksums,
 		false,
@@ -2230,7 +2230,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"huge_pages_required", PGC_INTERNAL, RESOURCES_MEM,
 			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
 			gettext_noop("-1 indicates that the value could not be determined."),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&huge_pages_required,
 		-1, -1, INT_MAX,
@@ -2355,7 +2355,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
 			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB | GUC_RUNTIME_COMPUTED
 		},
 		&shmem_size_mb,
 		0, 0, INT_MAX,
@@ -2420,7 +2420,7 @@ static struct config_int ConfigureNamesInt[] =
 						 "in the form accepted by the chmod and umask system "
 						 "calls. (To use the customary octal format the number "
 						 "must start with a 0 (zero).)"),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_directory_mode,
 		0700, 0000, 0777,
@@ -3244,7 +3244,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"wal_segment_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of write ahead log segments."),
 			NULL,
-			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&wal_segment_size,
 		DEFAULT_XLOG_SEG_SIZE,
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index a7c3a4958e..aa18d304ac 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -229,6 +229,12 @@ typedef enum
 
 #define GUC_EXPLAIN			  0x100000	/* include in explain */
 
+/*
+ * GUC_RUNTIME_COMPUTED is intended for runtime-computed GUCs that are only
+ * available via 'postgres -C' if the server is not running.
+ */
+#define GUC_RUNTIME_COMPUTED  0x200000
+
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
 
-- 
2.16.6

From 46c6ada322517c4246ea16597475c392d0b33005 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Tue, 7 Sep 2021 16:46:52 +0000
Subject: [PATCH v7 2/3] Introduce huge_pages_required GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.  Like shared_memory_size,
it cannot be viewed with 'postgres -C' yet.
---
 doc/src/sgml/config.sgml       | 21 +++++++++++++++++++++
 src/backend/port/sysv_shmem.c  | 16 +++++++++++-----
 src/backend/port/win32_shmem.c | 14 ++++++++++++++
 src/backend/storage/ipc/ipci.c | 12 ++++++++++++
 src/backend/utils/misc/guc.c   | 12 ++++++++++++
 src/include/storage/pg_shmem.h |  1 +
 6 files changed, 71 insertions(+), 5 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..b27d8aff15 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10101,6 +10101,27 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-required" xreflabel="huge_pages_required">
+      <term><varname>huge_pages_required</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>huge_pages_required</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are required for the main
+        shared memory area based on the specified
+        <xref linkend="guc-huge-page-size"/>.  If huge pages are not supported,
+        this will be <literal>-1</literal>.
+       </para>
+       <para>
+        This setting is supported only on Linux.  It is always set to
+        <literal>-1</literal> on other platforms.  For more details about using
+        huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..125e2d47ec 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -456,8 +456,6 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 	return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
 }
 
-#ifdef MAP_HUGETLB
-
 /*
  * Identify the huge page size to use, and compute the related mmap flags.
  *
@@ -476,11 +474,19 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * such as increasing shared_buffers to absorb the extra space.
  *
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
- * and the hugepage-related mmap flags to use into *mmap_flags.
+ * and the hugepage-related mmap flags to use into *mmap_flags.  If huge pages
+ * is not supported, *hugepagesize and *mmap_flags will be set to 0.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
+#ifndef MAP_HUGETLB
+
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+
+#else
+
 	Size		default_hugepagesize = 0;
 
 	/*
@@ -553,9 +559,9 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 		*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
 	}
 #endif
-}
 
 #endif							/* MAP_HUGETLB */
+}
 
 /*
  * Creates an anonymous mmap()ed shared memory segment.
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index d7a71992d8..90de2ab4e1 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -605,3 +605,17 @@ pgwin32_ReserveSharedMemoryRegion(HANDLE hChild)
 
 	return true;
 }
+
+/*
+ * This function is provided for consistency with sysv_shmem.c and does not
+ * provide any useful information for Windows.  To obtain the large page size,
+ * use GetLargePageMinimum() instead.
+ *
+ * This always sets *hugepagesize and *mmap_flags to 0.
+ */
+void
+GetHugePageSize(Size *hugepagesize, int *mmap_flags)
+{
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+}
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 4b7b2faa4c..14de7afe0a 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -326,6 +326,9 @@ InitializeShmemGUCs(void)
 	char	buf[64];
 	Size	size_b;
 	Size	size_mb;
+	Size	hp_size;
+	Size	hp_required;
+	int		unused;
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest
@@ -335,4 +338,13 @@ InitializeShmemGUCs(void)
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%lu", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+	/* Calculate the number of huge pages required */
+	GetHugePageSize(&hp_size, &unused);
+	if (hp_size != 0)
+	{
+		hp_required = (size_b / hp_size) + 1;
+		sprintf(buf, "%lu", hp_required);
+		SetConfigOption("huge_pages_required", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a11387c5ce..1a30d635fc 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -621,6 +621,7 @@ char	   *pgstat_temp_directory;
 char	   *application_name;
 
 int			shmem_size_mb;
+int			huge_pages_required;
 
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
@@ -2225,6 +2226,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_required", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the value could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_required,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* This is PGC_SUSET to prevent hiding from log_lock_waits. */
 		{"deadlock_timeout", PGC_SUSET, LOCK_MANAGEMENT,
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..518eb86065 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -87,5 +87,6 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 										   PGShmemHeader **shim);
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
 
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From 96d9d14dcc1626c3ff604bfa60b8c9549532a1ba Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Tue, 7 Sep 2021 16:42:46 +0000
Subject: [PATCH v7 1/3] Introduce shared_memory_size GUC.

This runtime-computed GUC shows the size of the server's main
shared memory area.  While this is intended to be useful for
calculating the number of huge pages required prior to startup, it
cannot be viewed via 'postgres -C' yet.  This may be addressed in a
future change.
---
 doc/src/sgml/config.sgml            | 14 ++++++++++++++
 src/backend/postmaster/postmaster.c |  7 +++++++
 src/backend/storage/ipc/ipci.c      | 23 +++++++++++++++++++++++
 src/backend/utils/misc/guc.c        | 13 +++++++++++++
 src/include/storage/ipc.h           |  1 +
 5 files changed, 58 insertions(+)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2c31c35a6b..ef0e2a7746 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10275,6 +10275,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size" xreflabel="shared_memory_size">
+      <term><varname>shared_memory_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the size of the main shared memory area, rounded up to the
+        nearest megabyte.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c2c98614a..963c03fa93 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1026,6 +1026,13 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	/*
+	 * Now that loadable modules have had their chance to request additional
+	 * shared memory, determine the value of any runtime-computed GUCs that
+	 * depend on the amount of shared memory required.
+	 */
+	InitializeShmemGUCs();
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 64bc16fa84..4b7b2faa4c 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -313,3 +313,26 @@ CreateSharedMemoryAndSemaphores(void)
 	if (shmem_startup_hook)
 		shmem_startup_hook();
 }
+
+/*
+ * InitializeShmemGUCs
+ *
+ * This function initializes runtime-computed GUCs related to the amount of
+ * shared memory required for the current configuration.
+ */
+void
+InitializeShmemGUCs(void)
+{
+	char	buf[64];
+	Size	size_b;
+	Size	size_mb;
+
+	/*
+	 * Calculate the shared memory size and round up to the nearest
+	 * megabyte.
+	 */
+	size_b = CalculateShmemSize(NULL);
+	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
+	sprintf(buf, "%lu", size_mb);
+	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe..a11387c5ce 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -620,6 +620,8 @@ char	   *pgstat_temp_directory;
 
 char	   *application_name;
 
+int			shmem_size_mb;
+
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
 int			tcp_keepalives_count;
@@ -2337,6 +2339,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+		},
+		&shmem_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 80e191d407..7a1ebc8559 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -79,5 +79,6 @@ extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
 extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
+extern void InitializeShmemGUCs(void);
 
 #endif							/* IPC_H */
-- 
2.16.6

From 3edf8cfb42c2d16352685f2bed8c39ac04ae1a2a Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Wed, 8 Sep 2021 17:23:14 +0000
Subject: [PATCH v8 2/2] Provide useful values for 'postgres -C' with
 runtime-computed GUCs.

The -C option is handled before a small subset of GUCs that are
computed at runtime are initialized.  Unfortunately, we cannot move
this handling to after they are initialized without disallowing
'postgres -C' on a running server.  One notable reason for this is
that loadable libraries' _PG_init() functions are called before all
runtime-computed GUCs are initialized, and this is not guaranteed
to be safe to do on running servers.

In order to provide useful values for 'postgres -C' for runtime-
computed GUCs, this change adds a new section for handling just
these GUCs just before shared memory is initialized.  While users
won't be able to use -C for runtime-computed GUCs on running
servers, providing a useful value with this restriction seems
better than not providing a useful value at all.
---
 doc/src/sgml/ref/postgres-ref.sgml  | 12 +++++++--
 doc/src/sgml/runtime.sgml           | 33 ++++++++----------------
 src/backend/postmaster/postmaster.c | 50 +++++++++++++++++++++++++++++++------
 src/backend/utils/misc/guc.c        | 10 ++++----
 src/include/utils/guc.h             |  6 +++++
 5 files changed, 75 insertions(+), 36 deletions(-)

diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index 4aaa7abe1a..89cc3cdb4e 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -133,13 +133,21 @@ PostgreSQL documentation
       <listitem>
        <para>
         Prints the value of the named run-time parameter, and exits.
-        (See the <option>-c</option> option above for details.)  This can
-        be used on a running server, and returns values from
+        (See the <option>-c</option> option above for details.)  This
+        returns values from
         <filename>postgresql.conf</filename>, modified by any parameters
         supplied in this invocation.  It does not reflect parameters
         supplied when the cluster was started.
        </para>
 
+       <para>
+        This can be used on a running server for most parameters.  However,
+        the server must be shut down for some runtime-computed parameters
+        (e.g., <xref linkend="guc-huge-pages-required"/>,
+        <xref linkend="guc-shared-memory-size"/>, and
+        <xref linkend="guc-wal-segment-size"/>).
+       </para>
+
        <para>
         This option is meant for other programs that interact with a server
         instance, such as <xref linkend="app-pg-ctl"/>, to query configuration
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..d955639900 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,32 +1442,21 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
-    This might look like:
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-huge-pages-required"/>.  This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C huge_pages_required</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
-     We can set this with:
+     While we need at least <literal>3170</literal> huge pages in this
+     example, a larger setting would be appropriate if other programs on
+     the machine also need huge pages.  We can allocate the huge pages
+     with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
 </programlisting>
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index b2fe420c3c..f601b9e5d1 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -896,15 +896,31 @@ PostmasterMain(int argc, char *argv[])
 	if (output_config_variable != NULL)
 	{
 		/*
-		 * "-C guc" was specified, so print GUC's value and exit.  No extra
-		 * permission check is needed because the user is reading inside the
-		 * data dir.
+		 * If this is a runtime-computed GUC, it hasn't yet been initialized,
+		 * and the present value is not useful.  However, this is a convenient
+		 * place to print the value for most GUCs because it is safe to run
+		 * postmaster startup to this point even if the server is already
+		 * running.  For the handful of runtime-computed GUCs that we can't
+		 * provide meaningful values for yet, we wait until later in postmaster
+		 * startup to print the value.  We won't be able to use -C on running
+		 * servers for those GUCs, but otherwise this option is unusable for
+		 * them.
 		 */
-		const char *config_val = GetConfigOption(output_config_variable,
-												 false, false);
+		int flags = GetConfigOptionFlags(output_config_variable, true);
 
-		puts(config_val ? config_val : "");
-		ExitPostmaster(0);
+		if ((flags & GUC_RUNTIME_COMPUTED) == 0)
+		{
+			/*
+			 * "-C guc" was specified, so print GUC's value and exit.  No extra
+			 * permission check is needed because the user is reading inside
+			 * the data dir.
+			 */
+			const char *config_val = GetConfigOption(output_config_variable,
+													 false, false);
+
+			puts(config_val ? config_val : "");
+			ExitPostmaster(0);
+		}
 	}
 
 	/* Verify that DataDir looks reasonable */
@@ -1033,6 +1049,26 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeShmemGUCs();
 
+	/*
+	 * If -C was specified with a runtime-computed GUC, we held off printing
+	 * the value earlier, as the GUC was not yet initialized.  We handle -C for
+	 * most GUCs before we lock the data directory so that the option may be
+	 * used on a running server.  However, a handful of GUCs are runtime-
+	 * computed and do not have meaningful values until after locking the data
+	 * directory, and we cannot safely calculate their values earlier on a
+	 * running server.  At this point, such GUCs should be properly
+	 * initialized, and we haven't yet set up shared memory, so this is a good
+	 * time to handle the -C option for these special GUCs.
+	 */
+	if (output_config_variable != NULL)
+	{
+		const char *config_val = GetConfigOption(output_config_variable,
+												 false, false);
+
+		puts(config_val ? config_val : "");
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 8edfd07340..50e60b270b 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1984,7 +1984,7 @@ static struct config_bool ConfigureNamesBool[] =
 		{"data_checksums", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows whether data checksums are turned on for this cluster."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_checksums,
 		false,
@@ -2229,7 +2229,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"huge_pages_required", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
 			gettext_noop("-1 indicates that the value could not be determined."),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&huge_pages_required,
 		-1, -1, INT_MAX,
@@ -2354,7 +2354,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"shared_memory_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB | GUC_RUNTIME_COMPUTED
 		},
 		&shared_memory_size_mb,
 		0, 0, INT_MAX,
@@ -2419,7 +2419,7 @@ static struct config_int ConfigureNamesInt[] =
 						 "in the form accepted by the chmod and umask system "
 						 "calls. (To use the customary octal format the number "
 						 "must start with a 0 (zero).)"),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_directory_mode,
 		0700, 0000, 0777,
@@ -3243,7 +3243,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"wal_segment_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of write ahead log segments."),
 			NULL,
-			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&wal_segment_size,
 		DEFAULT_XLOG_SEG_SIZE,
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index a7c3a4958e..aa18d304ac 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -229,6 +229,12 @@ typedef enum
 
 #define GUC_EXPLAIN			  0x100000	/* include in explain */
 
+/*
+ * GUC_RUNTIME_COMPUTED is intended for runtime-computed GUCs that are only
+ * available via 'postgres -C' if the server is not running.
+ */
+#define GUC_RUNTIME_COMPUTED  0x200000
+
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
 
-- 
2.16.6

From b7da8e594f52cd45487c5637c70b0cc11087fb2f Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Wed, 8 Sep 2021 17:18:39 +0000
Subject: [PATCH v8 1/2] Introduce huge_pages_required GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.
---
 doc/src/sgml/config.sgml       | 21 +++++++++++++++++++++
 src/backend/port/sysv_shmem.c  | 16 +++++++++++-----
 src/backend/port/win32_shmem.c | 14 ++++++++++++++
 src/backend/storage/ipc/ipci.c | 14 ++++++++++++++
 src/backend/utils/misc/guc.c   | 14 +++++++++++++-
 src/include/storage/pg_shmem.h |  1 +
 6 files changed, 74 insertions(+), 6 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..b27d8aff15 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10101,6 +10101,27 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-required" xreflabel="huge_pages_required">
+      <term><varname>huge_pages_required</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>huge_pages_required</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are required for the main
+        shared memory area based on the specified
+        <xref linkend="guc-huge-page-size"/>.  If huge pages are not supported,
+        this will be <literal>-1</literal>.
+       </para>
+       <para>
+        This setting is supported only on Linux.  It is always set to
+        <literal>-1</literal> on other platforms.  For more details about using
+        huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..125e2d47ec 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -456,8 +456,6 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 	return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
 }
 
-#ifdef MAP_HUGETLB
-
 /*
  * Identify the huge page size to use, and compute the related mmap flags.
  *
@@ -476,11 +474,19 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * such as increasing shared_buffers to absorb the extra space.
  *
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
- * and the hugepage-related mmap flags to use into *mmap_flags.
+ * and the hugepage-related mmap flags to use into *mmap_flags.  If huge pages
+ * is not supported, *hugepagesize and *mmap_flags will be set to 0.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
+#ifndef MAP_HUGETLB
+
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+
+#else
+
 	Size		default_hugepagesize = 0;
 
 	/*
@@ -553,9 +559,9 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 		*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
 	}
 #endif
-}
 
 #endif							/* MAP_HUGETLB */
+}
 
 /*
  * Creates an anonymous mmap()ed shared memory segment.
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index d7a71992d8..90de2ab4e1 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -605,3 +605,17 @@ pgwin32_ReserveSharedMemoryRegion(HANDLE hChild)
 
 	return true;
 }
+
+/*
+ * This function is provided for consistency with sysv_shmem.c and does not
+ * provide any useful information for Windows.  To obtain the large page size,
+ * use GetLargePageMinimum() instead.
+ *
+ * This always sets *hugepagesize and *mmap_flags to 0.
+ */
+void
+GetHugePageSize(Size *hugepagesize, int *mmap_flags)
+{
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+}
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 13f3926ff6..91490653cf 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -326,6 +326,9 @@ InitializeShmemGUCs(void)
 	char		buf[64];
 	Size		size_b;
 	Size		size_mb;
+	Size		hp_size;
+	Size		hp_required;
+	int			unused;
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest megabyte.
@@ -334,4 +337,15 @@ InitializeShmemGUCs(void)
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%zu", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+	/*
+	 * Calculate the number of huge pages required.
+	 */
+	GetHugePageSize(&hp_size, &unused);
+	if (hp_size != 0)
+	{
+		hp_required = (size_b / hp_size) + 1;
+		sprintf(buf, "%zu", hp_required);
+		SetConfigOption("huge_pages_required", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index c339acf067..8edfd07340 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -665,6 +665,7 @@ static int	max_identifier_length;
 static int	block_size;
 static int	segment_size;
 static int	shared_memory_size_mb;
+static int	huge_pages_required;
 static int	wal_block_size;
 static bool data_checksums;
 static bool integer_datetimes;
@@ -2224,6 +2225,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_required", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the value could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_required,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* This is PGC_SUSET to prevent hiding from log_lock_waits. */
 		{"deadlock_timeout", PGC_SUSET, LOCK_MANAGEMENT,
@@ -2339,7 +2351,7 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+		{"shared_memory_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
 			NULL,
 			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..518eb86065 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -87,5 +87,6 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 										   PGShmemHeader **shim);
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
 
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From 09b1f5a5e0e88a6d84bfc2f8a1bc410e21fb3775 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 9 Sep 2021 13:10:24 +0900
Subject: [PATCH v9 1/2] Introduce huge_pages_required GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.
---
 src/include/storage/pg_shmem.h |  1 +
 src/backend/port/sysv_shmem.c  | 16 +++++++++++-----
 src/backend/port/win32_shmem.c | 14 ++++++++++++++
 src/backend/storage/ipc/ipci.c | 14 ++++++++++++++
 src/backend/utils/misc/guc.c   | 12 ++++++++++++
 doc/src/sgml/config.sgml       | 21 +++++++++++++++++++++
 6 files changed, 73 insertions(+), 5 deletions(-)

diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..518eb86065 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -87,5 +87,6 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 										   PGShmemHeader **shim);
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
 
 #endif							/* PG_SHMEM_H */
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..125e2d47ec 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -456,8 +456,6 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 	return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
 }
 
-#ifdef MAP_HUGETLB
-
 /*
  * Identify the huge page size to use, and compute the related mmap flags.
  *
@@ -476,11 +474,19 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * such as increasing shared_buffers to absorb the extra space.
  *
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
- * and the hugepage-related mmap flags to use into *mmap_flags.
+ * and the hugepage-related mmap flags to use into *mmap_flags.  If huge pages
+ * is not supported, *hugepagesize and *mmap_flags will be set to 0.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
+#ifndef MAP_HUGETLB
+
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+
+#else
+
 	Size		default_hugepagesize = 0;
 
 	/*
@@ -553,9 +559,9 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 		*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
 	}
 #endif
-}
 
 #endif							/* MAP_HUGETLB */
+}
 
 /*
  * Creates an anonymous mmap()ed shared memory segment.
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index d7a71992d8..90de2ab4e1 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -605,3 +605,17 @@ pgwin32_ReserveSharedMemoryRegion(HANDLE hChild)
 
 	return true;
 }
+
+/*
+ * This function is provided for consistency with sysv_shmem.c and does not
+ * provide any useful information for Windows.  To obtain the large page size,
+ * use GetLargePageMinimum() instead.
+ *
+ * This always sets *hugepagesize and *mmap_flags to 0.
+ */
+void
+GetHugePageSize(Size *hugepagesize, int *mmap_flags)
+{
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+}
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 13f3926ff6..91490653cf 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -326,6 +326,9 @@ InitializeShmemGUCs(void)
 	char		buf[64];
 	Size		size_b;
 	Size		size_mb;
+	Size		hp_size;
+	Size		hp_required;
+	int			unused;
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest megabyte.
@@ -334,4 +337,15 @@ InitializeShmemGUCs(void)
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%zu", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+	/*
+	 * Calculate the number of huge pages required.
+	 */
+	GetHugePageSize(&hp_size, &unused);
+	if (hp_size != 0)
+	{
+		hp_required = (size_b / hp_size) + 1;
+		sprintf(buf, "%zu", hp_required);
+		SetConfigOption("huge_pages_required", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index fd4ca83be1..8edfd07340 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -665,6 +665,7 @@ static int	max_identifier_length;
 static int	block_size;
 static int	segment_size;
 static int	shared_memory_size_mb;
+static int	huge_pages_required;
 static int	wal_block_size;
 static bool data_checksums;
 static bool integer_datetimes;
@@ -2224,6 +2225,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_required", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the value could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_required,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* This is PGC_SUSET to prevent hiding from log_lock_waits. */
 		{"deadlock_timeout", PGC_SUSET, LOCK_MANAGEMENT,
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..b27d8aff15 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10101,6 +10101,27 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-required" xreflabel="huge_pages_required">
+      <term><varname>huge_pages_required</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>huge_pages_required</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are required for the main
+        shared memory area based on the specified
+        <xref linkend="guc-huge-page-size"/>.  If huge pages are not supported,
+        this will be <literal>-1</literal>.
+       </para>
+       <para>
+        This setting is supported only on Linux.  It is always set to
+        <literal>-1</literal> on other platforms.  For more details about using
+        huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>
-- 
2.33.0

From f4763273a7f4649d4c50699b590c0b4b8077cb95 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 9 Sep 2021 13:10:38 +0900
Subject: [PATCH v9 2/2] Provide useful values for 'postgres -C' with
 runtime-computed GUCs.

The -C option is handled before a small subset of GUCs that are
computed at runtime are initialized.  Unfortunately, we cannot move
this handling to after they are initialized without disallowing
'postgres -C' on a running server.  One notable reason for this is
that loadable libraries' _PG_init() functions are called before all
runtime-computed GUCs are initialized, and this is not guaranteed
to be safe to do on running servers.

In order to provide useful values for 'postgres -C' for runtime-
computed GUCs, this change adds a new section for handling just
these GUCs just before shared memory is initialized.  While users
won't be able to use -C for runtime-computed GUCs on running
servers, providing a useful value with this restriction seems
better than not providing a useful value at all.
---
 src/include/utils/guc.h             |  6 ++++
 src/backend/postmaster/postmaster.c | 50 +++++++++++++++++++++++++----
 src/backend/utils/misc/guc.c        | 10 +++---
 doc/src/sgml/ref/postgres-ref.sgml  | 12 +++++--
 doc/src/sgml/runtime.sgml           | 33 +++++++------------
 5 files changed, 75 insertions(+), 36 deletions(-)

diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index a7c3a4958e..aa18d304ac 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -229,6 +229,12 @@ typedef enum
 
 #define GUC_EXPLAIN			  0x100000	/* include in explain */
 
+/*
+ * GUC_RUNTIME_COMPUTED is intended for runtime-computed GUCs that are only
+ * available via 'postgres -C' if the server is not running.
+ */
+#define GUC_RUNTIME_COMPUTED  0x200000
+
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
 
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index b2fe420c3c..f601b9e5d1 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -896,15 +896,31 @@ PostmasterMain(int argc, char *argv[])
 	if (output_config_variable != NULL)
 	{
 		/*
-		 * "-C guc" was specified, so print GUC's value and exit.  No extra
-		 * permission check is needed because the user is reading inside the
-		 * data dir.
+		 * If this is a runtime-computed GUC, it hasn't yet been initialized,
+		 * and the present value is not useful.  However, this is a convenient
+		 * place to print the value for most GUCs because it is safe to run
+		 * postmaster startup to this point even if the server is already
+		 * running.  For the handful of runtime-computed GUCs that we can't
+		 * provide meaningful values for yet, we wait until later in postmaster
+		 * startup to print the value.  We won't be able to use -C on running
+		 * servers for those GUCs, but otherwise this option is unusable for
+		 * them.
 		 */
-		const char *config_val = GetConfigOption(output_config_variable,
-												 false, false);
+		int flags = GetConfigOptionFlags(output_config_variable, true);
 
-		puts(config_val ? config_val : "");
-		ExitPostmaster(0);
+		if ((flags & GUC_RUNTIME_COMPUTED) == 0)
+		{
+			/*
+			 * "-C guc" was specified, so print GUC's value and exit.  No extra
+			 * permission check is needed because the user is reading inside
+			 * the data dir.
+			 */
+			const char *config_val = GetConfigOption(output_config_variable,
+													 false, false);
+
+			puts(config_val ? config_val : "");
+			ExitPostmaster(0);
+		}
 	}
 
 	/* Verify that DataDir looks reasonable */
@@ -1033,6 +1049,26 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeShmemGUCs();
 
+	/*
+	 * If -C was specified with a runtime-computed GUC, we held off printing
+	 * the value earlier, as the GUC was not yet initialized.  We handle -C for
+	 * most GUCs before we lock the data directory so that the option may be
+	 * used on a running server.  However, a handful of GUCs are runtime-
+	 * computed and do not have meaningful values until after locking the data
+	 * directory, and we cannot safely calculate their values earlier on a
+	 * running server.  At this point, such GUCs should be properly
+	 * initialized, and we haven't yet set up shared memory, so this is a good
+	 * time to handle the -C option for these special GUCs.
+	 */
+	if (output_config_variable != NULL)
+	{
+		const char *config_val = GetConfigOption(output_config_variable,
+												 false, false);
+
+		puts(config_val ? config_val : "");
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 8edfd07340..50e60b270b 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1984,7 +1984,7 @@ static struct config_bool ConfigureNamesBool[] =
 		{"data_checksums", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows whether data checksums are turned on for this cluster."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_checksums,
 		false,
@@ -2229,7 +2229,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"huge_pages_required", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
 			gettext_noop("-1 indicates that the value could not be determined."),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&huge_pages_required,
 		-1, -1, INT_MAX,
@@ -2354,7 +2354,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"shared_memory_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB | GUC_RUNTIME_COMPUTED
 		},
 		&shared_memory_size_mb,
 		0, 0, INT_MAX,
@@ -2419,7 +2419,7 @@ static struct config_int ConfigureNamesInt[] =
 						 "in the form accepted by the chmod and umask system "
 						 "calls. (To use the customary octal format the number "
 						 "must start with a 0 (zero).)"),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_directory_mode,
 		0700, 0000, 0777,
@@ -3243,7 +3243,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"wal_segment_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of write ahead log segments."),
 			NULL,
-			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&wal_segment_size,
 		DEFAULT_XLOG_SEG_SIZE,
diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index 4aaa7abe1a..89cc3cdb4e 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -133,13 +133,21 @@ PostgreSQL documentation
       <listitem>
        <para>
         Prints the value of the named run-time parameter, and exits.
-        (See the <option>-c</option> option above for details.)  This can
-        be used on a running server, and returns values from
+        (See the <option>-c</option> option above for details.)  This
+        returns values from
         <filename>postgresql.conf</filename>, modified by any parameters
         supplied in this invocation.  It does not reflect parameters
         supplied when the cluster was started.
        </para>
 
+       <para>
+        This can be used on a running server for most parameters.  However,
+        the server must be shut down for some runtime-computed parameters
+        (e.g., <xref linkend="guc-huge-pages-required"/>,
+        <xref linkend="guc-shared-memory-size"/>, and
+        <xref linkend="guc-wal-segment-size"/>).
+       </para>
+
        <para>
         This option is meant for other programs that interact with a server
         instance, such as <xref linkend="app-pg-ctl"/>, to query configuration
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..d955639900 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,32 +1442,21 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
-    This might look like:
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-huge-pages-required"/>.  This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C huge_pages_required</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
-     We can set this with:
+     While we need at least <literal>3170</literal> huge pages in this
+     example, a larger setting would be appropriate if other programs on
+     the machine also need huge pages.  We can allocate the huge pages
+     with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
 </programlisting>
-- 
2.33.0

From aba09219bbcbdb5d9747c63dbf75380c774873e7 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Thu, 9 Sep 2021 21:42:41 +0000
Subject: [PATCH 1/1] Provide useful values for 'postgres -C' with
 runtime-computed GUCs.

The -C option is handled before a small subset of GUCs that are
computed at runtime are initialized.  Unfortunately, we cannot move
this handling to after they are initialized without disallowing
'postgres -C' on a running server.  One notable reason for this is
that loadable libraries' _PG_init() functions are called before all
runtime-computed GUCs are initialized, and this is not guaranteed
to be safe to do on running servers.

In order to provide useful values for 'postgres -C' for runtime-
computed GUCs, this change adds a new section for handling just
these GUCs just before shared memory is initialized.  While users
won't be able to use -C for runtime-computed GUCs on running
servers, providing a useful value with this restriction seems
better than not providing a useful value at all.
---
 doc/src/sgml/ref/postgres-ref.sgml  | 12 ++++++--
 doc/src/sgml/runtime.sgml           | 33 +++++++-------------
 src/backend/bootstrap/bootstrap.c   |  2 +-
 src/backend/postmaster/postmaster.c | 52 +++++++++++++++++++++++++++-----
 src/backend/tcop/postgres.c         |  2 +-
 src/backend/utils/init/miscinit.c   | 60 ++++++++++++++++++++++++++++---------
 src/backend/utils/misc/guc.c        |  8 ++---
 src/include/miscadmin.h             |  3 +-
 src/include/utils/guc.h             |  6 ++++
 9 files changed, 125 insertions(+), 53 deletions(-)

diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index 4aaa7abe1a..89cc3cdb4e 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -133,13 +133,21 @@ PostgreSQL documentation
       <listitem>
        <para>
         Prints the value of the named run-time parameter, and exits.
-        (See the <option>-c</option> option above for details.)  This can
-        be used on a running server, and returns values from
+        (See the <option>-c</option> option above for details.)  This
+        returns values from
         <filename>postgresql.conf</filename>, modified by any parameters
         supplied in this invocation.  It does not reflect parameters
         supplied when the cluster was started.
        </para>
 
+       <para>
+        This can be used on a running server for most parameters.  However,
+        the server must be shut down for some runtime-computed parameters
+        (e.g., <xref linkend="guc-huge-pages-required"/>,
+        <xref linkend="guc-shared-memory-size"/>, and
+        <xref linkend="guc-wal-segment-size"/>).
+       </para>
+
        <para>
         This option is meant for other programs that interact with a server
         instance, such as <xref linkend="app-pg-ctl"/>, to query configuration
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..d955639900 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,32 +1442,21 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
-    This might look like:
+    To estimate the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-huge-pages-required"/>.  This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C huge_pages_required</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
-     We can set this with:
+     While we need at least <literal>3170</literal> huge pages in this
+     example, a larger setting would be appropriate if other programs on
+     the machine also need huge pages.  We can allocate the huge pages
+     with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
 </programlisting>
diff --git a/src/backend/bootstrap/bootstrap.c b/src/backend/bootstrap/bootstrap.c
index 48615c0ebc..2c9e832951 100644
--- a/src/backend/bootstrap/bootstrap.c
+++ b/src/backend/bootstrap/bootstrap.c
@@ -317,7 +317,7 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 	checkDataDir();
 	ChangeToDataDir();
 
-	CreateDataDirLockFile(false);
+	CreateDataDirLockFile(false, NULL);
 
 	SetProcessingMode(BootstrapProcessing);
 	IgnoreSystemIndexes = true;
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index b2fe420c3c..58136aac09 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -896,15 +896,31 @@ PostmasterMain(int argc, char *argv[])
 	if (output_config_variable != NULL)
 	{
 		/*
-		 * "-C guc" was specified, so print GUC's value and exit.  No extra
-		 * permission check is needed because the user is reading inside the
-		 * data dir.
+		 * If this is a runtime-computed GUC, it hasn't yet been initialized,
+		 * and the present value is not useful.  However, this is a convenient
+		 * place to print the value for most GUCs because it is safe to run
+		 * postmaster startup to this point even if the server is already
+		 * running.  For the handful of runtime-computed GUCs that we can't
+		 * provide meaningful values for yet, we wait until later in postmaster
+		 * startup to print the value.  We won't be able to use -C on running
+		 * servers for those GUCs, but otherwise this option is unusable for
+		 * them.
 		 */
-		const char *config_val = GetConfigOption(output_config_variable,
-												 false, false);
+		int flags = GetConfigOptionFlags(output_config_variable, true);
 
-		puts(config_val ? config_val : "");
-		ExitPostmaster(0);
+		if ((flags & GUC_RUNTIME_COMPUTED) == 0)
+		{
+			/*
+			 * "-C guc" was specified, so print GUC's value and exit.  No extra
+			 * permission check is needed because the user is reading inside
+			 * the data dir.
+			 */
+			const char *config_val = GetConfigOption(output_config_variable,
+													 false, false);
+
+			puts(config_val ? config_val : "");
+			ExitPostmaster(0);
+		}
 	}
 
 	/* Verify that DataDir looks reasonable */
@@ -983,7 +999,7 @@ PostmasterMain(int argc, char *argv[])
 	 * so it must happen before opening sockets so that at exit, the socket
 	 * lockfiles go away after CloseServerPorts runs.
 	 */
-	CreateDataDirLockFile(true);
+	CreateDataDirLockFile(true, output_config_variable);
 
 	/*
 	 * Read the control file (for error checking and config info).
@@ -1033,6 +1049,26 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeShmemGUCs();
 
+	/*
+	 * If -C was specified with a runtime-computed GUC, we held off printing
+	 * the value earlier, as the GUC was not yet initialized.  We handle -C for
+	 * most GUCs before we lock the data directory so that the option may be
+	 * used on a running server.  However, a handful of GUCs are runtime-
+	 * computed and do not have meaningful values until after locking the data
+	 * directory, and we cannot safely calculate their values earlier on a
+	 * running server.  At this point, such GUCs should be properly
+	 * initialized, and we haven't yet set up shared memory, so this is a good
+	 * time to handle the -C option for these special GUCs.
+	 */
+	if (output_config_variable != NULL)
+	{
+		const char *config_val = GetConfigOption(output_config_variable,
+												 false, false);
+
+		puts(config_val ? config_val : "");
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 58b5960e27..d84759ae59 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -4043,7 +4043,7 @@ PostgresMain(int argc, char *argv[],
 		/*
 		 * Create lockfile for data directory.
 		 */
-		CreateDataDirLockFile(false);
+		CreateDataDirLockFile(false, NULL);
 
 		/* read control file (error checking and contains config ) */
 		LocalProcessControlFile(false);
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 88801374b5..a63aba6f57 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -952,6 +952,7 @@ static void
 UnlinkLockFiles(int status, Datum arg)
 {
 	ListCell   *l;
+	bool		logShutDown = DatumGetBool(arg);
 
 	foreach(l, lock_files)
 	{
@@ -968,10 +969,10 @@ UnlinkLockFiles(int status, Datum arg)
 	 * of a postmaster or standalone backend, while we won't come here at all
 	 * when exiting postmaster child processes.  Therefore, this is a good
 	 * place to log completion of shutdown.  We could alternatively teach
-	 * proc_exit() to do it, but that seems uglier.  In a standalone backend,
-	 * use NOTICE elevel to be less chatty.
+	 * proc_exit() to do it, but that seems uglier.  In a standalone backend
+	 * and when logShutDown is false, use NOTICE elevel to be less chatty.
 	 */
-	ereport(IsPostmasterEnvironment ? LOG : NOTICE,
+	ereport(IsPostmasterEnvironment && logShutDown ? LOG : NOTICE,
 			(errmsg("database system is shut down")));
 }
 
@@ -986,7 +987,8 @@ UnlinkLockFiles(int status, Datum arg)
 static void
 CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
-			   bool isDDLock, const char *refName)
+			   bool isDDLock, const char *refName,
+			   const char *output_config_variable)
 {
 	int			fd;
 	char		buffer[MAXPGPATH * 2 + 256];
@@ -999,6 +1001,18 @@ CreateLockFile(const char *filename, bool amPostmaster,
 				my_gp_pid;
 	const char *envvar;
 
+	/*
+	 * If output_config_variable is not NULL, we try to add some useful context
+	 * to the FATAL messages to make it clear why 'postgres -C' is failing for
+	 * this particular GUC (only a small set of runtime-computed GUCs require
+	 * the server to be shut down).
+	 */
+#define LOCK_FILE_ERRDETAIL (output_config_variable ? \
+							 errdetail("Runtime-computed GUC \"%s\" cannot " \
+									   "be viewed on a running server.", \
+									   output_config_variable) : \
+							 0)
+
 	/*
 	 * If the PID in the lockfile is our own PID or our parent's or
 	 * grandparent's PID, then the file must be stale (probably left over from
@@ -1060,7 +1074,8 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			ereport(FATAL,
 					(errcode_for_file_access(),
 					 errmsg("could not create lock file \"%s\": %m",
-							filename)));
+							filename),
+					 LOCK_FILE_ERRDETAIL));
 
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
@@ -1074,14 +1089,16 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			ereport(FATAL,
 					(errcode_for_file_access(),
 					 errmsg("could not open lock file \"%s\": %m",
-							filename)));
+							filename),
+					 LOCK_FILE_ERRDETAIL));
 		}
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
 					(errcode_for_file_access(),
 					 errmsg("could not read lock file \"%s\": %m",
-							filename)));
+							filename),
+					 LOCK_FILE_ERRDETAIL));
 		pgstat_report_wait_end();
 		close(fd);
 
@@ -1090,6 +1107,7 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			ereport(FATAL,
 					(errcode(ERRCODE_LOCK_FILE_EXISTS),
 					 errmsg("lock file \"%s\" is empty", filename),
+					 LOCK_FILE_ERRDETAIL,
 					 errhint("Either another server is starting, or the lock file is the remnant of a previous server startup crash.")));
 		}
 
@@ -1136,6 +1154,7 @@ CreateLockFile(const char *filename, bool amPostmaster,
 						(errcode(ERRCODE_LOCK_FILE_EXISTS),
 						 errmsg("lock file \"%s\" already exists",
 								filename),
+						 LOCK_FILE_ERRDETAIL,
 						 isDDLock ?
 						 (encoded_pid < 0 ?
 						  errhint("Is another postgres (PID %d) running in data directory \"%s\"?",
@@ -1183,6 +1202,7 @@ CreateLockFile(const char *filename, bool amPostmaster,
 							(errcode(ERRCODE_LOCK_FILE_EXISTS),
 							 errmsg("pre-existing shared memory block (key %lu, ID %lu) is still in use",
 									id1, id2),
+							 LOCK_FILE_ERRDETAIL,
 							 errhint("Terminate any old server processes associated with data directory \"%s\".",
 									 refName)));
 			}
@@ -1198,6 +1218,7 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					(errcode_for_file_access(),
 					 errmsg("could not remove old lock file \"%s\": %m",
 							filename),
+					 LOCK_FILE_ERRDETAIL,
 					 errhint("The file seems accidentally left over, but "
 							 "it could not be removed. Please remove the file "
 							 "by hand and try again.")));
@@ -1235,7 +1256,8 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		errno = save_errno ? save_errno : ENOSPC;
 		ereport(FATAL,
 				(errcode_for_file_access(),
-				 errmsg("could not write lock file \"%s\": %m", filename)));
+				 errmsg("could not write lock file \"%s\": %m", filename),
+				 LOCK_FILE_ERRDETAIL));
 	}
 	pgstat_report_wait_end();
 
@@ -1249,7 +1271,8 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		errno = save_errno;
 		ereport(FATAL,
 				(errcode_for_file_access(),
-				 errmsg("could not write lock file \"%s\": %m", filename)));
+				 errmsg("could not write lock file \"%s\": %m", filename),
+				 LOCK_FILE_ERRDETAIL));
 	}
 	pgstat_report_wait_end();
 	if (close(fd) != 0)
@@ -1260,16 +1283,21 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		errno = save_errno;
 		ereport(FATAL,
 				(errcode_for_file_access(),
-				 errmsg("could not write lock file \"%s\": %m", filename)));
+				 errmsg("could not write lock file \"%s\": %m", filename),
+				 LOCK_FILE_ERRDETAIL));
 	}
 
 	/*
 	 * Arrange to unlink the lock file(s) at proc_exit.  If this is the first
 	 * one, set up the on_proc_exit function to do it; then add this lock file
 	 * to the list of files to unlink.
+	 *
+	 * If output_config_variable is not NULL, we ask UnlinkLockFiles to be less
+	 * chatty to improve the user experience with 'postgres -C'.
 	 */
 	if (lock_files == NIL)
-		on_proc_exit(UnlinkLockFiles, 0);
+		on_proc_exit(UnlinkLockFiles,
+					 BoolGetDatum(output_config_variable == NULL));
 
 	/*
 	 * Use lcons so that the lock files are unlinked in reverse order of
@@ -1287,11 +1315,15 @@ CreateLockFile(const char *filename, bool amPostmaster,
  *
  * Note that the socket directory path line is initially written as empty.
  * postmaster.c will rewrite it upon creating the first Unix socket.
+ *
+ * If output_config_variable is not NULL (i.e., we are running 'postgres
+ * -C'), the logs we emit will be slightly adjusted to provide a better
+ * user experience.
  */
 void
-CreateDataDirLockFile(bool amPostmaster)
+CreateDataDirLockFile(bool amPostmaster, const char *output_config_variable)
 {
-	CreateLockFile(DIRECTORY_LOCK_FILE, amPostmaster, "", true, DataDir);
+	CreateLockFile(DIRECTORY_LOCK_FILE, amPostmaster, "", true, DataDir, output_config_variable);
 }
 
 /*
@@ -1304,7 +1336,7 @@ CreateSocketLockFile(const char *socketfile, bool amPostmaster,
 	char		lockfile[MAXPGPATH];
 
 	snprintf(lockfile, sizeof(lockfile), "%s.lock", socketfile);
-	CreateLockFile(lockfile, amPostmaster, socketDir, false, socketfile);
+	CreateLockFile(lockfile, amPostmaster, socketDir, false, socketfile, NULL);
 }
 
 /*
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 23236fa4c3..a6e4fcc24e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1983,7 +1983,7 @@ static struct config_bool ConfigureNamesBool[] =
 		{"data_checksums", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows whether data checksums are turned on for this cluster."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_checksums,
 		false,
@@ -2342,7 +2342,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"shared_memory_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB | GUC_RUNTIME_COMPUTED
 		},
 		&shared_memory_size_mb,
 		0, 0, INT_MAX,
@@ -2407,7 +2407,7 @@ static struct config_int ConfigureNamesInt[] =
 						 "in the form accepted by the chmod and umask system "
 						 "calls. (To use the customary octal format the number "
 						 "must start with a 0 (zero).)"),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_directory_mode,
 		0700, 0000, 0777,
@@ -3231,7 +3231,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"wal_segment_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of write ahead log segments."),
 			NULL,
-			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&wal_segment_size,
 		DEFAULT_XLOG_SEG_SIZE,
diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index 2e2e9a364a..d630ffa310 100644
--- a/src/include/miscadmin.h
+++ b/src/include/miscadmin.h
@@ -466,7 +466,8 @@ extern char *session_preload_libraries_string;
 extern char *shared_preload_libraries_string;
 extern char *local_preload_libraries_string;
 
-extern void CreateDataDirLockFile(bool amPostmaster);
+extern void CreateDataDirLockFile(bool amPostmaster,
+								  const char *output_config_variable);
 extern void CreateSocketLockFile(const char *socketfile, bool amPostmaster,
 								 const char *socketDir);
 extern void TouchSocketLockFiles(void);
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index a7c3a4958e..aa18d304ac 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -229,6 +229,12 @@ typedef enum
 
 #define GUC_EXPLAIN			  0x100000	/* include in explain */
 
+/*
+ * GUC_RUNTIME_COMPUTED is intended for runtime-computed GUCs that are only
+ * available via 'postgres -C' if the server is not running.
+ */
+#define GUC_RUNTIME_COMPUTED  0x200000
+
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
 
-- 
2.16.6

From e24d151efe45e1bc7048ec72ea66097c94f633be Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 15 Sep 2021 12:02:32 +0900
Subject: [PATCH v10] Provide useful values for 'postgres -C' with 
 runtime-computed GUCs

The -C option is handled before a small subset of GUCs that are
computed at runtime are initialized.  Unfortunately, we cannot move
this handling to after they are initialized without disallowing
'postgres -C' on a running server.  One notable reason for this is
that loadable libraries' _PG_init() functions are called before all
runtime-computed GUCs are initialized, and this is not guaranteed
to be safe to do on running servers.

In order to provide useful values for 'postgres -C' for runtime-
computed GUCs, this change adds a new section for handling just
these GUCs just before shared memory is initialized.  While users
won't be able to use -C for runtime-computed GUCs on running
servers, providing a useful value with this restriction seems
better than not providing a useful value at all.
---
 src/include/utils/guc.h               |  6 ++++
 src/backend/postmaster/postmaster.c   | 50 +++++++++++++++++++++++----
 src/backend/utils/misc/guc.c          |  8 ++---
 src/bin/pg_checksums/t/002_actions.pl | 21 ++++++++++-
 doc/src/sgml/ref/postgres-ref.sgml    | 11 ++++--
 5 files changed, 82 insertions(+), 14 deletions(-)

diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index a7c3a4958e..aa18d304ac 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -229,6 +229,12 @@ typedef enum
 
 #define GUC_EXPLAIN			  0x100000	/* include in explain */
 
+/*
+ * GUC_RUNTIME_COMPUTED is intended for runtime-computed GUCs that are only
+ * available via 'postgres -C' if the server is not running.
+ */
+#define GUC_RUNTIME_COMPUTED  0x200000
+
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
 
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index b2fe420c3c..f5b91c7af7 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -896,15 +896,31 @@ PostmasterMain(int argc, char *argv[])
 	if (output_config_variable != NULL)
 	{
 		/*
-		 * "-C guc" was specified, so print GUC's value and exit.  No extra
-		 * permission check is needed because the user is reading inside the
-		 * data dir.
+		 * If this is a runtime-computed GUC, it hasn't yet been initialized,
+		 * and the present value is not useful.  However, this is a convenient
+		 * place to print the value for most GUCs because it is safe to run
+		 * postmaster startup to this point even if the server is already
+		 * running.  For the handful of runtime-computed GUCs that we can't
+		 * provide meaningful values for yet, we wait until later in
+		 * postmaster startup to print the value.  We won't be able to use -C
+		 * on running servers for those GUCs, but otherwise this option is
+		 * unusable for them.
 		 */
-		const char *config_val = GetConfigOption(output_config_variable,
-												 false, false);
+		int			flags = GetConfigOptionFlags(output_config_variable, true);
 
-		puts(config_val ? config_val : "");
-		ExitPostmaster(0);
+		if ((flags & GUC_RUNTIME_COMPUTED) == 0)
+		{
+			/*
+			 * "-C guc" was specified, so print GUC's value and exit.  No
+			 * extra permission check is needed because the user is reading
+			 * inside the data dir.
+			 */
+			const char *config_val = GetConfigOption(output_config_variable,
+													 false, false);
+
+			puts(config_val ? config_val : "");
+			ExitPostmaster(0);
+		}
 	}
 
 	/* Verify that DataDir looks reasonable */
@@ -1033,6 +1049,26 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeShmemGUCs();
 
+	/*
+	 * If -C was specified with a runtime-computed GUC, we held off printing
+	 * the value earlier, as the GUC was not yet initialized.  We handle -C
+	 * for most GUCs before we lock the data directory so that the option may
+	 * be used on a running server.  However, a handful of GUCs are runtime-
+	 * computed and do not have meaningful values until after locking the data
+	 * directory, and we cannot safely calculate their values earlier on a
+	 * running server.  At this point, such GUCs should be properly
+	 * initialized, and we haven't yet set up shared memory, so this is a good
+	 * time to handle the -C option for these special GUCs.
+	 */
+	if (output_config_variable != NULL)
+	{
+		const char *config_val = GetConfigOption(output_config_variable,
+												 false, false);
+
+		puts(config_val ? config_val : "");
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 23236fa4c3..a6e4fcc24e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1983,7 +1983,7 @@ static struct config_bool ConfigureNamesBool[] =
 		{"data_checksums", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows whether data checksums are turned on for this cluster."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_checksums,
 		false,
@@ -2342,7 +2342,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"shared_memory_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
 			NULL,
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB | GUC_RUNTIME_COMPUTED
 		},
 		&shared_memory_size_mb,
 		0, 0, INT_MAX,
@@ -2407,7 +2407,7 @@ static struct config_int ConfigureNamesInt[] =
 						 "in the form accepted by the chmod and umask system "
 						 "calls. (To use the customary octal format the number "
 						 "must start with a 0 (zero).)"),
-			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&data_directory_mode,
 		0700, 0000, 0777,
@@ -3231,7 +3231,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"wal_segment_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of write ahead log segments."),
 			NULL,
-			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
 		},
 		&wal_segment_size,
 		DEFAULT_XLOG_SEG_SIZE,
diff --git a/src/bin/pg_checksums/t/002_actions.pl b/src/bin/pg_checksums/t/002_actions.pl
index a18c104a94..e98586c3f8 100644
--- a/src/bin/pg_checksums/t/002_actions.pl
+++ b/src/bin/pg_checksums/t/002_actions.pl
@@ -10,7 +10,7 @@ use PostgresNode;
 use TestLib;
 
 use Fcntl qw(:seek);
-use Test::More tests => 63;
+use Test::More tests => 69;
 
 
 # Utility routine to create and check a table with corrupted checksums
@@ -178,11 +178,30 @@ command_fails(
 	[ 'pg_checksums', '--enable', '--filenode', '1234', '-D', $pgdata ],
 	"fails when relfilenodes are requested and action is --enable");
 
+# Test postgres -C for an offline cluster.
+# Run-time GUCs are safe to query here.  Note that a lock file is created,
+# then unlinked, leading to an extra LOG entry showing in stderr.
+command_checks_all(
+	[ 'postgres', '-D', $pgdata, '-C', 'data_checksums' ],
+	0,
+	[qr/^on$/],
+	# LOG entry when unlinking lock file.
+	[qr/database system is shut down/],
+	'data_checksums=on is reported on an offline cluster');
+
 # Checks cannot happen with an online cluster
 $node->start;
 command_fails([ 'pg_checksums', '--check', '-D', $pgdata ],
 	"fails with online cluster");
 
+# Test postgres -C on an online cluster.
+command_fails_like(
+	[ 'postgres', '-D', $pgdata, '-C', 'data_checksums' ],
+	qr/lock file .* already exists/,
+	'data_checksums is not reported on an online cluster');
+command_ok([ 'postgres', '-D', $pgdata, '-C', 'work_mem' ],
+	'non-runtime parameter is reported on an online cluster');
+
 # Check corruption of table on default tablespace.
 check_relation_corruption($node, 'corrupt1', 'pg_default');
 
diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index 4aaa7abe1a..02142ffab2 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -133,13 +133,20 @@ PostgreSQL documentation
       <listitem>
        <para>
         Prints the value of the named run-time parameter, and exits.
-        (See the <option>-c</option> option above for details.)  This can
-        be used on a running server, and returns values from
+        (See the <option>-c</option> option above for details.)  This
+        returns values from
         <filename>postgresql.conf</filename>, modified by any parameters
         supplied in this invocation.  It does not reflect parameters
         supplied when the cluster was started.
        </para>
 
+       <para>
+        This can be used on a running server for most parameters.  However,
+        the server must be shut down for some runtime-computed parameters
+        (e.g., <xref linkend="guc-shared-memory-size"/>, and
+        <xref linkend="guc-wal-segment-size"/>).
+       </para>
+
        <para>
         This option is meant for other programs that interact with a server
         instance, such as <xref linkend="app-pg-ctl"/>, to query configuration
-- 
2.33.0

From 4ef008bd7fb4ad7a6c7c0fbd070ef571429c046c Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Thu, 16 Sep 2021 16:35:55 +0000
Subject: [PATCH v11 1/1] Introduce shared_memory_size_in_huge_pages GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.
---
 doc/src/sgml/config.sgml           | 20 ++++++++++++++++++++
 doc/src/sgml/ref/postgres-ref.sgml |  3 ++-
 doc/src/sgml/runtime.sgml          | 30 ++++++++++--------------------
 src/backend/port/sysv_shmem.c      | 16 +++++++++++-----
 src/backend/port/win32_shmem.c     | 14 ++++++++++++++
 src/backend/storage/ipc/ipci.c     | 14 ++++++++++++++
 src/backend/utils/misc/guc.c       | 12 ++++++++++++
 src/include/storage/pg_shmem.h     |  1 +
 8 files changed, 84 insertions(+), 26 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..6e8839cd1b 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10289,6 +10289,26 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size-in-huge-pages" xreflabel="shared_memory_size_in_huge_pages">
+      <term><varname>shared_memory_size_in_huge_pages</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size_in_huge_pages</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are needed for the main shared
+        memory area based on the specified <xref linkend="guc-huge-page-size"/>.
+        If huge pages are not supported, this will be <literal>-1</literal>.
+       </para>
+       <para>
+        This setting is supported only on Linux.  It is always set to
+        <literal>-1</literal> on other platforms.  For more details about using
+        huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index f72c3b04e4..55a3f6c69d 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -143,7 +143,8 @@ PostgreSQL documentation
        <para>
         This can be used on a running server for most parameters.  However,
         the server must be shut down for some runtime-computed parameters
-        (e.g., <xref linkend="guc-shared-memory-size"/> and
+        (e.g., <xref linkend="guc-shared-memory-size"/>,
+        <xref linkend="guc-shared-memory-size-in-huge-pages"/>, and
         <xref linkend="guc-wal-segment-size"/>).
        </para>
 
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..9310859506 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,31 +1442,21 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
+    To determine the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-shared-memory-size-in-huge-pages"/>.
     This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
+     While we need at least <literal>3170</literal> huge pages in this example,
+     a larger setting would be appropriate if other programs on the machine
+     also need huge pages.
      We can set this with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..125e2d47ec 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -456,8 +456,6 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 	return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
 }
 
-#ifdef MAP_HUGETLB
-
 /*
  * Identify the huge page size to use, and compute the related mmap flags.
  *
@@ -476,11 +474,19 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * such as increasing shared_buffers to absorb the extra space.
  *
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
- * and the hugepage-related mmap flags to use into *mmap_flags.
+ * and the hugepage-related mmap flags to use into *mmap_flags.  If huge pages
+ * is not supported, *hugepagesize and *mmap_flags will be set to 0.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
+#ifndef MAP_HUGETLB
+
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+
+#else
+
 	Size		default_hugepagesize = 0;
 
 	/*
@@ -553,9 +559,9 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 		*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
 	}
 #endif
-}
 
 #endif							/* MAP_HUGETLB */
+}
 
 /*
  * Creates an anonymous mmap()ed shared memory segment.
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index d7a71992d8..90de2ab4e1 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -605,3 +605,17 @@ pgwin32_ReserveSharedMemoryRegion(HANDLE hChild)
 
 	return true;
 }
+
+/*
+ * This function is provided for consistency with sysv_shmem.c and does not
+ * provide any useful information for Windows.  To obtain the large page size,
+ * use GetLargePageMinimum() instead.
+ *
+ * This always sets *hugepagesize and *mmap_flags to 0.
+ */
+void
+GetHugePageSize(Size *hugepagesize, int *mmap_flags)
+{
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+}
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 13f3926ff6..805d807906 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -326,6 +326,9 @@ InitializeShmemGUCs(void)
 	char		buf[64];
 	Size		size_b;
 	Size		size_mb;
+	Size		hp_size;
+	Size		hp_required;
+	int			unused;
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest megabyte.
@@ -334,4 +337,15 @@ InitializeShmemGUCs(void)
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%zu", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+	/*
+	 * Calculate the number of huge pages required.
+	 */
+	GetHugePageSize(&hp_size, &unused);
+	if (hp_size != 0)
+	{
+		hp_required = add_size(size_b / hp_size, 1);
+		sprintf(buf, "%zu", hp_required);
+		SetConfigOption("shared_memory_size_in_huge_pages", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a6e4fcc24e..d2ce4a8450 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -665,6 +665,7 @@ static int	max_identifier_length;
 static int	block_size;
 static int	segment_size;
 static int	shared_memory_size_mb;
+static int	shared_memory_size_in_huge_pages;
 static int	wal_block_size;
 static bool data_checksums;
 static bool integer_datetimes;
@@ -2349,6 +2350,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size_in_huge_pages", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the value could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
+		},
+		&shared_memory_size_in_huge_pages,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..518eb86065 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -87,5 +87,6 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 										   PGShmemHeader **shim);
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
 
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From de31b31eb48f1c918a266b1a902ca6ab553a5675 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Thu, 16 Sep 2021 21:23:04 +0000
Subject: [PATCH v12 1/1] Introduce shared_memory_size_in_huge_pages GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.
---
 doc/src/sgml/config.sgml           | 20 ++++++++++++++++++++
 doc/src/sgml/ref/postgres-ref.sgml |  3 ++-
 doc/src/sgml/runtime.sgml          | 30 ++++++++++--------------------
 src/backend/port/sysv_shmem.c      | 16 +++++++++++-----
 src/backend/port/win32_shmem.c     | 14 ++++++++++++++
 src/backend/storage/ipc/ipci.c     | 14 ++++++++++++++
 src/backend/utils/misc/guc.c       | 12 ++++++++++++
 src/include/storage/pg_shmem.h     |  1 +
 8 files changed, 84 insertions(+), 26 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..6e8839cd1b 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10289,6 +10289,26 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size-in-huge-pages" xreflabel="shared_memory_size_in_huge_pages">
+      <term><varname>shared_memory_size_in_huge_pages</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size_in_huge_pages</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are needed for the main shared
+        memory area based on the specified <xref linkend="guc-huge-page-size"/>.
+        If huge pages are not supported, this will be <literal>-1</literal>.
+       </para>
+       <para>
+        This setting is supported only on Linux.  It is always set to
+        <literal>-1</literal> on other platforms.  For more details about using
+        huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index f72c3b04e4..55a3f6c69d 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -143,7 +143,8 @@ PostgreSQL documentation
        <para>
         This can be used on a running server for most parameters.  However,
         the server must be shut down for some runtime-computed parameters
-        (e.g., <xref linkend="guc-shared-memory-size"/> and
+        (e.g., <xref linkend="guc-shared-memory-size"/>,
+        <xref linkend="guc-shared-memory-size-in-huge-pages"/>, and
         <xref linkend="guc-wal-segment-size"/>).
        </para>
 
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..9310859506 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,31 +1442,21 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
+    To determine the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-shared-memory-size-in-huge-pages"/>.
     This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
+     While we need at least <literal>3170</literal> huge pages in this example,
+     a larger setting would be appropriate if other programs on the machine
+     also need huge pages.
      We can set this with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..bc15e2649c 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -456,8 +456,6 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 	return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
 }
 
-#ifdef MAP_HUGETLB
-
 /*
  * Identify the huge page size to use, and compute the related mmap flags.
  *
@@ -476,11 +474,19 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * such as increasing shared_buffers to absorb the extra space.
  *
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
- * and the hugepage-related mmap flags to use into *mmap_flags.
+ * and the hugepage-related mmap flags to use into *mmap_flags.  If huge pages
+ * are not supported, *hugepagesize and *mmap_flags will be set to 0.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
+#ifndef MAP_HUGETLB
+
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+
+#else
+
 	Size		default_hugepagesize = 0;
 
 	/*
@@ -553,9 +559,9 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 		*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
 	}
 #endif
-}
 
 #endif							/* MAP_HUGETLB */
+}
 
 /*
  * Creates an anonymous mmap()ed shared memory segment.
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index d7a71992d8..90de2ab4e1 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -605,3 +605,17 @@ pgwin32_ReserveSharedMemoryRegion(HANDLE hChild)
 
 	return true;
 }
+
+/*
+ * This function is provided for consistency with sysv_shmem.c and does not
+ * provide any useful information for Windows.  To obtain the large page size,
+ * use GetLargePageMinimum() instead.
+ *
+ * This always sets *hugepagesize and *mmap_flags to 0.
+ */
+void
+GetHugePageSize(Size *hugepagesize, int *mmap_flags)
+{
+	*hugepagesize = 0;
+	*mmap_flags = 0;
+}
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 13f3926ff6..805d807906 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -326,6 +326,9 @@ InitializeShmemGUCs(void)
 	char		buf[64];
 	Size		size_b;
 	Size		size_mb;
+	Size		hp_size;
+	Size		hp_required;
+	int			unused;
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest megabyte.
@@ -334,4 +337,15 @@ InitializeShmemGUCs(void)
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%zu", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+	/*
+	 * Calculate the number of huge pages required.
+	 */
+	GetHugePageSize(&hp_size, &unused);
+	if (hp_size != 0)
+	{
+		hp_required = add_size(size_b / hp_size, 1);
+		sprintf(buf, "%zu", hp_required);
+		SetConfigOption("shared_memory_size_in_huge_pages", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a6e4fcc24e..d2ce4a8450 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -665,6 +665,7 @@ static int	max_identifier_length;
 static int	block_size;
 static int	segment_size;
 static int	shared_memory_size_mb;
+static int	shared_memory_size_in_huge_pages;
 static int	wal_block_size;
 static bool data_checksums;
 static bool integer_datetimes;
@@ -2349,6 +2350,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size_in_huge_pages", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the value could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
+		},
+		&shared_memory_size_in_huge_pages,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..518eb86065 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -87,5 +87,6 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 										   PGShmemHeader **shim);
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
 
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From 4a86d93c6c75d3e692bc66d026b68338452c8ead Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Fri, 17 Sep 2021 16:23:59 +0000
Subject: [PATCH v13 1/1] Introduce shared_memory_size_in_huge_pages GUC.

This runtime-computed GUC shows the number of huge pages required
for the server's main shared memory area.
---
 doc/src/sgml/config.sgml           | 21 ++++++++++++++++++
 doc/src/sgml/ref/postgres-ref.sgml |  3 ++-
 doc/src/sgml/runtime.sgml          | 31 ++++++++++-----------------
 src/backend/port/sysv_shmem.c      | 44 ++++++++++++++++++++++++--------------
 src/backend/port/win32_shmem.c     | 17 +++++++++++++++
 src/backend/storage/ipc/ipci.c     | 14 ++++++++++++
 src/backend/utils/misc/guc.c       | 12 +++++++++++
 src/include/storage/pg_shmem.h     |  1 +
 8 files changed, 106 insertions(+), 37 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ef0e2a7746..0a8e35c59f 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10289,6 +10289,27 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size-in-huge-pages" xreflabel="shared_memory_size_in_huge_pages">
+      <term><varname>shared_memory_size_in_huge_pages</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size_in_huge_pages</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of huge pages that are needed for the main shared
+        memory area based on the specified <xref linkend="guc-huge-page-size"/>.
+        If huge pages are not supported, this will be <literal>-1</literal>.
+       </para>
+       <para>
+        This setting is supported only on <productname>Linux</productname>.  It
+        is always set to <literal>-1</literal> on other platforms.  For more
+        details about using huge pages on <productname>Linux</productname>, see
+        <xref linkend="linux-huge-pages"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/doc/src/sgml/ref/postgres-ref.sgml b/doc/src/sgml/ref/postgres-ref.sgml
index f72c3b04e4..55a3f6c69d 100644
--- a/doc/src/sgml/ref/postgres-ref.sgml
+++ b/doc/src/sgml/ref/postgres-ref.sgml
@@ -143,7 +143,8 @@ PostgreSQL documentation
        <para>
         This can be used on a running server for most parameters.  However,
         the server must be shut down for some runtime-computed parameters
-        (e.g., <xref linkend="guc-shared-memory-size"/> and
+        (e.g., <xref linkend="guc-shared-memory-size"/>,
+        <xref linkend="guc-shared-memory-size-in-huge-pages"/>, and
         <xref linkend="guc-wal-segment-size"/>).
        </para>
 
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f1cbc1d9e9..aa2291a326 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1442,31 +1442,22 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To estimate the number of huge pages needed, start
-    <productname>PostgreSQL</productname> without huge pages enabled and check
-    the postmaster's anonymous shared memory segment size, as well as the
-    system's default and supported huge page sizes, using the
-    <filename>/proc</filename> and <filename>/sys</filename> file systems.
+    To determine the number of huge pages needed, use the
+    <command>postgres</command> command to see the value of
+    <xref linkend="guc-shared-memory-size-in-huge-pages"/>.  Note that the
+    server must be shut down to view this runtime-computed parameter.
     This might look like:
 <programlisting>
-$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
-4170
-$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
-6490428K
-$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
-Hugepagesize:       2048 kB
-$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
-hugepages-1048576kB  hugepages-2048kB
+$ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages</userinput>
+3170
 </programlisting>
 
-     In this example the default is 2MB, but you can also explicitly request
-     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+     Note that you can explicitly request either 2MB or 1GB huge pages with
+     <xref linkend="guc-huge-page-size"/>.
 
-     Assuming <literal>2MB</literal> huge pages,
-     <literal>6490428</literal> / <literal>2048</literal> gives approximately
-     <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages.  A larger setting would be
-     appropriate if other programs on the machine also need huge pages.
+     While we need at least <literal>3170</literal> huge pages in this example,
+     a larger setting would be appropriate if other programs on the machine
+     also need huge pages.
      We can set this with:
 <programlisting>
 # <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 9de96edf6a..1ad70a3600 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -456,8 +456,6 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 	return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
 }
 
-#ifdef MAP_HUGETLB
-
 /*
  * Identify the huge page size to use, and compute the related mmap flags.
  *
@@ -476,11 +474,22 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * such as increasing shared_buffers to absorb the extra space.
  *
  * Returns the (real, assumed or config provided) page size into *hugepagesize,
- * and the hugepage-related mmap flags to use into *mmap_flags.
+ * and the hugepage-related mmap flags to use into *mmap_flags.  If huge pages
+ * are not supported, *hugepagesize and *mmap_flags will be set to 0.  It is
+ * safe to set mmap_flags to NULL if its value is not needed.
  */
-static void
+void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
+#ifndef MAP_HUGETLB
+
+	*hugepagesize = 0;
+
+	if (mmap_flags != NULL)
+		*mmap_flags = 0;
+
+#else
+
 	Size		default_hugepagesize = 0;
 
 	/*
@@ -539,23 +548,26 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 		*hugepagesize = 2 * 1024 * 1024;
 	}
 
-	*mmap_flags = MAP_HUGETLB;
+	/* If requested, determine appropriate mmap flags. */
+	if (mmap_flags != NULL)
+	{
+		*mmap_flags = MAP_HUGETLB;
 
-	/*
-	 * On recent enough Linux, also include the explicit page size, if
-	 * necessary.
-	 */
+		/*
+		 * On recent enough Linux, also include the explicit page size, if
+		 * necessary.
+		 */
 #if defined(MAP_HUGE_MASK) && defined(MAP_HUGE_SHIFT)
-	if (*hugepagesize != default_hugepagesize)
-	{
-		int			shift = pg_ceil_log2_64(*hugepagesize);
+		if (*hugepagesize != default_hugepagesize)
+		{
+			int			shift = pg_ceil_log2_64(*hugepagesize);
 
-		*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
-	}
+			*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
+		}
 #endif
-}
-
+	}
 #endif							/* MAP_HUGETLB */
+}
 
 /*
  * Creates an anonymous mmap()ed shared memory segment.
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index d7a71992d8..1d5d902eb8 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -605,3 +605,20 @@ pgwin32_ReserveSharedMemoryRegion(HANDLE hChild)
 
 	return true;
 }
+
+/*
+ * This function is provided for consistency with sysv_shmem.c and does not
+ * provide any useful information for Windows.  To obtain the large page size,
+ * use GetLargePageMinimum() instead.
+ *
+ * This function sets *hugepagesize to 0.  If mmap_flags is not NULL, it also
+ * sets *mmap_flags to 0.
+ */
+void
+GetHugePageSize(Size *hugepagesize, int *mmap_flags)
+{
+	*hugepagesize = 0;
+
+	if (mmap_flags != NULL)
+		*mmap_flags = 0;
+}
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 13f3926ff6..9fa3e0631e 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -326,6 +326,7 @@ InitializeShmemGUCs(void)
 	char		buf[64];
 	Size		size_b;
 	Size		size_mb;
+	Size		hp_size;
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest megabyte.
@@ -334,4 +335,17 @@ InitializeShmemGUCs(void)
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%zu", size_mb);
 	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+
+	/*
+	 * Calculate the number of huge pages required.
+	 */
+	GetHugePageSize(&hp_size, NULL);
+	if (hp_size != 0)
+	{
+		Size		hp_required;
+
+		hp_required = add_size(size_b / hp_size, 1);
+		sprintf(buf, "%zu", hp_required);
+		SetConfigOption("shared_memory_size_in_huge_pages", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a6e4fcc24e..d2ce4a8450 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -665,6 +665,7 @@ static int	max_identifier_length;
 static int	block_size;
 static int	segment_size;
 static int	shared_memory_size_mb;
+static int	shared_memory_size_in_huge_pages;
 static int	wal_block_size;
 static bool data_checksums;
 static bool integer_datetimes;
@@ -2349,6 +2350,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size_in_huge_pages", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Shows the number of huge pages needed for the main shared memory area."),
+			gettext_noop("-1 indicates that the value could not be determined."),
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
+		},
+		&shared_memory_size_in_huge_pages,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 059df1b72c..518eb86065 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -87,5 +87,6 @@ extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 										   PGShmemHeader **shim);
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
+extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
 
 #endif							/* PG_SHMEM_H */
-- 
2.16.6

From 7ee7b176c5280349631426ff047a9df394e26d59 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandbossart@gmail.com>
Date: Mon, 14 Mar 2022 10:24:48 -0700
Subject: [PATCH v1 1/1] send stderr to /dev/null in Linux Huge Pages
 documentation

---
 doc/src/sgml/runtime.sgml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index f77ed24204..85b3ffcd71 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1448,7 +1448,7 @@ export PG_OOM_ADJUST_VALUE=0
     server must be shut down to view this runtime-computed parameter.
     This might look like:
 <programlisting>
-$ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages</userinput>
+$ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages 2> /dev/null</userinput>
 3170
 $ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
 Hugepagesize:       2048 kB
-- 
2.25.1

From cdfd1ad00ca1d8afbfcbafc1f684b5ba9cc43eb6 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandbossart@gmail.com>
Date: Tue, 15 Mar 2022 15:36:41 -0700
Subject: [PATCH v2 1/1] don't emit shutdown messages for 'postgres -C' with
 runtime-computed GUCs

---
 src/backend/postmaster/postmaster.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 80bb269599..bf48bc6326 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1066,6 +1066,10 @@ PostmasterMain(int argc, char *argv[])
 												 false, false);
 
 		puts(config_val ? config_val : "");
+
+		/* don't emit shutdown messages */
+		SetConfigOption("log_min_messages", "PANIC", PGC_INTERNAL, PGC_S_OVERRIDE);
+
 		ExitPostmaster(0);
 	}
 
-- 
2.25.1

From f36d04f5d19c673a7be788d6d4955b148f580095 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandbossart@gmail.com>
Date: Mon, 28 Mar 2022 10:12:23 -0700
Subject: [PATCH v3 1/1] Silence extra logging with 'postgres -C'.

Presently, the server may emit a variety of extra log messages when inspecting
GUC values.  For example, when inspecting a runtime-computed GUC, the server
will always emit a "database system is shut down" LOG (unless the user has set
log_min_messages higher than LOG).  To avoid these extra log messages, this
change sets log_min_messages to FATAL when -C is used (unless the user has set
it to PANIC).  At FATAL, the user will still receive messages explaining why a
GUC's value cannot be inspected.
---
 src/backend/postmaster/postmaster.c | 34 +++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 80bb269599..7ee20d7c69 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -897,6 +897,26 @@ PostmasterMain(int argc, char *argv[])
 
 	if (output_config_variable != NULL)
 	{
+		const char *logging_setting;
+		int			flags;
+
+		/*
+		 * Silence any extra log messages when -C is used.  Before adjusting
+		 * log_min_messages, we save its value in case the user is inspecting
+		 * it.  If we are inspecting a runtime-computed GUC, it is possible that
+		 * a preloaded library's _PG_init() will override our custom setting for
+		 * log_min_messages, but trying to deal with that is probably more
+		 * trouble than it's worth.
+		 *
+		 * If the user has set log_min_messages to PANIC, we leave it at PANIC.
+		 * This has the side effect of silencing helpful FATAL messages (e.g.,
+		 * when trying to inspect a runtime-computed GUC on a running server),
+		 * but presumably the user has set it there for good reason.
+		 */
+		logging_setting = GetConfigOption("log_min_messages", false, false);
+		if (log_min_messages < FATAL)
+			SetConfigOption("log_min_messages", "FATAL", PGC_INTERNAL, PGC_S_OVERRIDE);
+
 		/*
 		 * If this is a runtime-computed GUC, it hasn't yet been initialized,
 		 * and the present value is not useful.  However, this is a convenient
@@ -908,17 +928,23 @@ PostmasterMain(int argc, char *argv[])
 		 * on running servers for those GUCs, but using this option now would
 		 * lead to incorrect results for them.
 		 */
-		int			flags = GetConfigOptionFlags(output_config_variable, true);
+		flags = GetConfigOptionFlags(output_config_variable, true);
 
 		if ((flags & GUC_RUNTIME_COMPUTED) == 0)
 		{
+			const char *config_val;
+
 			/*
 			 * "-C guc" was specified, so print GUC's value and exit.  No
 			 * extra permission check is needed because the user is reading
-			 * inside the data dir.
+			 * inside the data dir.  If we are inspecting log_min_messages,
+			 * print the saved value.
 			 */
-			const char *config_val = GetConfigOption(output_config_variable,
-													 false, false);
+			if (pg_strcasecmp(output_config_variable, "log_min_messages") == 0)
+				config_val = logging_setting;
+			else
+				config_val = GetConfigOption(output_config_variable,
+											 false, false);
 
 			puts(config_val ? config_val : "");
 			ExitPostmaster(0);
-- 
2.25.1

From 3b8a7f8079955cd59a5a318adf6938cdd3c29c6b Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 9 May 2022 15:50:19 +0900
Subject: [PATCH v4] Silence extra logging with 'postgres -C'.

Presently, the server may emit a variety of extra log messages when
inspecting GUC values.  For example, when inspecting a runtime-computed
GUC, the server will always emit a "database system is shut down" LOG
(unless the user has set log_min_messages higher than LOG).  To avoid
these extra log messages, this change sets log_min_messages to FATAL
when -C is used (even if set to PANIC in postgresql.conf).  At FATAL,
the user will still receive messages explaining why a GUC's value cannot
be inspected.
---
 src/backend/postmaster/postmaster.c | 10 ++++++++++
 doc/src/sgml/runtime.sgml           |  2 +-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index ce4007bb2c..38b63bc215 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -913,6 +913,16 @@ PostmasterMain(int argc, char *argv[])
 			puts(config_val ? config_val : "");
 			ExitPostmaster(0);
 		}
+
+		/*
+		 * A runtime-computed GUC will be printed later on.  As we initialize
+		 * a server startup sequence, silence any log messages that may show up
+		 * in the output generated.  FATAL and more severe messages are useful
+		 * to show, even if one would only expect at least PANIC.  LOG entries
+		 * are hidden.
+		 */
+		SetConfigOption("log_min_messages", "FATAL", PGC_INTERNAL,
+						PGC_S_OVERRIDE);
 	}
 
 	/* Verify that DataDir looks reasonable */
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 4465c876b1..62cec614d3 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1448,7 +1448,7 @@ export PG_OOM_ADJUST_VALUE=0
     server must be shut down to view this runtime-computed parameter.
     This might look like:
 <programlisting>
-$ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages 2> /dev/null</userinput>
+$ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages</userinput>
 3170
 $ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
 Hugepagesize:       2048 kB
-- 
2.36.0