allow changing autovacuum_max_workers without restarting

From 612e3ccc66f689a2a16e9dbf027541b71454bad3 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 13 Apr 2024 15:00:08 -0500
Subject: [PATCH v1 1/4] Rename autovacuum_max_workers to
 autovacuum_max_worker_slots.

---
 doc/src/sgml/config.sgml                             |  8 ++++----
 doc/src/sgml/maintenance.sgml                        |  4 ++--
 doc/src/sgml/runtime.sgml                            | 12 ++++++------
 src/backend/access/transam/xlog.c                    |  2 +-
 src/backend/postmaster/autovacuum.c                  |  8 ++++----
 src/backend/postmaster/postmaster.c                  |  2 +-
 src/backend/storage/lmgr/proc.c                      |  6 +++---
 src/backend/utils/init/postinit.c                    | 12 ++++++------
 src/backend/utils/misc/guc_tables.c                  |  6 +++---
 src/backend/utils/misc/postgresql.conf.sample        |  2 +-
 src/include/postmaster/autovacuum.h                  |  2 +-
 src/include/utils/guc_hooks.h                        |  4 ++--
 .../modules/xid_wraparound/t/001_emergency_vacuum.pl |  2 +-
 src/test/modules/xid_wraparound/t/003_wraparounds.pl |  2 +-
 14 files changed, 36 insertions(+), 36 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index d8e1282e12..b4d67a93b6 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1914,7 +1914,7 @@ include_dir 'conf.d'
        </para>
        <para>
         Note that when autovacuum runs, up to
-        <xref linkend="guc-autovacuum-max-workers"/> times this memory
+        <xref linkend="guc-autovacuum-max-worker-slots"/> times this memory
         may be allocated, so be careful not to set the default value
         too high.  It may be useful to control for this by separately
         setting <xref linkend="guc-autovacuum-work-mem"/>.
@@ -8534,10 +8534,10 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
-      <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
+     <varlistentry id="guc-autovacuum-max-worker-slots" xreflabel="autovacuum_max_worker_slots">
+      <term><varname>autovacuum_max_worker_slots</varname> (<type>integer</type>)
       <indexterm>
-       <primary><varname>autovacuum_max_workers</varname> configuration parameter</primary>
+       <primary><varname>autovacuum_max_worker_slots</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml
index 2bfa05b8bc..7b4b3f0087 100644
--- a/doc/src/sgml/maintenance.sgml
+++ b/doc/src/sgml/maintenance.sgml
@@ -864,9 +864,9 @@ HINT:  Execute a database-wide VACUUM in that database.
     seconds.  (Therefore, if the installation has <replaceable>N</replaceable> databases,
     a new worker will be launched every
     <varname>autovacuum_naptime</varname>/<replaceable>N</replaceable> seconds.)
-    A maximum of <xref linkend="guc-autovacuum-max-workers"/> worker processes
+    A maximum of <xref linkend="guc-autovacuum-max-worker-slots"/> worker processes
     are allowed to run at the same time. If there are more than
-    <varname>autovacuum_max_workers</varname> databases to be processed,
+    <varname>autovacuum_max_worker_slots</varname> databases to be processed,
     the next database will be processed as soon as the first worker finishes.
     Each worker process will check each table within its database and
     execute <command>VACUUM</command> and/or <command>ANALYZE</command> as needed.
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 6047b8171d..26a02034c8 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil((max_connections + autovacuum_max_worker_slots + max_wal_senders + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil((max_connections + autovacuum_max_worker_slots + max_wal_senders + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -838,7 +838,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
+    (<xref linkend="guc-autovacuum-max-worker-slots"/>) and allowed background
     process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
     Each such set will
     also contain a 17th semaphore which contains a <quote>magic
@@ -846,13 +846,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
     as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
+    <varname>autovacuum_max_worker_slots</varname> plus <varname>max_wal_senders</varname>,
     plus <varname>max_worker_processes</varname>, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal>.
+    least <literal>ceil((max_connections + autovacuum_max_worker_slots + max_wal_senders + max_worker_processes + 5) / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
@@ -883,7 +883,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
+    (<xref linkend="guc-autovacuum-max-worker-slots"/>) and allowed background
     process (<xref linkend="guc-max-worker-processes"/>).
     On the platforms where this option is preferred, there is no specific
     kernel limit on the number of POSIX semaphores.
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 34a2c71812..9f9ce5da7d 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5362,7 +5362,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_max_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index c367ede6f8..af3d1e218e 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -114,7 +114,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
-int			autovacuum_max_workers;
+int			autovacuum_max_worker_slots;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
 int			autovacuum_vac_thresh;
@@ -209,7 +209,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_max_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -3262,7 +3262,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_max_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3299,7 +3299,7 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_max_worker_slots; i++)
 		{
 			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
 							&worker[i].wi_links);
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 7f3170a8f0..0faec534c0 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + autovacuum_max_worker_slots + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 162b1f919d..84339655e9 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -142,7 +142,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_max_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -242,13 +242,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_max_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_max_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 0805398e24..c05653262f 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -577,7 +577,7 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_max_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	/* internal error because the values were all checked previously */
@@ -591,17 +591,17 @@ InitializeMaxBackends(void)
 bool
 check_max_connections(int *newval, void **extra, GucSource source)
 {
-	if (*newval + autovacuum_max_workers + 1 +
+	if (*newval + autovacuum_max_worker_slots + 1 +
 		max_worker_processes + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
 }
 
 /*
- * GUC check_hook for autovacuum_max_workers
+ * GUC check_hook for autovacuum_max_worker_slots
  */
 bool
-check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
+check_autovacuum_max_worker_slots(int *newval, void **extra, GucSource source)
 {
 	if (MaxConnections + *newval + 1 +
 		max_worker_processes + max_wal_senders > MAX_BACKENDS)
@@ -615,7 +615,7 @@ check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
 bool
 check_max_worker_processes(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + autovacuum_max_worker_slots + 1 +
 		*newval + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
@@ -627,7 +627,7 @@ check_max_worker_processes(int *newval, void **extra, GucSource source)
 bool
 check_max_wal_senders(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + autovacuum_max_worker_slots + 1 +
 		max_worker_processes + *newval > MAX_BACKENDS)
 		return false;
 	return true;
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index c68fdc008b..92dea7061a 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3402,13 +3402,13 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_max_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
-		&autovacuum_max_workers,
+		&autovacuum_max_worker_slots,
 		3, 1, MAX_BACKENDS,
-		check_autovacuum_max_workers, NULL, NULL
+		check_autovacuum_max_worker_slots, NULL, NULL
 	},
 
 	{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 2166ea4a87..c37767cecf 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -658,7 +658,7 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+#autovacuum_max_worker_slots = 3	# max number of autovacuum subprocesses
 					# (change requires restart)
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..754d04485d 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,7 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
-extern PGDLLIMPORT int autovacuum_max_workers;
+extern PGDLLIMPORT int autovacuum_max_worker_slots;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
 extern PGDLLIMPORT int autovacuum_vac_thresh;
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index d64dc5fcdb..22d4c50bc6 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -29,8 +29,8 @@ extern bool check_application_name(char **newval, void **extra,
 								   GucSource source);
 extern void assign_application_name(const char *newval, void *extra);
 extern const char *show_archive_command(void);
-extern bool check_autovacuum_max_workers(int *newval, void **extra,
-										 GucSource source);
+extern bool check_autovacuum_max_worker_slots(int *newval, void **extra,
+											  GucSource source);
 extern bool check_autovacuum_work_mem(int *newval, void **extra,
 									  GucSource source);
 extern bool check_vacuum_buffer_usage_limit(int *newval, void **extra,
diff --git a/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl b/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
index 37550b67a4..f9cdd50c19 100644
--- a/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
+++ b/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
@@ -21,7 +21,7 @@ $node->append_conf(
 autovacuum = off # run autovacuum only when to anti wraparound
 autovacuum_naptime = 1s
 # so it's easier to verify the order of operations
-autovacuum_max_workers = 1
+autovacuum_max_worker_slots = 1
 log_autovacuum_min_duration = 0
 ]);
 $node->start;
diff --git a/src/test/modules/xid_wraparound/t/003_wraparounds.pl b/src/test/modules/xid_wraparound/t/003_wraparounds.pl
index 88063b4b52..99f76229d5 100644
--- a/src/test/modules/xid_wraparound/t/003_wraparounds.pl
+++ b/src/test/modules/xid_wraparound/t/003_wraparounds.pl
@@ -24,7 +24,7 @@ $node->append_conf(
 autovacuum = off # run autovacuum only when to anti wraparound
 autovacuum_naptime = 1s
 # so it's easier to verify the order of operations
-autovacuum_max_workers = 1
+autovacuum_max_worker_slots = 1
 log_autovacuum_min_duration = 0
 ]);
 $node->start;
-- 
2.25.1

From 2da218f260afdd68820ec2708fe7279ec2339d8a Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 13 Apr 2024 21:48:53 -0500
Subject: [PATCH v1 2/4] Convert autovacuum's free workers list to a dclist.

---
 src/backend/postmaster/autovacuum.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index af3d1e218e..e925eff1e4 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -289,7 +289,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -575,7 +575,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
+		launcher_determine_sleep(!dclist_is_empty(&AutoVacuumShmem->av_freeWorkers),
 								 false, &nap);
 
 		/*
@@ -636,7 +636,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = !dclist_is_empty(&AutoVacuumShmem->av_freeWorkers);
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -679,8 +679,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -1087,7 +1087,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (dclist_is_empty(&AutoVacuumShmem->av_freeWorkers))
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1240,7 +1240,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1609,8 +1609,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3289,7 +3289,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3301,8 +3301,8 @@ AutoVacuumShmemInit(void)
 		/* initialize the WorkerInfo free list */
 		for (i = 0; i < autovacuum_max_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
-- 
2.25.1

From b54ccd2f461fd325037f13806be16bea58dad321 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sun, 14 Apr 2024 09:04:01 -0500
Subject: [PATCH v1 3/4] Move free autovacuum worker checks to a helper
 function.

---
 src/backend/postmaster/autovacuum.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index e925eff1e4..f80365faff 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -347,6 +347,7 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
 
 
 
@@ -575,8 +576,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dclist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -636,7 +636,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dclist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -1087,7 +1087,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dclist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -3338,3 +3338,14 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	const dclist_head *freelist = &AutoVacuumShmem->av_freeWorkers;
+
+	return dclist_count(freelist) > 0;
+}
-- 
2.25.1

From 259615d1b03ce2f27ddd17d9210147e39cd7a4cf Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 13 Apr 2024 21:42:33 -0500
Subject: [PATCH v1 4/4] Reintroduce autovacuum_max_workers as a PGC_SIGHUP
 parameter.

---
 doc/src/sgml/config.sgml                      | 25 ++++++++++++++++++-
 doc/src/sgml/maintenance.sgml                 |  4 +--
 src/backend/postmaster/autovacuum.c           |  4 ++-
 src/backend/utils/misc/guc_tables.c           | 15 ++++++++---
 src/backend/utils/misc/postgresql.conf.sample |  3 ++-
 src/include/postmaster/autovacuum.h           |  1 +
 .../xid_wraparound/t/001_emergency_vacuum.pl  |  2 +-
 .../xid_wraparound/t/003_wraparounds.pl       |  2 +-
 8 files changed, 46 insertions(+), 10 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index b4d67a93b6..569b090593 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1914,7 +1914,7 @@ include_dir 'conf.d'
        </para>
        <para>
         Note that when autovacuum runs, up to
-        <xref linkend="guc-autovacuum-max-worker-slots"/> times this memory
+        <xref linkend="guc-autovacuum-max-workers"/> times this memory
         may be allocated, so be careful not to set the default value
         too high.  It may be useful to control for this by separately
         setting <xref linkend="guc-autovacuum-work-mem"/>.
@@ -8540,12 +8540,35 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <primary><varname>autovacuum_max_worker_slots</varname> configuration parameter</primary>
       </indexterm>
       </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 32.  This parameter can only be set at server
+        start.
+       </para>
+       <para>
+        Note that the value of <xref linkend="guc-autovacuum-max-workers"/> is
+        silently capped to this value.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
+      <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_max_workers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
       <listitem>
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
         is three.  This parameter can only be set at server start.
        </para>
+       <para>
+        Note that this value is silently capped to the value of
+        <xref linkend="guc-autovacuum-max-worker-slots"/>.
+       </para>
       </listitem>
      </varlistentry>
 
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml
index 7b4b3f0087..2bfa05b8bc 100644
--- a/doc/src/sgml/maintenance.sgml
+++ b/doc/src/sgml/maintenance.sgml
@@ -864,9 +864,9 @@ HINT:  Execute a database-wide VACUUM in that database.
     seconds.  (Therefore, if the installation has <replaceable>N</replaceable> databases,
     a new worker will be launched every
     <varname>autovacuum_naptime</varname>/<replaceable>N</replaceable> seconds.)
-    A maximum of <xref linkend="guc-autovacuum-max-worker-slots"/> worker processes
+    A maximum of <xref linkend="guc-autovacuum-max-workers"/> worker processes
     are allowed to run at the same time. If there are more than
-    <varname>autovacuum_max_worker_slots</varname> databases to be processed,
+    <varname>autovacuum_max_workers</varname> databases to be processed,
     the next database will be processed as soon as the first worker finishes.
     Each worker process will check each table within its database and
     execute <command>VACUUM</command> and/or <command>ANALYZE</command> as needed.
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index f80365faff..ed7e2b462f 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -115,6 +115,7 @@
  */
 bool		autovacuum_start_daemon = false;
 int			autovacuum_max_worker_slots;
+int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
 int			autovacuum_vac_thresh;
@@ -3346,6 +3347,7 @@ static bool
 av_worker_available(void)
 {
 	const dclist_head *freelist = &AutoVacuumShmem->av_freeWorkers;
+	int			reserved_slots = autovacuum_max_worker_slots - autovacuum_max_workers;
 
-	return dclist_count(freelist) > 0;
+	return dclist_count(freelist) > Max(0, reserved_slots);
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 92dea7061a..92d4d10fe9 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3403,13 +3403,22 @@ struct config_int ConfigureNamesInt[] =
 	{
 		/* see max_connections */
 		{"autovacuum_max_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
-			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
-			NULL
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			gettext_noop("autovacuum_max_workers is silently capped to this value.")
 		},
 		&autovacuum_max_worker_slots,
-		3, 1, MAX_BACKENDS,
+		32, 1, MAX_BACKENDS,
 		check_autovacuum_max_worker_slots, NULL, NULL
 	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
+			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
+			gettext_noop("This value is silently capped to autovacuum_max_worker_slots.")
+		},
+		&autovacuum_max_workers,
+		3, 1, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
 
 	{
 		{"max_parallel_maintenance_workers", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index c37767cecf..c46d245153 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -658,8 +658,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_worker_slots = 3	# max number of autovacuum subprocesses
+autovacuum_max_worker_slots = 32	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index 754d04485d..598782fd34 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -29,6 +29,7 @@ typedef enum
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
 extern PGDLLIMPORT int autovacuum_max_worker_slots;
+extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
 extern PGDLLIMPORT int autovacuum_vac_thresh;
diff --git a/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl b/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
index f9cdd50c19..37550b67a4 100644
--- a/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
+++ b/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
@@ -21,7 +21,7 @@ $node->append_conf(
 autovacuum = off # run autovacuum only when to anti wraparound
 autovacuum_naptime = 1s
 # so it's easier to verify the order of operations
-autovacuum_max_worker_slots = 1
+autovacuum_max_workers = 1
 log_autovacuum_min_duration = 0
 ]);
 $node->start;
diff --git a/src/test/modules/xid_wraparound/t/003_wraparounds.pl b/src/test/modules/xid_wraparound/t/003_wraparounds.pl
index 99f76229d5..88063b4b52 100644
--- a/src/test/modules/xid_wraparound/t/003_wraparounds.pl
+++ b/src/test/modules/xid_wraparound/t/003_wraparounds.pl
@@ -24,7 +24,7 @@ $node->append_conf(
 autovacuum = off # run autovacuum only when to anti wraparound
 autovacuum_naptime = 1s
 # so it's easier to verify the order of operations
-autovacuum_max_worker_slots = 1
+autovacuum_max_workers = 1
 log_autovacuum_min_duration = 0
 ]);
 $node->start;
-- 
2.25.1

From 466f31a23605755a8e3d17c362c9f4940bf91da0 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 13 Apr 2024 15:00:08 -0500
Subject: [PATCH v2 1/4] Rename autovacuum_max_workers to
 autovacuum_max_worker_slots.

---
 doc/src/sgml/config.sgml                             |  8 ++++----
 doc/src/sgml/maintenance.sgml                        |  4 ++--
 doc/src/sgml/runtime.sgml                            | 12 ++++++------
 src/backend/access/transam/xlog.c                    |  2 +-
 src/backend/postmaster/autovacuum.c                  |  8 ++++----
 src/backend/postmaster/postmaster.c                  |  2 +-
 src/backend/storage/lmgr/proc.c                      |  6 +++---
 src/backend/utils/init/postinit.c                    | 12 ++++++------
 src/backend/utils/misc/guc_tables.c                  |  6 +++---
 src/backend/utils/misc/postgresql.conf.sample        |  2 +-
 src/include/postmaster/autovacuum.h                  |  2 +-
 src/include/utils/guc_hooks.h                        |  4 ++--
 .../modules/xid_wraparound/t/001_emergency_vacuum.pl |  2 +-
 src/test/modules/xid_wraparound/t/003_wraparounds.pl |  2 +-
 14 files changed, 36 insertions(+), 36 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index d8e1282e12..b4d67a93b6 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1914,7 +1914,7 @@ include_dir 'conf.d'
        </para>
        <para>
         Note that when autovacuum runs, up to
-        <xref linkend="guc-autovacuum-max-workers"/> times this memory
+        <xref linkend="guc-autovacuum-max-worker-slots"/> times this memory
         may be allocated, so be careful not to set the default value
         too high.  It may be useful to control for this by separately
         setting <xref linkend="guc-autovacuum-work-mem"/>.
@@ -8534,10 +8534,10 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
-      <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
+     <varlistentry id="guc-autovacuum-max-worker-slots" xreflabel="autovacuum_max_worker_slots">
+      <term><varname>autovacuum_max_worker_slots</varname> (<type>integer</type>)
       <indexterm>
-       <primary><varname>autovacuum_max_workers</varname> configuration parameter</primary>
+       <primary><varname>autovacuum_max_worker_slots</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml
index 0be90bdc7e..5373acba41 100644
--- a/doc/src/sgml/maintenance.sgml
+++ b/doc/src/sgml/maintenance.sgml
@@ -864,9 +864,9 @@ HINT:  Execute a database-wide VACUUM in that database.
     seconds.  (Therefore, if the installation has <replaceable>N</replaceable> databases,
     a new worker will be launched every
     <varname>autovacuum_naptime</varname>/<replaceable>N</replaceable> seconds.)
-    A maximum of <xref linkend="guc-autovacuum-max-workers"/> worker processes
+    A maximum of <xref linkend="guc-autovacuum-max-worker-slots"/> worker processes
     are allowed to run at the same time. If there are more than
-    <varname>autovacuum_max_workers</varname> databases to be processed,
+    <varname>autovacuum_max_worker_slots</varname> databases to be processed,
     the next database will be processed as soon as the first worker finishes.
     Each worker process will check each table within its database and
     execute <command>VACUUM</command> and/or <command>ANALYZE</command> as needed.
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 6047b8171d..26a02034c8 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil((max_connections + autovacuum_max_worker_slots + max_wal_senders + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil((max_connections + autovacuum_max_worker_slots + max_wal_senders + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -838,7 +838,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
+    (<xref linkend="guc-autovacuum-max-worker-slots"/>) and allowed background
     process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
     Each such set will
     also contain a 17th semaphore which contains a <quote>magic
@@ -846,13 +846,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
     as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
+    <varname>autovacuum_max_worker_slots</varname> plus <varname>max_wal_senders</varname>,
     plus <varname>max_worker_processes</varname>, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal>.
+    least <literal>ceil((max_connections + autovacuum_max_worker_slots + max_wal_senders + max_worker_processes + 5) / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
@@ -883,7 +883,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
+    (<xref linkend="guc-autovacuum-max-worker-slots"/>) and allowed background
     process (<xref linkend="guc-max-worker-processes"/>).
     On the platforms where this option is preferred, there is no specific
     kernel limit on the number of POSIX semaphores.
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 34a2c71812..9f9ce5da7d 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5362,7 +5362,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_max_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index c367ede6f8..af3d1e218e 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -114,7 +114,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
-int			autovacuum_max_workers;
+int			autovacuum_max_worker_slots;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
 int			autovacuum_vac_thresh;
@@ -209,7 +209,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_max_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -3262,7 +3262,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_max_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3299,7 +3299,7 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_max_worker_slots; i++)
 		{
 			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
 							&worker[i].wi_links);
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 7f3170a8f0..0faec534c0 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + autovacuum_max_worker_slots + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index e4f256c63c..4587c5a508 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -142,7 +142,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_max_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -242,13 +242,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_max_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_max_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 0805398e24..c05653262f 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -577,7 +577,7 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_max_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	/* internal error because the values were all checked previously */
@@ -591,17 +591,17 @@ InitializeMaxBackends(void)
 bool
 check_max_connections(int *newval, void **extra, GucSource source)
 {
-	if (*newval + autovacuum_max_workers + 1 +
+	if (*newval + autovacuum_max_worker_slots + 1 +
 		max_worker_processes + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
 }
 
 /*
- * GUC check_hook for autovacuum_max_workers
+ * GUC check_hook for autovacuum_max_worker_slots
  */
 bool
-check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
+check_autovacuum_max_worker_slots(int *newval, void **extra, GucSource source)
 {
 	if (MaxConnections + *newval + 1 +
 		max_worker_processes + max_wal_senders > MAX_BACKENDS)
@@ -615,7 +615,7 @@ check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
 bool
 check_max_worker_processes(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + autovacuum_max_worker_slots + 1 +
 		*newval + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
@@ -627,7 +627,7 @@ check_max_worker_processes(int *newval, void **extra, GucSource source)
 bool
 check_max_wal_senders(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + autovacuum_max_worker_slots + 1 +
 		max_worker_processes + *newval > MAX_BACKENDS)
 		return false;
 	return true;
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index c68fdc008b..92dea7061a 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3402,13 +3402,13 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_max_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
-		&autovacuum_max_workers,
+		&autovacuum_max_worker_slots,
 		3, 1, MAX_BACKENDS,
-		check_autovacuum_max_workers, NULL, NULL
+		check_autovacuum_max_worker_slots, NULL, NULL
 	},
 
 	{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 2166ea4a87..c37767cecf 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -658,7 +658,7 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+#autovacuum_max_worker_slots = 3	# max number of autovacuum subprocesses
 					# (change requires restart)
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..754d04485d 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,7 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
-extern PGDLLIMPORT int autovacuum_max_workers;
+extern PGDLLIMPORT int autovacuum_max_worker_slots;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
 extern PGDLLIMPORT int autovacuum_vac_thresh;
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index d64dc5fcdb..22d4c50bc6 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -29,8 +29,8 @@ extern bool check_application_name(char **newval, void **extra,
 								   GucSource source);
 extern void assign_application_name(const char *newval, void *extra);
 extern const char *show_archive_command(void);
-extern bool check_autovacuum_max_workers(int *newval, void **extra,
-										 GucSource source);
+extern bool check_autovacuum_max_worker_slots(int *newval, void **extra,
+											  GucSource source);
 extern bool check_autovacuum_work_mem(int *newval, void **extra,
 									  GucSource source);
 extern bool check_vacuum_buffer_usage_limit(int *newval, void **extra,
diff --git a/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl b/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
index 37550b67a4..f9cdd50c19 100644
--- a/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
+++ b/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
@@ -21,7 +21,7 @@ $node->append_conf(
 autovacuum = off # run autovacuum only when to anti wraparound
 autovacuum_naptime = 1s
 # so it's easier to verify the order of operations
-autovacuum_max_workers = 1
+autovacuum_max_worker_slots = 1
 log_autovacuum_min_duration = 0
 ]);
 $node->start;
diff --git a/src/test/modules/xid_wraparound/t/003_wraparounds.pl b/src/test/modules/xid_wraparound/t/003_wraparounds.pl
index 88063b4b52..99f76229d5 100644
--- a/src/test/modules/xid_wraparound/t/003_wraparounds.pl
+++ b/src/test/modules/xid_wraparound/t/003_wraparounds.pl
@@ -24,7 +24,7 @@ $node->append_conf(
 autovacuum = off # run autovacuum only when to anti wraparound
 autovacuum_naptime = 1s
 # so it's easier to verify the order of operations
-autovacuum_max_workers = 1
+autovacuum_max_worker_slots = 1
 log_autovacuum_min_duration = 0
 ]);
 $node->start;
-- 
2.25.1

From 6ba708a66dfaf964f8330a81417e3efdfeef9c92 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 13 Apr 2024 21:48:53 -0500
Subject: [PATCH v2 2/4] Convert autovacuum's free workers list to a dclist.

---
 src/backend/postmaster/autovacuum.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index af3d1e218e..e925eff1e4 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -289,7 +289,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -575,7 +575,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
+		launcher_determine_sleep(!dclist_is_empty(&AutoVacuumShmem->av_freeWorkers),
 								 false, &nap);
 
 		/*
@@ -636,7 +636,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = !dclist_is_empty(&AutoVacuumShmem->av_freeWorkers);
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -679,8 +679,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -1087,7 +1087,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (dclist_is_empty(&AutoVacuumShmem->av_freeWorkers))
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1240,7 +1240,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1609,8 +1609,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3289,7 +3289,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3301,8 +3301,8 @@ AutoVacuumShmemInit(void)
 		/* initialize the WorkerInfo free list */
 		for (i = 0; i < autovacuum_max_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
-- 
2.25.1

From 6ecfb0aebb0b1d0e7d2d9f8b65038ae8c2c98f88 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sun, 14 Apr 2024 09:04:01 -0500
Subject: [PATCH v2 3/4] Move free autovacuum worker checks to a helper
 function.

---
 src/backend/postmaster/autovacuum.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index e925eff1e4..f80365faff 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -347,6 +347,7 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
 
 
 
@@ -575,8 +576,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dclist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -636,7 +636,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dclist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -1087,7 +1087,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dclist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -3338,3 +3338,14 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	const dclist_head *freelist = &AutoVacuumShmem->av_freeWorkers;
+
+	return dclist_count(freelist) > 0;
+}
-- 
2.25.1

From 915e1594439328d47a04096c9811dd43a4526efe Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 13 Apr 2024 21:42:33 -0500
Subject: [PATCH v2 4/4] Reintroduce autovacuum_max_workers as a PGC_SIGHUP
 parameter.

---
 doc/src/sgml/config.sgml                      | 24 +++++++++++++++++--
 doc/src/sgml/maintenance.sgml                 |  4 ++--
 src/backend/postmaster/autovacuum.c           |  4 +++-
 src/backend/utils/misc/guc_tables.c           | 15 +++++++++---
 src/backend/utils/misc/postgresql.conf.sample |  3 ++-
 src/include/postmaster/autovacuum.h           |  1 +
 .../xid_wraparound/t/001_emergency_vacuum.pl  |  2 +-
 .../xid_wraparound/t/003_wraparounds.pl       |  2 +-
 8 files changed, 44 insertions(+), 11 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index b4d67a93b6..0f022a8056 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1914,7 +1914,7 @@ include_dir 'conf.d'
        </para>
        <para>
         Note that when autovacuum runs, up to
-        <xref linkend="guc-autovacuum-max-worker-slots"/> times this memory
+        <xref linkend="guc-autovacuum-max-workers"/> times this memory
         may be allocated, so be careful not to set the default value
         too high.  It may be useful to control for this by separately
         setting <xref linkend="guc-autovacuum-work-mem"/>.
@@ -8540,11 +8540,31 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <primary><varname>autovacuum_max_worker_slots</varname> configuration parameter</primary>
       </indexterm>
       </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 32.  This parameter can only be set at server
+        start.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
+      <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_max_workers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
       <listitem>
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        Note that this value is silently capped to the value of
+        <xref linkend="guc-autovacuum-max-worker-slots"/>.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml
index 5373acba41..0be90bdc7e 100644
--- a/doc/src/sgml/maintenance.sgml
+++ b/doc/src/sgml/maintenance.sgml
@@ -864,9 +864,9 @@ HINT:  Execute a database-wide VACUUM in that database.
     seconds.  (Therefore, if the installation has <replaceable>N</replaceable> databases,
     a new worker will be launched every
     <varname>autovacuum_naptime</varname>/<replaceable>N</replaceable> seconds.)
-    A maximum of <xref linkend="guc-autovacuum-max-worker-slots"/> worker processes
+    A maximum of <xref linkend="guc-autovacuum-max-workers"/> worker processes
     are allowed to run at the same time. If there are more than
-    <varname>autovacuum_max_worker_slots</varname> databases to be processed,
+    <varname>autovacuum_max_workers</varname> databases to be processed,
     the next database will be processed as soon as the first worker finishes.
     Each worker process will check each table within its database and
     execute <command>VACUUM</command> and/or <command>ANALYZE</command> as needed.
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index f80365faff..ed7e2b462f 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -115,6 +115,7 @@
  */
 bool		autovacuum_start_daemon = false;
 int			autovacuum_max_worker_slots;
+int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
 int			autovacuum_vac_thresh;
@@ -3346,6 +3347,7 @@ static bool
 av_worker_available(void)
 {
 	const dclist_head *freelist = &AutoVacuumShmem->av_freeWorkers;
+	int			reserved_slots = autovacuum_max_worker_slots - autovacuum_max_workers;
 
-	return dclist_count(freelist) > 0;
+	return dclist_count(freelist) > Max(0, reserved_slots);
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 92dea7061a..92d4d10fe9 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3403,13 +3403,22 @@ struct config_int ConfigureNamesInt[] =
 	{
 		/* see max_connections */
 		{"autovacuum_max_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
-			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
-			NULL
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			gettext_noop("autovacuum_max_workers is silently capped to this value.")
 		},
 		&autovacuum_max_worker_slots,
-		3, 1, MAX_BACKENDS,
+		32, 1, MAX_BACKENDS,
 		check_autovacuum_max_worker_slots, NULL, NULL
 	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
+			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
+			gettext_noop("This value is silently capped to autovacuum_max_worker_slots.")
+		},
+		&autovacuum_max_workers,
+		3, 1, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
 
 	{
 		{"max_parallel_maintenance_workers", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index c37767cecf..c46d245153 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -658,8 +658,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_worker_slots = 3	# max number of autovacuum subprocesses
+autovacuum_max_worker_slots = 32	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index 754d04485d..598782fd34 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -29,6 +29,7 @@ typedef enum
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
 extern PGDLLIMPORT int autovacuum_max_worker_slots;
+extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
 extern PGDLLIMPORT int autovacuum_vac_thresh;
diff --git a/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl b/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
index f9cdd50c19..37550b67a4 100644
--- a/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
+++ b/src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl
@@ -21,7 +21,7 @@ $node->append_conf(
 autovacuum = off # run autovacuum only when to anti wraparound
 autovacuum_naptime = 1s
 # so it's easier to verify the order of operations
-autovacuum_max_worker_slots = 1
+autovacuum_max_workers = 1
 log_autovacuum_min_duration = 0
 ]);
 $node->start;
diff --git a/src/test/modules/xid_wraparound/t/003_wraparounds.pl b/src/test/modules/xid_wraparound/t/003_wraparounds.pl
index 99f76229d5..88063b4b52 100644
--- a/src/test/modules/xid_wraparound/t/003_wraparounds.pl
+++ b/src/test/modules/xid_wraparound/t/003_wraparounds.pl
@@ -24,7 +24,7 @@ $node->append_conf(
 autovacuum = off # run autovacuum only when to anti wraparound
 autovacuum_naptime = 1s
 # so it's easier to verify the order of operations
-autovacuum_max_worker_slots = 1
+autovacuum_max_workers = 1
 log_autovacuum_min_duration = 0
 ]);
 $node->start;
-- 
2.25.1

From 72e0496294ef0390c77cef8031ae51c1a44ebde8 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 7 May 2024 10:59:24 -0500
Subject: [PATCH v3 1/1] allow changing autovacuum_max_workers without
 restarting

---
 doc/src/sgml/config.sgml                      |  3 +-
 doc/src/sgml/runtime.sgml                     | 15 ++++---
 src/backend/access/transam/xlog.c             |  2 +-
 src/backend/postmaster/autovacuum.c           | 44 ++++++++++++-------
 src/backend/postmaster/postmaster.c           |  2 +-
 src/backend/storage/lmgr/proc.c               |  9 ++--
 src/backend/utils/init/postinit.c             | 20 ++-------
 src/backend/utils/misc/guc_tables.c           |  7 ++-
 src/backend/utils/misc/postgresql.conf.sample |  1 -
 src/include/postmaster/autovacuum.h           |  8 ++++
 src/include/utils/guc_hooks.h                 |  2 -
 11 files changed, 58 insertions(+), 55 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index e93208b2e6..8e2a1d6902 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8528,7 +8528,8 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 6047b8171d..8a672a8383 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil((max_connections + max_wal_senders + max_worker_processes + 1029) / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil((max_connections + max_wal_senders + max_worker_processes + 1029) / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -838,7 +838,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
+    (1024) and allowed background
     process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
     Each such set will
     also contain a 17th semaphore which contains a <quote>magic
@@ -846,13 +846,14 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
     as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
-    plus <varname>max_worker_processes</varname>, plus one extra for each 16
+    <varname>max_wal_senders</varname>,
+    plus <varname>max_worker_processes</varname>, plus 1024 for autovacuum
+    worker processes, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal>.
+    least <literal>ceil((max_connections + max_wal_senders + max_worker_processes + 1029) / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
@@ -883,7 +884,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
+    (1024) and allowed background
     process (<xref linkend="guc-max-worker-processes"/>).
     On the platforms where this option is preferred, there is no specific
     kernel limit on the number of POSIX semaphores.
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index c3fd9c1eae..5b0312b2a6 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5362,7 +5362,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum workers when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 9a925a10cd..1ab97b9903 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -208,8 +208,7 @@ typedef struct autovac_table
 
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
- * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * an array of these in shared memory.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -289,7 +288,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -347,6 +346,7 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
 
 
 
@@ -575,8 +575,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -636,7 +635,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -679,8 +678,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -1087,7 +1086,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1240,7 +1239,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1609,8 +1608,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3265,7 +3264,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(AUTOVAC_MAX_WORKER_SLOTS,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3292,7 +3291,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3302,10 +3301,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < AUTOVAC_MAX_WORKER_SLOTS; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3341,3 +3340,14 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			reserved_slots = AUTOVAC_MAX_WORKER_SLOTS - autovacuum_max_workers;
+
+	return dclist_count(&AutoVacuumShmem->av_freeWorkers) > reserved_slots;
+}
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 7f3170a8f0..cab8a8a5f4 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index e4f256c63c..fdf7373232 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -141,9 +141,8 @@ ProcGlobalSemas(void)
  *	  running out when trying to start another backend is a common failure.
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
- *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
- *	  find out sooner rather than later.
+ *	  MaxConnections, max_worker_processes, or max_wal_senders higher than
+ *    his kernel will support, he'll find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
  *	  implementation typically requires us to create semaphores in the
@@ -242,13 +241,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 0805398e24..684b22e3a5 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -577,7 +577,7 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 		max_worker_processes + max_wal_senders;
 
 	/* internal error because the values were all checked previously */
@@ -591,19 +591,7 @@ InitializeMaxBackends(void)
 bool
 check_max_connections(int *newval, void **extra, GucSource source)
 {
-	if (*newval + autovacuum_max_workers + 1 +
-		max_worker_processes + max_wal_senders > MAX_BACKENDS)
-		return false;
-	return true;
-}
-
-/*
- * GUC check_hook for autovacuum_max_workers
- */
-bool
-check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
-{
-	if (MaxConnections + *newval + 1 +
+	if (*newval + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 		max_worker_processes + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
@@ -615,7 +603,7 @@ check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
 bool
 check_max_worker_processes(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 		*newval + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
@@ -627,7 +615,7 @@ check_max_worker_processes(int *newval, void **extra, GucSource source)
 bool
 check_max_wal_senders(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 		max_worker_processes + *newval > MAX_BACKENDS)
 		return false;
 	return true;
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index ea2b0577bc..1e255fe263 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3380,14 +3380,13 @@ struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 	{
-		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
 		&autovacuum_max_workers,
-		3, 1, MAX_BACKENDS,
-		check_autovacuum_max_workers, NULL, NULL
+		3, 1, AUTOVAC_MAX_WORKER_SLOTS,
+		NULL, NULL, NULL
 	},
 
 	{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 83d5df8e46..439859e3ff 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -659,7 +659,6 @@
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
 #autovacuum_max_workers = 3		# max number of autovacuum subprocesses
-					# (change requires restart)
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..23a7a729aa 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -66,4 +66,12 @@ extern bool AutoVacuumRequestWork(AutoVacuumWorkItemType type,
 extern Size AutoVacuumShmemSize(void);
 extern void AutoVacuumShmemInit(void);
 
+/*
+ * Number of autovacuum worker slots to allocate.  This is the upper limit of
+ * autovacuum_max_workers.
+ *
+ * NB: This must be less than MAX_BACKENDS.
+ */
+#define AUTOVAC_MAX_WORKER_SLOTS	(1024)
+
 #endif							/* AUTOVACUUM_H */
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index d64dc5fcdb..76346eb05e 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -29,8 +29,6 @@ extern bool check_application_name(char **newval, void **extra,
 								   GucSource source);
 extern void assign_application_name(const char *newval, void *extra);
 extern const char *show_archive_command(void);
-extern bool check_autovacuum_max_workers(int *newval, void **extra,
-										 GucSource source);
 extern bool check_autovacuum_work_mem(int *newval, void **extra,
 									  GucSource source);
 extern bool check_vacuum_buffer_usage_limit(int *newval, void **extra,
-- 
2.25.1

From 056ad035c5d213f7ae49f5feb28229f35086430f Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 7 May 2024 10:59:24 -0500
Subject: [PATCH v4 1/1] allow changing autovacuum_max_workers without
 restarting

---
 doc/src/sgml/config.sgml                      |  3 +-
 doc/src/sgml/runtime.sgml                     | 15 ++++---
 src/backend/access/transam/xlog.c             |  1 -
 src/backend/postmaster/autovacuum.c           | 44 ++++++++++++-------
 src/backend/postmaster/postmaster.c           |  2 +-
 src/backend/storage/lmgr/proc.c               |  9 ++--
 src/backend/utils/init/postinit.c             | 20 ++-------
 src/backend/utils/misc/guc_tables.c           |  7 ++-
 src/backend/utils/misc/postgresql.conf.sample |  1 -
 src/include/postmaster/autovacuum.h           |  8 ++++
 src/include/utils/guc_hooks.h                 |  2 -
 11 files changed, 57 insertions(+), 55 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 698169afdb..b79a855729 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8535,7 +8535,8 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 883a849e6f..862f4f1f45 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil((max_connections + max_wal_senders + max_worker_processes + 263) / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil((max_connections + max_wal_senders + max_worker_processes + 263) / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -838,7 +838,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (256), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), and allowed background
     process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
     Each such set will
@@ -847,13 +847,14 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
     as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
-    plus <varname>max_worker_processes</varname>, plus one extra for each 16
+    <varname>max_wal_senders</varname>,
+    plus <varname>max_worker_processes</varname>, plus 256 for autovacuum
+    worker processes, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
+    least <literal>ceil((max_connections + max_wal_senders + max_worker_processes + 263) / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
@@ -884,7 +885,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
+    (256) and allowed background
     process (<xref linkend="guc-max-worker-processes"/>).
     On the platforms where this option is preferred, there is no specific
     kernel limit on the number of POSIX semaphores.
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 330e058c5f..b6511d0658 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5362,7 +5362,6 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 9a925a10cd..1ab97b9903 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -208,8 +208,7 @@ typedef struct autovac_table
 
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
- * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * an array of these in shared memory.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -289,7 +288,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -347,6 +346,7 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
 
 
 
@@ -575,8 +575,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -636,7 +635,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -679,8 +678,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -1087,7 +1086,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1240,7 +1239,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1609,8 +1608,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3265,7 +3264,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(AUTOVAC_MAX_WORKER_SLOTS,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3292,7 +3291,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3302,10 +3301,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < AUTOVAC_MAX_WORKER_SLOTS; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3341,3 +3340,14 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			reserved_slots = AUTOVAC_MAX_WORKER_SLOTS - autovacuum_max_workers;
+
+	return dclist_count(&AutoVacuumShmem->av_freeWorkers) > reserved_slots;
+}
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index bf0241aed0..efa15954ec 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index ce29da9012..90cd770caf 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -141,9 +141,8 @@ ProcGlobalSemas(void)
  *	  running out when trying to start another backend is a common failure.
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
- *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
- *	  find out sooner rather than later.
+ *	  MaxConnections, max_worker_processes, or max_wal_senders higher than
+ *    his kernel will support, he'll find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
  *	  implementation typically requires us to create semaphores in the
@@ -242,13 +241,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 0805398e24..684b22e3a5 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -577,7 +577,7 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 		max_worker_processes + max_wal_senders;
 
 	/* internal error because the values were all checked previously */
@@ -591,19 +591,7 @@ InitializeMaxBackends(void)
 bool
 check_max_connections(int *newval, void **extra, GucSource source)
 {
-	if (*newval + autovacuum_max_workers + 1 +
-		max_worker_processes + max_wal_senders > MAX_BACKENDS)
-		return false;
-	return true;
-}
-
-/*
- * GUC check_hook for autovacuum_max_workers
- */
-bool
-check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
-{
-	if (MaxConnections + *newval + 1 +
+	if (*newval + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 		max_worker_processes + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
@@ -615,7 +603,7 @@ check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
 bool
 check_max_worker_processes(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 		*newval + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
@@ -627,7 +615,7 @@ check_max_worker_processes(int *newval, void **extra, GucSource source)
 bool
 check_max_wal_senders(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + AUTOVAC_MAX_WORKER_SLOTS + 1 +
 		max_worker_processes + *newval > MAX_BACKENDS)
 		return false;
 	return true;
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 46c258be28..d97ad55c4a 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3380,14 +3380,13 @@ struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 	{
-		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
 		&autovacuum_max_workers,
-		3, 1, MAX_BACKENDS,
-		check_autovacuum_max_workers, NULL, NULL
+		3, 1, AUTOVAC_MAX_WORKER_SLOTS,
+		NULL, NULL, NULL
 	},
 
 	{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 83d5df8e46..439859e3ff 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -659,7 +659,6 @@
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
 #autovacuum_max_workers = 3		# max number of autovacuum subprocesses
-					# (change requires restart)
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..f71c82830c 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -66,4 +66,12 @@ extern bool AutoVacuumRequestWork(AutoVacuumWorkItemType type,
 extern Size AutoVacuumShmemSize(void);
 extern void AutoVacuumShmemInit(void);
 
+/*
+ * Number of autovacuum worker slots to allocate.  This is the upper limit of
+ * autovacuum_max_workers.
+ *
+ * NB: This must be less than MAX_BACKENDS.
+ */
+#define AUTOVAC_MAX_WORKER_SLOTS	(256)
+
 #endif							/* AUTOVACUUM_H */
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index d64dc5fcdb..76346eb05e 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -29,8 +29,6 @@ extern bool check_application_name(char **newval, void **extra,
 								   GucSource source);
 extern void assign_application_name(const char *newval, void *extra);
 extern const char *show_archive_command(void);
-extern bool check_autovacuum_max_workers(int *newval, void **extra,
-										 GucSource source);
 extern bool check_autovacuum_work_mem(int *newval, void **extra,
 									  GucSource source);
 extern bool check_vacuum_buffer_usage_limit(int *newval, void **extra,
-- 
2.39.3 (Apple Git-146)

From 69245426585052af1317c1bf3e564cae0c019f52 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 21 May 2024 14:02:22 -0500
Subject: [PATCH v5 1/3] add num_os_semaphores GUC

---
 doc/src/sgml/config.sgml            | 14 +++++++++++
 doc/src/sgml/runtime.sgml           | 39 ++++++++++++++++-------------
 src/backend/storage/ipc/ipci.c      |  6 ++++-
 src/backend/utils/misc/guc_tables.c | 12 +++++++++
 4 files changed, 53 insertions(+), 18 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 0c7a9082c5..087385cb4e 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -11234,6 +11234,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-num-os-semaphores" xreflabel="num_os_semaphores">
+      <term><varname>num_os_semaphores</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>num_os_semaphores</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of semaphores that are needed for the server based
+        on the number of allowed connections, worker processes, etc.
+       </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 2f7c618886..26f3cbe555 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil(num_os_semaphores / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil(num_os_semaphores / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -836,30 +836,38 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
 
    <para>
     When using System V semaphores,
-    <productname>PostgreSQL</productname> uses one semaphore per allowed connection
-    (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
-    (<xref linkend="guc-max-wal-senders"/>), and allowed background
-    process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
+    <productname>PostgreSQL</productname> uses one semaphore per allowed connection,
+    worker process, etc., in sets of 16.
     Each such set will
     also contain a 17th semaphore which contains a <quote>magic
     number</quote>, to detect collision with semaphore sets used by
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
-    as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
-    plus <varname>max_worker_processes</varname>, plus one extra for each 16
-    allowed connections plus workers (see the formula in <xref
+    as high as <xref linkend="guc-num-os-semaphores"/> plus one extra for
+    each set of 16 required semaphores (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
+    least <literal>ceil(num_os_semaphores / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
     left on device</quote>, from the function <function>semget</function>.
    </para>
 
+   <para>
+    The number of semaphores required by <productname>PostgreSQL</productname>
+    is provided by the runtime-computed parameter
+    <varname>num_os_semaphores</varname>, which can be determined before
+    starting the server with a <command>postgres</command> command like:
+<programlisting>
+$ <userinput>postgres -D $PGDATA -C num_os_semaphores</userinput>
+</programlisting>
+    The value of <varname>num_os_semaphores</varname> should be input into
+    the aforementioned formulas to determine appropriate values for
+    <varname>SEMMNI</varname> and <varname>SEMMNS</varname>.
+   </para>
+
    <para>
     In some cases it might also be necessary to increase
     <varname>SEMMAP</varname> to be at least on the order of
@@ -882,11 +890,8 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
 
    <para>
     When using POSIX semaphores, the number of semaphores needed is the
-    same as for System V, that is one semaphore per allowed connection
-    (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
-    (<xref linkend="guc-max-wal-senders"/>), and allowed background
-    process (<xref linkend="guc-max-worker-processes"/>).
+    same as for System V, that is one semaphore per allowed connection,
+    worker process, etc.
     On the platforms where this option is preferred, there is no specific
     kernel limit on the number of POSIX semaphores.
    </para>
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 521ed5418c..0314513aa6 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -372,11 +372,12 @@ InitializeShmemGUCs(void)
 	Size		size_b;
 	Size		size_mb;
 	Size		hp_size;
+	int			num_semas;
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest megabyte.
 	 */
-	size_b = CalculateShmemSize(NULL);
+	size_b = CalculateShmemSize(&num_semas);
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%zu", size_mb);
 	SetConfigOption("shared_memory_size", buf,
@@ -395,4 +396,7 @@ InitializeShmemGUCs(void)
 		SetConfigOption("shared_memory_size_in_huge_pages", buf,
 						PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 	}
+
+	sprintf(buf, "%d", num_semas);
+	SetConfigOption("num_os_semaphores", buf, PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 46c258be28..80e77cbac9 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -599,6 +599,7 @@ static int	segment_size;
 static int	shared_memory_size_mb;
 static int	shared_memory_size_in_huge_pages;
 static int	wal_block_size;
+static int	num_os_semaphores;
 static bool data_checksums;
 static bool integer_datetimes;
 
@@ -2291,6 +2292,17 @@ struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"num_os_semaphores", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Shows the number of semaphores required for the server."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
+		},
+		&num_os_semaphores,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"commit_timestamp_buffers", PGC_POSTMASTER, RESOURCES_MEM,
 			gettext_noop("Sets the size of the dedicated buffer pool used for the commit timestamp cache."),
-- 
2.39.3 (Apple Git-146)

From b65ae84fc36c440451c42ef61b383bcd98a825bd Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Wed, 19 Jun 2024 13:39:18 -0500
Subject: [PATCH v5 2/3] remove check hooks for GUCs that contribute to
 MaxBackends

---
 src/backend/utils/init/postinit.c   | 57 ++++-------------------------
 src/backend/utils/misc/guc_tables.c |  8 ++--
 src/include/utils/guc_hooks.h       |  6 ---
 3 files changed, 11 insertions(+), 60 deletions(-)

diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 0805398e24..8a629982c4 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -580,57 +580,14 @@ InitializeMaxBackends(void)
 	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
 		max_worker_processes + max_wal_senders;
 
-	/* internal error because the values were all checked previously */
 	if (MaxBackends > MAX_BACKENDS)
-		elog(ERROR, "too many backends configured");
-}
-
-/*
- * GUC check_hook for max_connections
- */
-bool
-check_max_connections(int *newval, void **extra, GucSource source)
-{
-	if (*newval + autovacuum_max_workers + 1 +
-		max_worker_processes + max_wal_senders > MAX_BACKENDS)
-		return false;
-	return true;
-}
-
-/*
- * GUC check_hook for autovacuum_max_workers
- */
-bool
-check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
-{
-	if (MaxConnections + *newval + 1 +
-		max_worker_processes + max_wal_senders > MAX_BACKENDS)
-		return false;
-	return true;
-}
-
-/*
- * GUC check_hook for max_worker_processes
- */
-bool
-check_max_worker_processes(int *newval, void **extra, GucSource source)
-{
-	if (MaxConnections + autovacuum_max_workers + 1 +
-		*newval + max_wal_senders > MAX_BACKENDS)
-		return false;
-	return true;
-}
-
-/*
- * GUC check_hook for max_wal_senders
- */
-bool
-check_max_wal_senders(int *newval, void **extra, GucSource source)
-{
-	if (MaxConnections + autovacuum_max_workers + 1 +
-		max_worker_processes + *newval > MAX_BACKENDS)
-		return false;
-	return true;
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("too many backends configured"),
+				 errdetail("\"max_connections\" (%d) plus \"autovacuum_max_workers\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
+						   MaxConnections, autovacuum_max_workers,
+						   max_worker_processes, max_wal_senders,
+						   MAX_BACKENDS - 1)));
 }
 
 /*
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 80e77cbac9..57df7767ad 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -2208,7 +2208,7 @@ struct config_int ConfigureNamesInt[] =
 		},
 		&MaxConnections,
 		100, 1, MAX_BACKENDS,
-		check_max_connections, NULL, NULL
+		NULL, NULL, NULL
 	},
 
 	{
@@ -2935,7 +2935,7 @@ struct config_int ConfigureNamesInt[] =
 		},
 		&max_wal_senders,
 		10, 0, MAX_BACKENDS,
-		check_max_wal_senders, NULL, NULL
+		NULL, NULL, NULL
 	},
 
 	{
@@ -3165,7 +3165,7 @@ struct config_int ConfigureNamesInt[] =
 		},
 		&max_worker_processes,
 		8, 0, MAX_BACKENDS,
-		check_max_worker_processes, NULL, NULL
+		NULL, NULL, NULL
 	},
 
 	{
@@ -3399,7 +3399,7 @@ struct config_int ConfigureNamesInt[] =
 		},
 		&autovacuum_max_workers,
 		3, 1, MAX_BACKENDS,
-		check_autovacuum_max_workers, NULL, NULL
+		NULL, NULL, NULL
 	},
 
 	{
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index d64dc5fcdb..6304f0679b 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -29,8 +29,6 @@ extern bool check_application_name(char **newval, void **extra,
 								   GucSource source);
 extern void assign_application_name(const char *newval, void *extra);
 extern const char *show_archive_command(void);
-extern bool check_autovacuum_max_workers(int *newval, void **extra,
-										 GucSource source);
 extern bool check_autovacuum_work_mem(int *newval, void **extra,
 									  GucSource source);
 extern bool check_vacuum_buffer_usage_limit(int *newval, void **extra,
@@ -84,13 +82,9 @@ extern const char *show_log_timezone(void);
 extern bool check_maintenance_io_concurrency(int *newval, void **extra,
 											 GucSource source);
 extern void assign_maintenance_io_concurrency(int newval, void *extra);
-extern bool check_max_connections(int *newval, void **extra, GucSource source);
-extern bool check_max_wal_senders(int *newval, void **extra, GucSource source);
 extern bool check_max_slot_wal_keep_size(int *newval, void **extra,
 										 GucSource source);
 extern void assign_max_wal_size(int newval, void *extra);
-extern bool check_max_worker_processes(int *newval, void **extra,
-									   GucSource source);
 extern bool check_max_stack_depth(int *newval, void **extra, GucSource source);
 extern void assign_max_stack_depth(int newval, void *extra);
 extern bool check_multixact_member_buffers(int *newval, void **extra,
-- 
2.39.3 (Apple Git-146)

From 48f7d56b99f2d98533c8767d5891a78c8411872d Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Fri, 21 Jun 2024 15:29:28 -0500
Subject: [PATCH v5 3/3] allow changing autovacuum_max_workers without
 restarting

---
 doc/src/sgml/config.sgml                      |  8 +++-
 src/backend/postmaster/autovacuum.c           | 25 ++++++++++---
 src/backend/postmaster/postmaster.c           |  2 +-
 src/backend/storage/lmgr/proc.c               |  4 +-
 src/backend/utils/init/postinit.c             | 37 +++++++++++++++++--
 src/backend/utils/misc/guc_tables.c           |  4 +-
 src/backend/utils/misc/postgresql.conf.sample |  1 -
 src/include/postmaster/autovacuum.h           |  1 +
 src/include/utils/guc_hooks.h                 |  2 +
 9 files changed, 69 insertions(+), 15 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 087385cb4e..289c2b63c4 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8554,7 +8554,13 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        This parameter is limited to 50% of
+        <xref linkend="guc-max-connections"/> (but no less than 1 or greater
+        than 64).
        </para>
       </listitem>
      </varlistentry>
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 9a925a10cd..8bf95981a0 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -130,6 +130,7 @@ double		autovacuum_vac_cost_delay;
 int			autovacuum_vac_cost_limit;
 
 int			Log_autovacuum_min_duration = 600000;
+int			autovacuum_worker_slots = -1;
 
 /* the minimum allowed time between two awakenings of the launcher */
 #define MIN_AUTOVAC_SLEEPTIME 100.0 /* milliseconds */
@@ -347,6 +348,7 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
 
 
 
@@ -575,7 +577,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
+		launcher_determine_sleep(av_worker_available(),
 								 false, &nap);
 
 		/*
@@ -636,7 +638,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -1087,7 +1089,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -3265,7 +3267,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3302,7 +3304,7 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
 			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
 							&worker[i].wi_links);
@@ -3341,3 +3343,16 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+static bool
+av_worker_available(void)
+{
+	int			reserved = autovacuum_worker_slots - autovacuum_max_workers;
+	int			count = 0;
+	dlist_iter	iter;
+
+	dlist_foreach(iter, &AutoVacuumShmem->av_freeWorkers)
+		count++;
+
+	return count > reserved;
+}
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index bf0241aed0..2588a68fbc 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + autovacuum_worker_slots + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index ce29da9012..10e0b681cf 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -242,13 +242,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 8a629982c4..d206dd1533 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -576,20 +576,51 @@ InitializeMaxBackends(void)
 {
 	Assert(MaxBackends == 0);
 
+	autovacuum_worker_slots = Max(1, Min(64, MaxConnections / 2));
+	if (autovacuum_max_workers > autovacuum_worker_slots)
+	{
+		/* keep in sync with check_autovacuum_max_workers() */
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid setting for \"autovacuum_max_workers\""),
+				 errdetail("\"autovacuum_max_workers\" (%d) is limited to 50%% of \"max_connections\" (but no less than 1 or greater than 64) (%d)",
+						   autovacuum_max_workers, autovacuum_worker_slots)));
+	}
+
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	if (MaxBackends > MAX_BACKENDS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("too many backends configured"),
-				 errdetail("\"max_connections\" (%d) plus \"autovacuum_max_workers\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
-						   MaxConnections, autovacuum_max_workers,
+				 errdetail("\"max_connections\" (%d) plus autovacuum worker slots (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
+						   MaxConnections, autovacuum_worker_slots,
 						   max_worker_processes, max_wal_senders,
 						   MAX_BACKENDS - 1)));
 }
 
+/*
+ * GUC check_hook for autovacuum_max_workers
+ */
+bool
+check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
+{
+	/*
+	 * If autovacuum_worker_slots has not yet been initialized, just return
+	 * true.  We'll fail startup in PostmasterMain() if needed.
+	 */
+	if (autovacuum_worker_slots == -1 || *newval <= autovacuum_worker_slots)
+		return true;
+
+	/* keep in sync with matching error message in InitializeMaxBackends() */
+	GUC_check_errmsg("invalid setting for \"autovacuum_max_workers\"");
+	GUC_check_errdetail("\"autovacuum_max_workers\" (%d) is limited to 50%% of \"max_connections\" (but no less than 1 or greater than 64) (%d)",
+						autovacuum_max_workers, autovacuum_worker_slots);
+	return false;
+}
+
 /*
  * Early initialization of a backend (either standalone or under postmaster).
  * This happens even before InitPostgres.
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 57df7767ad..f1babff58e 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3393,13 +3393,13 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
 		&autovacuum_max_workers,
 		3, 1, MAX_BACKENDS,
-		NULL, NULL, NULL
+		check_autovacuum_max_workers, NULL, NULL
 	},
 
 	{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index e0567de219..fec719fe79 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -659,7 +659,6 @@
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
 #autovacuum_max_workers = 3		# max number of autovacuum subprocesses
-					# (change requires restart)
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..2b2aa1b713 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -46,6 +46,7 @@ extern PGDLLIMPORT int autovacuum_vac_cost_limit;
 extern PGDLLIMPORT int AutovacuumLauncherPid;
 
 extern PGDLLIMPORT int Log_autovacuum_min_duration;
+extern PGDLLIMPORT int autovacuum_worker_slots;
 
 /* Status inquiry functions */
 extern bool AutoVacuumingActive(void);
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index 6304f0679b..b3e1f9f2cb 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -29,6 +29,8 @@ extern bool check_application_name(char **newval, void **extra,
 								   GucSource source);
 extern void assign_application_name(const char *newval, void *extra);
 extern const char *show_archive_command(void);
+extern bool check_autovacuum_max_workers(int *newval, void **extra,
+										 GucSource source);
 extern bool check_autovacuum_work_mem(int *newval, void **extra,
 									  GucSource source);
 extern bool check_vacuum_buffer_usage_limit(int *newval, void **extra,
-- 
2.39.3 (Apple Git-146)

From c1c33c6c157a7cec81180714369b2978b09e402f Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 22 Jun 2024 15:05:44 -0500
Subject: [PATCH v6 1/1] allow changing autovacuum_max_workers without
 restarting

---
 doc/src/sgml/config.sgml                      | 28 +++++++++++-
 doc/src/sgml/runtime.sgml                     | 12 ++---
 src/backend/access/transam/xlog.c             |  2 +-
 src/backend/postmaster/autovacuum.c           | 44 ++++++++++++-------
 src/backend/postmaster/postmaster.c           |  2 +-
 src/backend/storage/lmgr/proc.c               |  6 +--
 src/backend/utils/init/postinit.c             | 12 ++---
 src/backend/utils/misc/guc_tables.c           | 13 +++++-
 src/backend/utils/misc/postgresql.conf.sample |  3 +-
 src/include/postmaster/autovacuum.h           |  1 +
 src/include/utils/guc_hooks.h                 |  4 +-
 src/test/perl/PostgreSQL/Test/Cluster.pm      |  1 +
 12 files changed, 89 insertions(+), 39 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 0c7a9082c5..64411918a4 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8544,6 +8544,25 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-autovacuum-worker-slots" xreflabel="autovacuum_worker_slots">
+      <term><varname>autovacuum_worker_slots</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_worker_slots</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 16.  This parameter can only be set at server
+        start.
+       </para>
+       <para>
+        When changing this value, consider also adjusting
+        <xref linkend="guc-autovacuum-max-workers"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
       <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
       <indexterm>
@@ -8554,7 +8573,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        Note that a setting for this value which is higher than
+        <xref linkend="guc-autovacuum-worker-slots"/> will have no effect,
+        since autovacuum workers are taken from the pool of slots established
+        by that setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 2f7c618886..4bb37faffe 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -838,7 +838,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), and allowed background
     process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
     Each such set will
@@ -847,13 +847,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
     as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
+    <varname>autovacuum_worker_slots</varname> plus <varname>max_wal_senders</varname>,
     plus <varname>max_worker_processes</varname>, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
+    least <literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
@@ -884,7 +884,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), and allowed background
     process (<xref linkend="guc-max-worker-processes"/>).
     On the platforms where this option is preferred, there is no specific
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 330e058c5f..fec72086b9 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5362,7 +5362,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 9a925a10cd..5f413305a2 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -114,6 +114,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
+int			autovacuum_worker_slots;
 int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
@@ -209,7 +210,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -289,7 +290,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -347,6 +348,7 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
 
 
 
@@ -575,8 +577,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -636,7 +637,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -679,8 +680,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -1087,7 +1088,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1240,7 +1241,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1609,8 +1610,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3265,7 +3266,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3292,7 +3293,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3302,10 +3303,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3341,3 +3342,14 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			reserved = autovacuum_worker_slots - autovacuum_max_workers;
+
+	return dclist_count(&AutoVacuumShmem->av_freeWorkers) > Max(0, reserved);
+}
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index bf0241aed0..2588a68fbc 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + autovacuum_worker_slots + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index ce29da9012..80dbd793b5 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -142,7 +142,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -242,13 +242,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 0805398e24..8fb7753fe0 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -577,7 +577,7 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	/* internal error because the values were all checked previously */
@@ -591,17 +591,17 @@ InitializeMaxBackends(void)
 bool
 check_max_connections(int *newval, void **extra, GucSource source)
 {
-	if (*newval + autovacuum_max_workers + 1 +
+	if (*newval + autovacuum_worker_slots + 1 +
 		max_worker_processes + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
 }
 
 /*
- * GUC check_hook for autovacuum_max_workers
+ * GUC check_hook for autovacuum_worker_slots
  */
 bool
-check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
+check_autovacuum_worker_slots(int *newval, void **extra, GucSource source)
 {
 	if (MaxConnections + *newval + 1 +
 		max_worker_processes + max_wal_senders > MAX_BACKENDS)
@@ -615,7 +615,7 @@ check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
 bool
 check_max_worker_processes(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + autovacuum_worker_slots + 1 +
 		*newval + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
@@ -627,7 +627,7 @@ check_max_worker_processes(int *newval, void **extra, GucSource source)
 bool
 check_max_wal_senders(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 +
+	if (MaxConnections + autovacuum_worker_slots + 1 +
 		max_worker_processes + *newval > MAX_BACKENDS)
 		return false;
 	return true;
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 46c258be28..d9fa3bc8db 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3381,13 +3381,22 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			NULL
+		},
+		&autovacuum_worker_slots,
+		16, 1, MAX_BACKENDS,
+		check_autovacuum_worker_slots, NULL, NULL
+	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
 		&autovacuum_max_workers,
 		3, 1, MAX_BACKENDS,
-		check_autovacuum_max_workers, NULL, NULL
+		NULL, NULL, NULL
 	},
 
 	{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index e0567de219..4084e80cb9 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -658,8 +658,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..190baa699d 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,6 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
+extern PGDLLIMPORT int autovacuum_worker_slots;
 extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index d64dc5fcdb..aa5da2c286 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -29,8 +29,8 @@ extern bool check_application_name(char **newval, void **extra,
 								   GucSource source);
 extern void assign_application_name(const char *newval, void *extra);
 extern const char *show_archive_command(void);
-extern bool check_autovacuum_max_workers(int *newval, void **extra,
-										 GucSource source);
+extern bool check_autovacuum_worker_slots(int *newval, void **extra,
+										  GucSource source);
 extern bool check_autovacuum_work_mem(int *newval, void **extra,
 									  GucSource source);
 extern bool check_vacuum_buffer_usage_limit(int *newval, void **extra,
diff --git a/src/test/perl/PostgreSQL/Test/Cluster.pm b/src/test/perl/PostgreSQL/Test/Cluster.pm
index 83f385a487..9428e9cd1a 100644
--- a/src/test/perl/PostgreSQL/Test/Cluster.pm
+++ b/src/test/perl/PostgreSQL/Test/Cluster.pm
@@ -608,6 +608,7 @@ sub init
 		}
 		print $conf "max_wal_senders = 10\n";
 		print $conf "max_replication_slots = 10\n";
+		print $conf "autovacuum_worker_slots = 3\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters:
-- 
2.39.3 (Apple Git-146)

From bd486d1ab302c4654b9cfbc57230bcf9b140711e Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 22 Jun 2024 15:05:44 -0500
Subject: [PATCH v7 1/1] allow changing autovacuum_max_workers without
 restarting

---
 doc/src/sgml/config.sgml                      | 28 +++++++++++-
 doc/src/sgml/runtime.sgml                     | 12 ++---
 src/backend/access/transam/xlog.c             |  2 +-
 src/backend/postmaster/autovacuum.c           | 44 ++++++++++++-------
 src/backend/postmaster/postmaster.c           |  2 +-
 src/backend/storage/lmgr/proc.c               |  6 +--
 src/backend/utils/init/postinit.c             |  6 +--
 src/backend/utils/misc/guc_tables.c           | 11 ++++-
 src/backend/utils/misc/postgresql.conf.sample |  3 +-
 src/include/postmaster/autovacuum.h           |  1 +
 src/test/perl/PostgreSQL/Test/Cluster.pm      |  1 +
 11 files changed, 83 insertions(+), 33 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index f627a3e63c..da30d1ea4f 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8544,6 +8544,25 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-autovacuum-worker-slots" xreflabel="autovacuum_worker_slots">
+      <term><varname>autovacuum_worker_slots</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_worker_slots</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 16.  This parameter can only be set at server
+        start.
+       </para>
+       <para>
+        When changing this value, consider also adjusting
+        <xref linkend="guc-autovacuum-max-workers"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
       <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
       <indexterm>
@@ -8554,7 +8573,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        Note that a setting for this value which is higher than
+        <xref linkend="guc-autovacuum-worker-slots"/> will have no effect,
+        since autovacuum workers are taken from the pool of slots established
+        by that setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 2f7c618886..4bb37faffe 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -838,7 +838,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), and allowed background
     process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
     Each such set will
@@ -847,13 +847,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
     as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
+    <varname>autovacuum_worker_slots</varname> plus <varname>max_wal_senders</varname>,
     plus <varname>max_worker_processes</varname>, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
+    least <literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
@@ -884,7 +884,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), and allowed background
     process (<xref linkend="guc-max-worker-processes"/>).
     On the platforms where this option is preferred, there is no specific
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 33e27a6e72..816f9f2b4b 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5362,7 +5362,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 928754b51c..565e14ca9b 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -114,6 +114,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
+int			autovacuum_worker_slots;
 int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
@@ -209,7 +210,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -289,7 +290,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -347,6 +348,7 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
 
 
 
@@ -575,8 +577,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -636,7 +637,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -679,8 +680,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -1087,7 +1088,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1240,7 +1241,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1609,8 +1610,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3265,7 +3266,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3292,7 +3293,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3302,10 +3303,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3341,3 +3342,14 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			reserved = autovacuum_worker_slots - autovacuum_max_workers;
+
+	return dclist_count(&AutoVacuumShmem->av_freeWorkers) > Max(0, reserved);
+}
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 6f974a8d21..e4c824fcb1 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + autovacuum_worker_slots + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1b23efb26f..c20c9338ec 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -142,7 +142,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -242,13 +242,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 25867c8bd5..acbae29baf 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -577,15 +577,15 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	if (MaxBackends > MAX_BACKENDS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("too many server processes configured"),
-				 errdetail("\"max_connections\" (%d) plus \"autovacuum_max_workers\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
-						   MaxConnections, autovacuum_max_workers,
+				 errdetail("\"max_connections\" (%d) plus \"autovacuum_worker_slots\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
+						   MaxConnections, autovacuum_worker_slots,
 						   max_worker_processes, max_wal_senders,
 						   MAX_BACKENDS)));
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 630ed0f162..6ffca198e9 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3383,7 +3383,16 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			NULL
+		},
+		&autovacuum_worker_slots,
+		16, 1, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 9ec9f97e92..52a4d44b59 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -658,8 +658,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..190baa699d 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,6 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
+extern PGDLLIMPORT int autovacuum_worker_slots;
 extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
diff --git a/src/test/perl/PostgreSQL/Test/Cluster.pm b/src/test/perl/PostgreSQL/Test/Cluster.pm
index 0135c5a795..98a5039709 100644
--- a/src/test/perl/PostgreSQL/Test/Cluster.pm
+++ b/src/test/perl/PostgreSQL/Test/Cluster.pm
@@ -608,6 +608,7 @@ sub init
 		}
 		print $conf "max_wal_senders = 10\n";
 		print $conf "max_replication_slots = 10\n";
+		print $conf "autovacuum_worker_slots = 3\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters:
-- 
2.39.3 (Apple Git-146)

From e59c8199858b0331c2d9ec7a40d26f0e89657bf4 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 22 Jun 2024 15:05:44 -0500
Subject: [PATCH v8 1/1] allow changing autovacuum_max_workers without
 restarting

---
 doc/src/sgml/config.sgml                      | 28 ++++++-
 doc/src/sgml/runtime.sgml                     | 12 +--
 src/backend/access/transam/xlog.c             |  2 +-
 src/backend/postmaster/autovacuum.c           | 76 +++++++++++++++----
 src/backend/postmaster/postmaster.c           |  2 +-
 src/backend/storage/lmgr/proc.c               |  6 +-
 src/backend/utils/init/postinit.c             |  6 +-
 src/backend/utils/misc/guc_tables.c           | 11 ++-
 src/backend/utils/misc/postgresql.conf.sample |  3 +-
 src/include/postmaster/autovacuum.h           |  1 +
 src/test/perl/PostgreSQL/Test/Cluster.pm      |  1 +
 11 files changed, 115 insertions(+), 33 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 3dec0b7cfe..d5101dd76c 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8552,6 +8552,25 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-autovacuum-worker-slots" xreflabel="autovacuum_worker_slots">
+      <term><varname>autovacuum_worker_slots</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_worker_slots</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 16.  This parameter can only be set at server
+        start.
+       </para>
+       <para>
+        When changing this value, consider also adjusting
+        <xref linkend="guc-autovacuum-max-workers"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
       <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
       <indexterm>
@@ -8562,7 +8581,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        Note that a setting for this value which is higher than
+        <xref linkend="guc-autovacuum-worker-slots"/> will have no effect,
+        since autovacuum workers are taken from the pool of slots established
+        by that setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 2f7c618886..4bb37faffe 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -838,7 +838,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), and allowed background
     process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
     Each such set will
@@ -847,13 +847,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
     as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
+    <varname>autovacuum_worker_slots</varname> plus <varname>max_wal_senders</varname>,
     plus <varname>max_worker_processes</varname>, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
+    least <literal>ceil((max_connections + autovacuum_worker_slots + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
@@ -884,7 +884,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), and allowed background
     process (<xref linkend="guc-max-worker-processes"/>).
     On the platforms where this option is preferred, there is no specific
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 636be5ca4d..67cabd214d 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5362,7 +5362,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 4e4a0ccbef..937cd940aa 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -115,6 +115,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
+int			autovacuum_worker_slots;
 int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
@@ -210,7 +211,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -290,7 +291,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -348,6 +349,8 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
+static void CheckAutovacuumWorkerGUCs(void);
 
 
 
@@ -424,6 +427,12 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 										  ALLOCSET_DEFAULT_SIZES);
 	MemoryContextSwitchTo(AutovacMemCxt);
 
+	/*
+	 * Emit a WARNING if autovacuum_worker_slots < autovacuum_max_workers.  We
+	 * do this on startup and on subsequent configuration reloads as needed.
+	 */
+	CheckAutovacuumWorkerGUCs();
+
 	/*
 	 * If an exception is encountered, processing resumes here.
 	 *
@@ -576,8 +585,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -637,7 +645,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -680,8 +688,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -746,6 +754,8 @@ HandleAutoVacLauncherInterrupts(void)
 
 	if (ConfigReloadPending)
 	{
+		int			autovacuum_max_workers_prev = autovacuum_max_workers;
+
 		ConfigReloadPending = false;
 		ProcessConfigFile(PGC_SIGHUP);
 
@@ -753,6 +763,14 @@ HandleAutoVacLauncherInterrupts(void)
 		if (!AutoVacuumingActive())
 			AutoVacLauncherShutdown();
 
+		/*
+		 * If autovacuum_max_workers changed, emit a WARNING if
+		 * autovacuum_worker_slots < autovacuum_max_workers.  If it didn't
+		 * change, skip this to avoid too many repeated log messages.
+		 */
+		if (autovacuum_max_workers_prev != autovacuum_max_workers)
+			CheckAutovacuumWorkerGUCs();
+
 		/* rebuild the list in case the naptime changed */
 		rebuild_database_list(InvalidOid);
 	}
@@ -1088,7 +1106,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1241,7 +1259,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1610,8 +1628,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3272,7 +3290,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3299,7 +3317,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3309,10 +3327,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3348,3 +3366,29 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			reserved = autovacuum_worker_slots - autovacuum_max_workers;
+
+	return dclist_count(&AutoVacuumShmem->av_freeWorkers) > Max(0, reserved);
+}
+
+/*
+ * Emit a WARNING if autovacuum_worker_slots < autovacuum_max_workers.
+ */
+static void
+CheckAutovacuumWorkerGUCs(void)
+{
+	if (autovacuum_worker_slots < autovacuum_max_workers)
+		ereport(WARNING,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("\"autovacuum_max_workers\" (%d) should be less than or equal to \"autovacuum_worker_slots\" (%d)",
+						autovacuum_max_workers, autovacuum_worker_slots),
+				 errdetail("The server will continue running but will only start up to \"autovacuum_worker_slots\" (%d) autovacuum workers at a given time.",
+						   autovacuum_worker_slots)));
+}
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 6f974a8d21..e4c824fcb1 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + autovacuum_worker_slots + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1b23efb26f..c20c9338ec 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -142,7 +142,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -242,13 +242,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 25867c8bd5..acbae29baf 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -577,15 +577,15 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	if (MaxBackends > MAX_BACKENDS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("too many server processes configured"),
-				 errdetail("\"max_connections\" (%d) plus \"autovacuum_max_workers\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
-						   MaxConnections, autovacuum_max_workers,
+				 errdetail("\"max_connections\" (%d) plus \"autovacuum_worker_slots\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
+						   MaxConnections, autovacuum_worker_slots,
 						   max_worker_processes, max_wal_senders,
 						   MAX_BACKENDS)));
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 630ed0f162..6ffca198e9 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3383,7 +3383,16 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			NULL
+		},
+		&autovacuum_worker_slots,
+		16, 1, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 9ec9f97e92..52a4d44b59 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -658,8 +658,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..190baa699d 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,6 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
+extern PGDLLIMPORT int autovacuum_worker_slots;
 extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
diff --git a/src/test/perl/PostgreSQL/Test/Cluster.pm b/src/test/perl/PostgreSQL/Test/Cluster.pm
index 32ee98aebc..a5047038f3 100644
--- a/src/test/perl/PostgreSQL/Test/Cluster.pm
+++ b/src/test/perl/PostgreSQL/Test/Cluster.pm
@@ -617,6 +617,7 @@ sub init
 		}
 		print $conf "max_wal_senders = 10\n";
 		print $conf "max_replication_slots = 10\n";
+		print $conf "autovacuum_worker_slots = 3\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters:
-- 
2.39.3 (Apple Git-146)

From 61513f744012c2b9b59085ce8c4a960da9e56ee7 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 22 Jun 2024 15:05:44 -0500
Subject: [PATCH v9 1/1] allow changing autovacuum_max_workers without
 restarting

---
 doc/src/sgml/config.sgml                      | 28 ++++++-
 doc/src/sgml/runtime.sgml                     |  4 +-
 src/backend/access/transam/xlog.c             |  2 +-
 src/backend/postmaster/autovacuum.c           | 76 +++++++++++++++----
 src/backend/postmaster/postmaster.c           |  2 +-
 src/backend/storage/lmgr/proc.c               |  6 +-
 src/backend/utils/init/postinit.c             |  6 +-
 src/backend/utils/misc/guc_tables.c           | 11 ++-
 src/backend/utils/misc/postgresql.conf.sample |  3 +-
 src/include/postmaster/autovacuum.h           |  1 +
 src/test/perl/PostgreSQL/Test/Cluster.pm      |  1 +
 11 files changed, 111 insertions(+), 29 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 57cd7bb972..7fc270c0ae 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8552,6 +8552,25 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-autovacuum-worker-slots" xreflabel="autovacuum_worker_slots">
+      <term><varname>autovacuum_worker_slots</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_worker_slots</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 16.  This parameter can only be set at server
+        start.
+       </para>
+       <para>
+        When changing this value, consider also adjusting
+        <xref linkend="guc-autovacuum-max-workers"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
       <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
       <indexterm>
@@ -8562,7 +8581,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        Note that a setting for this value which is higher than
+        <xref linkend="guc-autovacuum-worker-slots"/> will have no effect,
+        since autovacuum workers are taken from the pool of slots established
+        by that setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 2c4d5ef640..a1f43556ac 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -838,7 +838,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), allowed background
     process (<xref linkend="guc-max-worker-processes"/>), etc., in sets of 16.
     The runtime-computed parameter <xref linkend="guc-num-os-semaphores"/>
@@ -891,7 +891,7 @@ $ <userinput>postgres -D $PGDATA -C num_os_semaphores</userinput>
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), allowed background
     process (<xref linkend="guc-max-worker-processes"/>), etc.
     On the platforms where this option is preferred, there is no specific
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index f86f4b5c4b..64c1edb798 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5373,7 +5373,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 4e4a0ccbef..937cd940aa 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -115,6 +115,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
+int			autovacuum_worker_slots;
 int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
@@ -210,7 +211,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -290,7 +291,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -348,6 +349,8 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
+static void CheckAutovacuumWorkerGUCs(void);
 
 
 
@@ -424,6 +427,12 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 										  ALLOCSET_DEFAULT_SIZES);
 	MemoryContextSwitchTo(AutovacMemCxt);
 
+	/*
+	 * Emit a WARNING if autovacuum_worker_slots < autovacuum_max_workers.  We
+	 * do this on startup and on subsequent configuration reloads as needed.
+	 */
+	CheckAutovacuumWorkerGUCs();
+
 	/*
 	 * If an exception is encountered, processing resumes here.
 	 *
@@ -576,8 +585,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -637,7 +645,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -680,8 +688,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -746,6 +754,8 @@ HandleAutoVacLauncherInterrupts(void)
 
 	if (ConfigReloadPending)
 	{
+		int			autovacuum_max_workers_prev = autovacuum_max_workers;
+
 		ConfigReloadPending = false;
 		ProcessConfigFile(PGC_SIGHUP);
 
@@ -753,6 +763,14 @@ HandleAutoVacLauncherInterrupts(void)
 		if (!AutoVacuumingActive())
 			AutoVacLauncherShutdown();
 
+		/*
+		 * If autovacuum_max_workers changed, emit a WARNING if
+		 * autovacuum_worker_slots < autovacuum_max_workers.  If it didn't
+		 * change, skip this to avoid too many repeated log messages.
+		 */
+		if (autovacuum_max_workers_prev != autovacuum_max_workers)
+			CheckAutovacuumWorkerGUCs();
+
 		/* rebuild the list in case the naptime changed */
 		rebuild_database_list(InvalidOid);
 	}
@@ -1088,7 +1106,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1241,7 +1259,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1610,8 +1628,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3272,7 +3290,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3299,7 +3317,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3309,10 +3327,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3348,3 +3366,29 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			reserved = autovacuum_worker_slots - autovacuum_max_workers;
+
+	return dclist_count(&AutoVacuumShmem->av_freeWorkers) > Max(0, reserved);
+}
+
+/*
+ * Emit a WARNING if autovacuum_worker_slots < autovacuum_max_workers.
+ */
+static void
+CheckAutovacuumWorkerGUCs(void)
+{
+	if (autovacuum_worker_slots < autovacuum_max_workers)
+		ereport(WARNING,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("\"autovacuum_max_workers\" (%d) should be less than or equal to \"autovacuum_worker_slots\" (%d)",
+						autovacuum_max_workers, autovacuum_worker_slots),
+				 errdetail("The server will continue running but will only start up to \"autovacuum_worker_slots\" (%d) autovacuum workers at a given time.",
+						   autovacuum_worker_slots)));
+}
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 6f974a8d21..e4c824fcb1 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -4144,7 +4144,7 @@ CreateOptsFile(int argc, char *argv[], char *fullprogname)
 int
 MaxLivePostmasterChildren(void)
 {
-	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
+	return 2 * (MaxConnections + autovacuum_worker_slots + 1 +
 				max_wal_senders + max_worker_processes);
 }
 
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1b23efb26f..c20c9338ec 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -142,7 +142,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -242,13 +242,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 25867c8bd5..acbae29baf 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -577,15 +577,15 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	if (MaxBackends > MAX_BACKENDS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("too many server processes configured"),
-				 errdetail("\"max_connections\" (%d) plus \"autovacuum_max_workers\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
-						   MaxConnections, autovacuum_max_workers,
+				 errdetail("\"max_connections\" (%d) plus \"autovacuum_worker_slots\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
+						   MaxConnections, autovacuum_worker_slots,
 						   max_worker_processes, max_wal_senders,
 						   MAX_BACKENDS)));
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 6a623f5f34..19f6384638 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3385,7 +3385,16 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			NULL
+		},
+		&autovacuum_worker_slots,
+		16, 1, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 9ec9f97e92..52a4d44b59 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -658,8 +658,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..190baa699d 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,6 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
+extern PGDLLIMPORT int autovacuum_worker_slots;
 extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
diff --git a/src/test/perl/PostgreSQL/Test/Cluster.pm b/src/test/perl/PostgreSQL/Test/Cluster.pm
index 32ee98aebc..a5047038f3 100644
--- a/src/test/perl/PostgreSQL/Test/Cluster.pm
+++ b/src/test/perl/PostgreSQL/Test/Cluster.pm
@@ -617,6 +617,7 @@ sub init
 		}
 		print $conf "max_wal_senders = 10\n";
 		print $conf "max_replication_slots = 10\n";
+		print $conf "autovacuum_worker_slots = 3\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters:
-- 
2.39.3 (Apple Git-146)

From e877271830e076338f999ee72b9d8148e469d5d2 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Sat, 22 Jun 2024 15:05:44 -0500
Subject: [PATCH v10 1/1] allow changing autovacuum_max_workers without
 restarting

---
 doc/src/sgml/config.sgml                      | 28 ++++++-
 doc/src/sgml/runtime.sgml                     |  4 +-
 src/backend/access/transam/xlog.c             |  2 +-
 src/backend/postmaster/autovacuum.c           | 76 +++++++++++++++----
 src/backend/postmaster/pmchild.c              |  4 +-
 src/backend/storage/lmgr/proc.c               |  6 +-
 src/backend/utils/init/postinit.c             |  6 +-
 src/backend/utils/misc/guc_tables.c           | 11 ++-
 src/backend/utils/misc/postgresql.conf.sample |  3 +-
 src/include/postmaster/autovacuum.h           |  1 +
 src/test/perl/PostgreSQL/Test/Cluster.pm      |  1 +
 11 files changed, 112 insertions(+), 30 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index a84e60c09b..7db171198a 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8590,6 +8590,25 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-autovacuum-worker-slots" xreflabel="autovacuum_worker_slots">
+      <term><varname>autovacuum_worker_slots</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_worker_slots</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 16.  This parameter can only be set at server
+        start.
+       </para>
+       <para>
+        When changing this value, consider also adjusting
+        <xref linkend="guc-autovacuum-max-workers"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
       <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
       <indexterm>
@@ -8600,7 +8619,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        Note that a setting for this value which is higher than
+        <xref linkend="guc-autovacuum-worker-slots"/> will have no effect,
+        since autovacuum workers are taken from the pool of slots established
+        by that setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index bcd81e2415..8b7ae27908 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -839,7 +839,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), allowed background
     process (<xref linkend="guc-max-worker-processes"/>), etc., in sets of 16.
     The runtime-computed parameter <xref linkend="guc-num-os-semaphores"/>
@@ -892,7 +892,7 @@ $ <userinput>postgres -D $PGDATA -C num_os_semaphores</userinput>
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), allowed background
     process (<xref linkend="guc-max-worker-processes"/>), etc.
     On the platforms where this option is preferred, there is no specific
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 6f58412bca..706f2127de 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5403,7 +5403,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index dc3cf87aba..963924cbc7 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -115,6 +115,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
+int			autovacuum_worker_slots;
 int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
@@ -210,7 +211,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -291,7 +292,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -349,6 +350,8 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
+static void CheckAutovacuumWorkerGUCs(void);
 
 
 
@@ -425,6 +428,12 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 										  ALLOCSET_DEFAULT_SIZES);
 	MemoryContextSwitchTo(AutovacMemCxt);
 
+	/*
+	 * Emit a WARNING if autovacuum_worker_slots < autovacuum_max_workers.  We
+	 * do this on startup and on subsequent configuration reloads as needed.
+	 */
+	CheckAutovacuumWorkerGUCs();
+
 	/*
 	 * If an exception is encountered, processing resumes here.
 	 *
@@ -577,8 +586,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -638,7 +646,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -681,8 +689,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -747,6 +755,8 @@ HandleAutoVacLauncherInterrupts(void)
 
 	if (ConfigReloadPending)
 	{
+		int			autovacuum_max_workers_prev = autovacuum_max_workers;
+
 		ConfigReloadPending = false;
 		ProcessConfigFile(PGC_SIGHUP);
 
@@ -754,6 +764,14 @@ HandleAutoVacLauncherInterrupts(void)
 		if (!AutoVacuumingActive())
 			AutoVacLauncherShutdown();
 
+		/*
+		 * If autovacuum_max_workers changed, emit a WARNING if
+		 * autovacuum_worker_slots < autovacuum_max_workers.  If it didn't
+		 * change, skip this to avoid too many repeated log messages.
+		 */
+		if (autovacuum_max_workers_prev != autovacuum_max_workers)
+			CheckAutovacuumWorkerGUCs();
+
 		/* rebuild the list in case the naptime changed */
 		rebuild_database_list(InvalidOid);
 	}
@@ -1089,7 +1107,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1242,7 +1260,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1611,8 +1629,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3273,7 +3291,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3300,7 +3318,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3310,10 +3328,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3349,3 +3367,29 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			reserved = autovacuum_worker_slots - autovacuum_max_workers;
+
+	return dclist_count(&AutoVacuumShmem->av_freeWorkers) > Max(0, reserved);
+}
+
+/*
+ * Emit a WARNING if autovacuum_worker_slots < autovacuum_max_workers.
+ */
+static void
+CheckAutovacuumWorkerGUCs(void)
+{
+	if (autovacuum_worker_slots < autovacuum_max_workers)
+		ereport(WARNING,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("\"autovacuum_max_workers\" (%d) should be less than or equal to \"autovacuum_worker_slots\" (%d)",
+						autovacuum_max_workers, autovacuum_worker_slots),
+				 errdetail("The server will only start up to \"autovacuum_worker_slots\" (%d) autovacuum workers at a given time.",
+						   autovacuum_worker_slots)));
+}
diff --git a/src/backend/postmaster/pmchild.c b/src/backend/postmaster/pmchild.c
index 381cf005a9..821c225aad 100644
--- a/src/backend/postmaster/pmchild.c
+++ b/src/backend/postmaster/pmchild.c
@@ -8,7 +8,7 @@
  * child process is allocated a PMChild struct from a fixed pool of structs.
  * The size of the pool is determined by various settings that configure how
  * many worker processes and backend connections are allowed, i.e.
- * autovacuum_max_workers, max_worker_processes, max_wal_senders, and
+ * autovacuum_worker_slots, max_worker_processes, max_wal_senders, and
  * max_connections.
  *
  * Dead-end backends are handled slightly differently.  There is no limit
@@ -99,7 +99,7 @@ InitPostmasterChildSlots(void)
 	 */
 	pmchild_pools[B_BACKEND].size = 2 * (MaxConnections + max_wal_senders);
 
-	pmchild_pools[B_AUTOVAC_WORKER].size = autovacuum_max_workers;
+	pmchild_pools[B_AUTOVAC_WORKER].size = autovacuum_worker_slots;
 	pmchild_pools[B_BG_WORKER].size = max_worker_processes;
 
 	/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 720ef99ee8..b617db1a8c 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -150,7 +150,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -282,13 +282,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 5b657a3f13..729756b84c 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -545,15 +545,15 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	if (MaxBackends > MAX_BACKENDS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("too many server processes configured"),
-				 errdetail("\"max_connections\" (%d) plus \"autovacuum_max_workers\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
-						   MaxConnections, autovacuum_max_workers,
+				 errdetail("\"max_connections\" (%d) plus \"autovacuum_worker_slots\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
+						   MaxConnections, autovacuum_worker_slots,
 						   max_worker_processes, max_wal_senders,
 						   MAX_BACKENDS)));
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 8a67f01200..4b37c0d43c 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3448,7 +3448,16 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			NULL
+		},
+		&autovacuum_worker_slots,
+		16, 1, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 39a3ac2312..6225a24102 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -659,8 +659,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b329..190baa699d 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,6 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
+extern PGDLLIMPORT int autovacuum_worker_slots;
 extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
diff --git a/src/test/perl/PostgreSQL/Test/Cluster.pm b/src/test/perl/PostgreSQL/Test/Cluster.pm
index 508e5e3917..e827da5342 100644
--- a/src/test/perl/PostgreSQL/Test/Cluster.pm
+++ b/src/test/perl/PostgreSQL/Test/Cluster.pm
@@ -707,6 +707,7 @@ sub init
 		}
 		print $conf "max_wal_senders = 10\n";
 		print $conf "max_replication_slots = 10\n";
+		print $conf "autovacuum_worker_slots = 3\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters:
-- 
2.39.5 (Apple Git-154)

From dad43c4138c2cc0712006565bb4984c2a211e8fe Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Fri, 20 Dec 2024 13:33:26 -0600
Subject: [PATCH v11 1/1] Allow changing autovacuum_max_workers without
 restarting.

This commit introduces a new parameter named
autovacuum_worker_slots that controls how many autovacuum worker
slots to reserve during server startup.  Modifying this new
parameter's value does require a server restart, but it should
typically be set to the upper bound of what you might realistically
need to set autovacuum_max_workers.  With that new parameter in
place, autovacuum_max_workers can now be changed with a SIGHUP
(e.g., pg_ctl reload).

If autovacuum_max_workers is set higher than
autovacuum_worker_slots, a WARNING is emitted, and the server will
only start up to autovacuum_worker_slots workers at a given time.
If autovacuum_max_workers is set to a value less than the number of
currently-running autovacuum workers, the existing workers will
continue running, but no new workers will be started until the
number of running autovacuum workers drops below
autovacuum_max_workers.

Reviewed-by: Sami Imseih, Justin Pryzby, Robert Haas, Andres Freund, Yogesh Sharma
Discussion: https://postgr.es/m/20240410212344.GA1824549%40nathanxps13
---
 doc/src/sgml/config.sgml                      | 28 ++++++-
 doc/src/sgml/runtime.sgml                     |  4 +-
 src/backend/access/transam/xlog.c             |  2 +-
 src/backend/postmaster/autovacuum.c           | 82 +++++++++++++++----
 src/backend/postmaster/pmchild.c              |  4 +-
 src/backend/storage/lmgr/proc.c               |  6 +-
 src/backend/utils/init/postinit.c             |  6 +-
 src/backend/utils/misc/guc_tables.c           | 11 ++-
 src/backend/utils/misc/postgresql.conf.sample |  3 +-
 src/include/postmaster/autovacuum.h           |  1 +
 src/test/perl/PostgreSQL/Test/Cluster.pm      |  1 +
 11 files changed, 117 insertions(+), 31 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index fbdd6ce5740..740ff5d5044 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8630,6 +8630,25 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-autovacuum-worker-slots" xreflabel="autovacuum_worker_slots">
+      <term><varname>autovacuum_worker_slots</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_worker_slots</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 16.  This parameter can only be set at server
+        start.
+       </para>
+       <para>
+        When changing this value, consider also adjusting
+        <xref linkend="guc-autovacuum-max-workers"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
       <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
       <indexterm>
@@ -8640,7 +8659,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        Note that a setting for this value which is higher than
+        <xref linkend="guc-autovacuum-worker-slots"/> will have no effect,
+        since autovacuum workers are taken from the pool of slots established
+        by that setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 94135e9d5ee..537e356315d 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -839,7 +839,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), allowed background
     process (<xref linkend="guc-max-worker-processes"/>), etc., in sets of 16.
     The runtime-computed parameter <xref linkend="guc-num-os-semaphores"/>
@@ -892,7 +892,7 @@ $ <userinput>postgres -D $PGDATA -C num_os_semaphores</userinput>
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), allowed background
     process (<xref linkend="guc-max-worker-processes"/>), etc.
     On the platforms where this option is preferred, there is no specific
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 6f58412bcab..706f2127def 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5403,7 +5403,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index dc3cf87abab..0349e51042f 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -115,6 +115,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
+int			autovacuum_worker_slots;
 int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
@@ -210,7 +211,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -291,7 +292,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -349,6 +350,8 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
+static void check_av_worker_gucs(void);
 
 
 
@@ -577,8 +580,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -638,7 +640,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -681,8 +683,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -747,6 +749,8 @@ HandleAutoVacLauncherInterrupts(void)
 
 	if (ConfigReloadPending)
 	{
+		int			autovacuum_max_workers_prev = autovacuum_max_workers;
+
 		ConfigReloadPending = false;
 		ProcessConfigFile(PGC_SIGHUP);
 
@@ -754,6 +758,14 @@ HandleAutoVacLauncherInterrupts(void)
 		if (!AutoVacuumingActive())
 			AutoVacLauncherShutdown();
 
+		/*
+		 * If autovacuum_max_workers changed, emit a WARNING if
+		 * autovacuum_worker_slots < autovacuum_max_workers.  If it didn't
+		 * change, skip this to avoid too many repeated log messages.
+		 */
+		if (autovacuum_max_workers_prev != autovacuum_max_workers)
+			check_av_worker_gucs();
+
 		/* rebuild the list in case the naptime changed */
 		rebuild_database_list(InvalidOid);
 	}
@@ -1089,7 +1101,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1242,7 +1254,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1611,8 +1623,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3253,10 +3265,14 @@ AutoVacuumRequestWork(AutoVacuumWorkItemType type, Oid relationId,
 void
 autovac_init(void)
 {
-	if (autovacuum_start_daemon && !pgstat_track_counts)
+	if (!autovacuum_start_daemon)
+		return;
+	else if (!pgstat_track_counts)
 		ereport(WARNING,
 				(errmsg("autovacuum not started because of misconfiguration"),
 				 errhint("Enable the \"track_counts\" option.")));
+	else
+		check_av_worker_gucs();
 }
 
 /*
@@ -3273,7 +3289,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3300,7 +3316,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3310,10 +3326,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3349,3 +3365,35 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			free_slots;
+	int			reserved_slots;
+
+	free_slots = dclist_count(&AutoVacuumShmem->av_freeWorkers);
+
+	reserved_slots = autovacuum_worker_slots - autovacuum_max_workers;
+	reserved_slots = Max(0, reserved_slots);
+
+	return free_slots > reserved_slots;
+}
+
+/*
+ * Emits a WARNING if autovacuum_worker_slots < autovacuum_max_workers.
+ */
+static void
+check_av_worker_gucs(void)
+{
+	if (autovacuum_worker_slots < autovacuum_max_workers)
+		ereport(WARNING,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("\"autovacuum_max_workers\" (%d) should be less than or equal to \"autovacuum_worker_slots\" (%d)",
+						autovacuum_max_workers, autovacuum_worker_slots),
+				 errdetail("The server will only start up to \"autovacuum_worker_slots\" (%d) autovacuum workers at a given time.",
+						   autovacuum_worker_slots)));
+}
diff --git a/src/backend/postmaster/pmchild.c b/src/backend/postmaster/pmchild.c
index 381cf005a9b..821c225aad3 100644
--- a/src/backend/postmaster/pmchild.c
+++ b/src/backend/postmaster/pmchild.c
@@ -8,7 +8,7 @@
  * child process is allocated a PMChild struct from a fixed pool of structs.
  * The size of the pool is determined by various settings that configure how
  * many worker processes and backend connections are allowed, i.e.
- * autovacuum_max_workers, max_worker_processes, max_wal_senders, and
+ * autovacuum_worker_slots, max_worker_processes, max_wal_senders, and
  * max_connections.
  *
  * Dead-end backends are handled slightly differently.  There is no limit
@@ -99,7 +99,7 @@ InitPostmasterChildSlots(void)
 	 */
 	pmchild_pools[B_BACKEND].size = 2 * (MaxConnections + max_wal_senders);
 
-	pmchild_pools[B_AUTOVAC_WORKER].size = autovacuum_max_workers;
+	pmchild_pools[B_AUTOVAC_WORKER].size = autovacuum_worker_slots;
 	pmchild_pools[B_BG_WORKER].size = max_worker_processes;
 
 	/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 720ef99ee83..b617db1a8ce 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -150,7 +150,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -282,13 +282,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 5b657a3f135..729756b84c9 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -545,15 +545,15 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+	MaxBackends = MaxConnections + autovacuum_worker_slots + 1 +
 		max_worker_processes + max_wal_senders;
 
 	if (MaxBackends > MAX_BACKENDS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("too many server processes configured"),
-				 errdetail("\"max_connections\" (%d) plus \"autovacuum_max_workers\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
-						   MaxConnections, autovacuum_max_workers,
+				 errdetail("\"max_connections\" (%d) plus \"autovacuum_worker_slots\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
+						   MaxConnections, autovacuum_worker_slots,
 						   max_worker_processes, max_wal_senders,
 						   MAX_BACKENDS)));
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 8cf1afbad20..e2da6487e43 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3467,7 +3467,16 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			NULL
+		},
+		&autovacuum_worker_slots,
+		16, 1, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index a2ac7575ca7..b2bc43383db 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -661,8 +661,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index cae1e8b3294..190baa699da 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,6 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
+extern PGDLLIMPORT int autovacuum_worker_slots;
 extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
diff --git a/src/test/perl/PostgreSQL/Test/Cluster.pm b/src/test/perl/PostgreSQL/Test/Cluster.pm
index 508e5e3917a..e827da53422 100644
--- a/src/test/perl/PostgreSQL/Test/Cluster.pm
+++ b/src/test/perl/PostgreSQL/Test/Cluster.pm
@@ -707,6 +707,7 @@ sub init
 		}
 		print $conf "max_wal_senders = 10\n";
 		print $conf "max_replication_slots = 10\n";
+		print $conf "autovacuum_worker_slots = 3\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters:
-- 
2.39.5 (Apple Git-154)

From 46ea1dbcbb3f344c656db7fead89e3e9753dfadc Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Fri, 20 Dec 2024 13:33:26 -0600
Subject: [PATCH v12 1/1] Allow changing autovacuum_max_workers without
 restarting.

This commit introduces a new parameter named
autovacuum_worker_slots that controls how many autovacuum worker
slots to reserve during server startup.  Modifying this new
parameter's value does require a server restart, but it should
typically be set to the upper bound of what you might realistically
need to set autovacuum_max_workers.  With that new parameter in
place, autovacuum_max_workers can now be changed with a SIGHUP
(e.g., pg_ctl reload).

If autovacuum_max_workers is set higher than
autovacuum_worker_slots, a WARNING is emitted, and the server will
only start up to autovacuum_worker_slots workers at a given time.
If autovacuum_max_workers is set to a value less than the number of
currently-running autovacuum workers, the existing workers will
continue running, but no new workers will be started until the
number of running autovacuum workers drops below
autovacuum_max_workers.

Reviewed-by: Sami Imseih, Justin Pryzby, Robert Haas, Andres Freund, Yogesh Sharma
Discussion: https://postgr.es/m/20240410212344.GA1824549%40nathanxps13
---
 doc/src/sgml/config.sgml                      | 28 ++++++-
 doc/src/sgml/runtime.sgml                     |  4 +-
 src/backend/access/transam/xlog.c             |  2 +-
 src/backend/postmaster/autovacuum.c           | 82 +++++++++++++++----
 src/backend/postmaster/pmchild.c              |  4 +-
 src/backend/storage/lmgr/proc.c               |  6 +-
 src/backend/utils/init/postinit.c             |  6 +-
 src/backend/utils/misc/guc_tables.c           | 11 ++-
 src/backend/utils/misc/postgresql.conf.sample |  3 +-
 src/include/postmaster/autovacuum.h           |  1 +
 src/test/perl/PostgreSQL/Test/Cluster.pm      |  1 +
 11 files changed, 117 insertions(+), 31 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index fbdd6ce5740..740ff5d5044 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8630,6 +8630,25 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-autovacuum-worker-slots" xreflabel="autovacuum_worker_slots">
+      <term><varname>autovacuum_worker_slots</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>autovacuum_worker_slots</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the number of backend slots to reserve for autovacuum worker
+        processes.  The default is 16.  This parameter can only be set at server
+        start.
+       </para>
+       <para>
+        When changing this value, consider also adjusting
+        <xref linkend="guc-autovacuum-max-workers"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
       <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
       <indexterm>
@@ -8640,7 +8659,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum number of autovacuum processes (other than the
         autovacuum launcher) that may be running at any one time.  The default
-        is three.  This parameter can only be set at server start.
+        is three.  This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        Note that a setting for this value which is higher than
+        <xref linkend="guc-autovacuum-worker-slots"/> will have no effect,
+        since autovacuum workers are taken from the pool of slots established
+        by that setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 8750044852d..59f39e89924 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -839,7 +839,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using System V semaphores,
     <productname>PostgreSQL</productname> uses one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), allowed background
     process (<xref linkend="guc-max-worker-processes"/>), etc., in sets of 19.
     The runtime-computed parameter <xref linkend="guc-num-os-semaphores"/>
@@ -892,7 +892,7 @@ $ <userinput>postgres -D $PGDATA -C num_os_semaphores</userinput>
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-autovacuum-worker-slots"/>), allowed WAL sender process
     (<xref linkend="guc-max-wal-senders"/>), allowed background
     process (<xref linkend="guc-max-worker-processes"/>), etc.
     On the platforms where this option is preferred, there is no specific
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index b9ea92a5427..bf3dbda901d 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5403,7 +5403,7 @@ CheckRequiredParameterValues(void)
 	 */
 	if (ArchiveRecoveryRequested && EnableHotStandby)
 	{
-		/* We ignore autovacuum_max_workers when we make this test. */
+		/* We ignore autovacuum_worker_slots when we make this test. */
 		RecoveryRequiresIntParameter("max_connections",
 									 MaxConnections,
 									 ControlFile->MaxConnections);
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 3f826532b88..0ab921a169b 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -115,6 +115,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
+int			autovacuum_worker_slots;
 int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
@@ -210,7 +211,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -291,7 +292,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -349,6 +350,8 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
+static void check_av_worker_gucs(void);
 
 
 
@@ -577,8 +580,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -638,7 +640,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -681,8 +683,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -747,6 +749,8 @@ HandleAutoVacLauncherInterrupts(void)
 
 	if (ConfigReloadPending)
 	{
+		int			autovacuum_max_workers_prev = autovacuum_max_workers;
+
 		ConfigReloadPending = false;
 		ProcessConfigFile(PGC_SIGHUP);
 
@@ -754,6 +758,14 @@ HandleAutoVacLauncherInterrupts(void)
 		if (!AutoVacuumingActive())
 			AutoVacLauncherShutdown();
 
+		/*
+		 * If autovacuum_max_workers changed, emit a WARNING if
+		 * autovacuum_worker_slots < autovacuum_max_workers.  If it didn't
+		 * change, skip this to avoid too many repeated log messages.
+		 */
+		if (autovacuum_max_workers_prev != autovacuum_max_workers)
+			check_av_worker_gucs();
+
 		/* rebuild the list in case the naptime changed */
 		rebuild_database_list(InvalidOid);
 	}
@@ -1089,7 +1101,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1242,7 +1254,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1615,8 +1627,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3248,10 +3260,14 @@ AutoVacuumRequestWork(AutoVacuumWorkItemType type, Oid relationId,
 void
 autovac_init(void)
 {
-	if (autovacuum_start_daemon && !pgstat_track_counts)
+	if (!autovacuum_start_daemon)
+		return;
+	else if (!pgstat_track_counts)
 		ereport(WARNING,
 				(errmsg("autovacuum not started because of misconfiguration"),
 				 errhint("Enable the \"track_counts\" option.")));
+	else
+		check_av_worker_gucs();
 }
 
 /*
@@ -3268,7 +3284,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3295,7 +3311,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3305,10 +3321,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3344,3 +3360,35 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			free_slots;
+	int			reserved_slots;
+
+	free_slots = dclist_count(&AutoVacuumShmem->av_freeWorkers);
+
+	reserved_slots = autovacuum_worker_slots - autovacuum_max_workers;
+	reserved_slots = Max(0, reserved_slots);
+
+	return free_slots > reserved_slots;
+}
+
+/*
+ * Emits a WARNING if autovacuum_worker_slots < autovacuum_max_workers.
+ */
+static void
+check_av_worker_gucs(void)
+{
+	if (autovacuum_worker_slots < autovacuum_max_workers)
+		ereport(WARNING,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("\"autovacuum_max_workers\" (%d) should be less than or equal to \"autovacuum_worker_slots\" (%d)",
+						autovacuum_max_workers, autovacuum_worker_slots),
+				 errdetail("The server will only start up to \"autovacuum_worker_slots\" (%d) autovacuum workers at a given time.",
+						   autovacuum_worker_slots)));
+}
diff --git a/src/backend/postmaster/pmchild.c b/src/backend/postmaster/pmchild.c
index 0d53812406c..0d473226c3a 100644
--- a/src/backend/postmaster/pmchild.c
+++ b/src/backend/postmaster/pmchild.c
@@ -8,7 +8,7 @@
  * child process is allocated a PMChild struct from a fixed pool of structs.
  * The size of the pool is determined by various settings that configure how
  * many worker processes and backend connections are allowed, i.e.
- * autovacuum_max_workers, max_worker_processes, max_wal_senders, and
+ * autovacuum_worker_slots, max_worker_processes, max_wal_senders, and
  * max_connections.
  *
  * Dead-end backends are handled slightly differently.  There is no limit
@@ -99,7 +99,7 @@ InitPostmasterChildSlots(void)
 	 */
 	pmchild_pools[B_BACKEND].size = 2 * (MaxConnections + max_wal_senders);
 
-	pmchild_pools[B_AUTOVAC_WORKER].size = autovacuum_max_workers;
+	pmchild_pools[B_AUTOVAC_WORKER].size = autovacuum_worker_slots;
 	pmchild_pools[B_BG_WORKER].size = max_worker_processes;
 
 	/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index c7a972df7dd..49204f91a20 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -150,7 +150,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -284,13 +284,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + NUM_SPECIAL_WORKER_PROCS)
+		else if (i < MaxConnections + autovacuum_worker_slots + NUM_SPECIAL_WORKER_PROCS)
 		{
 			/* PGPROC for AV or special worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + NUM_SPECIAL_WORKER_PROCS + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + NUM_SPECIAL_WORKER_PROCS + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 46dbc46a97e..01bb6a410cb 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -547,15 +547,15 @@ InitializeMaxBackends(void)
 	Assert(MaxBackends == 0);
 
 	/* Note that this does not include "auxiliary" processes */
-	MaxBackends = MaxConnections + autovacuum_max_workers +
+	MaxBackends = MaxConnections + autovacuum_worker_slots +
 		max_worker_processes + max_wal_senders + NUM_SPECIAL_WORKER_PROCS;
 
 	if (MaxBackends > MAX_BACKENDS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("too many server processes configured"),
-				 errdetail("\"max_connections\" (%d) plus \"autovacuum_max_workers\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
-						   MaxConnections, autovacuum_max_workers,
+				 errdetail("\"max_connections\" (%d) plus \"autovacuum_worker_slots\" (%d) plus \"max_worker_processes\" (%d) plus \"max_wal_senders\" (%d) must be less than %d.",
+						   MaxConnections, autovacuum_worker_slots,
 						   max_worker_processes, max_wal_senders,
 						   MAX_BACKENDS - (NUM_SPECIAL_WORKER_PROCS - 1))));
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 22f16a3b462..c9d8cd796a8 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3467,7 +3467,16 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		/* see max_connections */
-		{"autovacuum_max_workers", PGC_POSTMASTER, AUTOVACUUM,
+		{"autovacuum_worker_slots", PGC_POSTMASTER, AUTOVACUUM,
+			gettext_noop("Sets the number of backend slots to allocate for autovacuum workers."),
+			NULL
+		},
+		&autovacuum_worker_slots,
+		16, 1, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
+	{
+		{"autovacuum_max_workers", PGC_SIGHUP, AUTOVACUUM,
 			gettext_noop("Sets the maximum number of simultaneously running autovacuum worker processes."),
 			NULL
 		},
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index a2ac7575ca7..b2bc43383db 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -661,8 +661,9 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
+autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
 					# (change requires restart)
+#autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
 #autovacuum_vacuum_threshold = 50	# min number of row updates before
 					# vacuum
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index edac746f3cf..54e01c81d68 100644
--- a/src/include/postmaster/autovacuum.h
+++ b/src/include/postmaster/autovacuum.h
@@ -28,6 +28,7 @@ typedef enum
 
 /* GUC variables */
 extern PGDLLIMPORT bool autovacuum_start_daemon;
+extern PGDLLIMPORT int autovacuum_worker_slots;
 extern PGDLLIMPORT int autovacuum_max_workers;
 extern PGDLLIMPORT int autovacuum_work_mem;
 extern PGDLLIMPORT int autovacuum_naptime;
diff --git a/src/test/perl/PostgreSQL/Test/Cluster.pm b/src/test/perl/PostgreSQL/Test/Cluster.pm
index 74bf3408682..08b89a4cdff 100644
--- a/src/test/perl/PostgreSQL/Test/Cluster.pm
+++ b/src/test/perl/PostgreSQL/Test/Cluster.pm
@@ -707,6 +707,7 @@ sub init
 		}
 		print $conf "max_wal_senders = 10\n";
 		print $conf "max_replication_slots = 10\n";
+		print $conf "autovacuum_worker_slots = 3\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters:
-- 
2.39.5 (Apple Git-154)

From 3f3c783c025e2a931bb766dc6d13c7186c957ebc Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 6 Jan 2025 16:09:49 -0600
Subject: [PATCH v1 1/1] lower default of autovacuum_worker_slots to 5

---
 doc/src/sgml/config.sgml                      | 2 +-
 src/backend/utils/misc/guc_tables.c           | 2 +-
 src/backend/utils/misc/postgresql.conf.sample | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 740ff5d5044..4fda5e6ae79 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8639,7 +8639,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       <listitem>
        <para>
         Specifies the number of backend slots to reserve for autovacuum worker
-        processes.  The default is 16.  This parameter can only be set at server
+        processes.  The default is 5.  This parameter can only be set at server
         start.
        </para>
        <para>
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index c9d8cd796a8..c75359b6bc4 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3472,7 +3472,7 @@ struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&autovacuum_worker_slots,
-		16, 1, MAX_BACKENDS,
+		5, 1, MAX_BACKENDS,
 		NULL, NULL, NULL
 	},
 	{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index b2bc43383db..c7d3852c040 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -661,7 +661,7 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
+autovacuum_worker_slots = 5		# autovacuum worker slots to allocate
 					# (change requires restart)
 #autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
-- 
2.39.5 (Apple Git-154)

From e35b4c5425cbe1120d3c38619be4f1523ae1f2c9 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 6 Jan 2025 17:14:54 -0600
Subject: [PATCH v1 1/1] Lower default value of autovacuum_worker_slots in
 initdb as needed.

---
 doc/src/sgml/config.sgml |  5 +++--
 src/bin/initdb/initdb.c  | 41 +++++++++++++++++++++++++++++++++++-----
 2 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 740ff5d5044..8683f0bdf53 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8639,8 +8639,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       <listitem>
        <para>
         Specifies the number of backend slots to reserve for autovacuum worker
-        processes.  The default is 16.  This parameter can only be set at server
-        start.
+        processes.  The default is typically 16 slots, but might be less if
+        your kernel settings will not support it (as determined during initdb).
+        This parameter can only be set at server start.
        </para>
        <para>
         When changing this value, consider also adjusting
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index 4e4b7ede190..1214f1b492a 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -197,6 +197,7 @@ static char *pgdata_native;
 /* defaults */
 static int	n_connections = 10;
 static int	n_buffers = 50;
+static int	n_av_slots = 16;
 static const char *dynamic_shared_memory_type = NULL;
 static const char *default_timezone = NULL;
 
@@ -273,7 +274,8 @@ static void check_input(char *path);
 static void write_version_file(const char *extrapath);
 static void set_null_conf(void);
 static void test_config_settings(void);
-static bool test_specific_config_settings(int test_conns, int test_buffs);
+static bool test_specific_config_settings(int test_conns, int test_buffs,
+										  int test_av_slots);
 static void setup_config(void);
 static void bootstrap_template1(void);
 static void setup_auth(FILE *cmdfd);
@@ -1118,6 +1120,13 @@ test_config_settings(void)
 	 */
 #define MIN_BUFS_FOR_CONNS(nconns)	((nconns) * 10)
 
+	/*
+	 * This macro defines the minimum default autovacuum_worker_slots we are
+	 * willing to consider.  It should be kept >= the default for
+	 * autovacuum_max_workers.
+	 */
+#define MIN_AV_WORKER_SLOTS	(3)
+
 	static const int trial_conns[] = {
 		100, 50, 40, 30, 25
 	};
@@ -1155,7 +1164,9 @@ test_config_settings(void)
 		test_conns = trial_conns[i];
 		test_buffs = MIN_BUFS_FOR_CONNS(test_conns);
 
-		if (test_specific_config_settings(test_conns, test_buffs))
+		if (test_specific_config_settings(test_conns,
+										  test_buffs,
+										  MIN_AV_WORKER_SLOTS))
 		{
 			ok_buffers = test_buffs;
 			break;
@@ -1180,7 +1191,9 @@ test_config_settings(void)
 			break;
 		}
 
-		if (test_specific_config_settings(n_connections, test_buffs))
+		if (test_specific_config_settings(n_connections,
+										  test_buffs,
+										  MIN_AV_WORKER_SLOTS))
 			break;
 	}
 	n_buffers = test_buffs;
@@ -1190,6 +1203,19 @@ test_config_settings(void)
 	else
 		printf("%dkB\n", n_buffers * (BLCKSZ / 1024));
 
+	printf(_("selecting default \"autovacuum_worker_slots\" ... "));
+	fflush(stdout);
+
+	for (; n_av_slots > MIN_AV_WORKER_SLOTS; n_av_slots--)
+	{
+		if (test_specific_config_settings(n_connections,
+										  n_buffers,
+										  n_av_slots))
+			break;
+	}
+
+	printf("%d\n", n_av_slots);
+
 	printf(_("selecting default time zone ... "));
 	fflush(stdout);
 	default_timezone = select_default_timezone(share_path);
@@ -1200,7 +1226,7 @@ test_config_settings(void)
  * Test a specific combination of configuration settings.
  */
 static bool
-test_specific_config_settings(int test_conns, int test_buffs)
+test_specific_config_settings(int test_conns, int test_buffs, int test_av_slots)
 {
 	PQExpBufferData cmd;
 	_stringlist *gnames,
@@ -1214,9 +1240,10 @@ test_specific_config_settings(int test_conns, int test_buffs)
 					  "\"%s\" --check %s %s "
 					  "-c max_connections=%d "
 					  "-c shared_buffers=%d "
+					  "-c autovacuum_worker_slots=%d "
 					  "-c dynamic_shared_memory_type=%s",
 					  backend_exec, boot_options, extra_options,
-					  test_conns, test_buffs,
+					  test_conns, test_buffs, test_av_slots,
 					  dynamic_shared_memory_type);
 
 	/* Add any user-given setting overrides */
@@ -1289,6 +1316,10 @@ setup_config(void)
 	conflines = replace_guc_value(conflines, "shared_buffers",
 								  repltok, false);
 
+	snprintf(repltok, sizeof(repltok), "%d", n_av_slots);
+	conflines = replace_guc_value(conflines, "autovacuum_worker_slots",
+								  repltok, false);
+
 	conflines = replace_guc_value(conflines, "lc_messages",
 								  lc_messages, false);
 
-- 
2.39.5 (Apple Git-154)

From a2cb9161ccd7bf08c5273e4b4e012861a410ce51 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 6 Jan 2025 17:14:54 -0600
Subject: [PATCH v2 1/1] Lower default value of autovacuum_worker_slots in
 initdb as needed.

---
 doc/src/sgml/config.sgml                      |  5 +--
 src/backend/utils/misc/guc_tables.c           |  2 +-
 src/backend/utils/misc/postgresql.conf.sample |  2 +-
 src/bin/initdb/initdb.c                       | 34 +++++++++++++++----
 4 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 740ff5d5044..a47c4dbfcb2 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8639,8 +8639,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       <listitem>
        <para>
         Specifies the number of backend slots to reserve for autovacuum worker
-        processes.  The default is 16.  This parameter can only be set at server
-        start.
+        processes.  The default is typically 12 slots, but might be less if
+        your kernel settings will not support it (as determined during initdb).
+        This parameter can only be set at server start.
        </para>
        <para>
         When changing this value, consider also adjusting
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index c9d8cd796a8..8781495a622 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3472,7 +3472,7 @@ struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&autovacuum_worker_slots,
-		16, 1, MAX_BACKENDS,
+		12, 1, MAX_BACKENDS,
 		NULL, NULL, NULL
 	},
 	{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index b2bc43383db..3405ca0e726 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -661,7 +661,7 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
+autovacuum_worker_slots = 12	# autovacuum worker slots to allocate
 					# (change requires restart)
 #autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index 4e4b7ede190..b5023484771 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -196,6 +196,7 @@ static char *pgdata_native;
 
 /* defaults */
 static int	n_connections = 10;
+static int	n_av_slots = 12;
 static int	n_buffers = 50;
 static const char *dynamic_shared_memory_type = NULL;
 static const char *default_timezone = NULL;
@@ -273,7 +274,8 @@ static void check_input(char *path);
 static void write_version_file(const char *extrapath);
 static void set_null_conf(void);
 static void test_config_settings(void);
-static bool test_specific_config_settings(int test_conns, int test_buffs);
+static bool test_specific_config_settings(int test_conns, int test_av_slots,
+										  int test_buffs);
 static void setup_config(void);
 static void bootstrap_template1(void);
 static void setup_auth(FILE *cmdfd);
@@ -1118,6 +1120,12 @@ test_config_settings(void)
 	 */
 #define MIN_BUFS_FOR_CONNS(nconns)	((nconns) * 10)
 
+	/*
+	 * This macro defines the default value of autovacuum_worker_slots we want
+	 * for a given max_connections value.
+	 */
+#define AV_SLOTS_FOR_CONNS(nconns)	((nconns) / 8)
+
 	static const int trial_conns[] = {
 		100, 50, 40, 30, 25
 	};
@@ -1145,7 +1153,8 @@ test_config_settings(void)
 
 	/*
 	 * Probe for max_connections before shared_buffers, since it is subject to
-	 * more constraints than shared_buffers.
+	 * more constraints than shared_buffers.  We also choose the default
+	 * autovacuum_worker_slots here.
 	 */
 	printf(_("selecting default \"max_connections\" ... "));
 	fflush(stdout);
@@ -1154,8 +1163,9 @@ test_config_settings(void)
 	{
 		test_conns = trial_conns[i];
 		test_buffs = MIN_BUFS_FOR_CONNS(test_conns);
+		n_av_slots = AV_SLOTS_FOR_CONNS(test_conns);
 
-		if (test_specific_config_settings(test_conns, test_buffs))
+		if (test_specific_config_settings(test_conns, n_av_slots, test_buffs))
 		{
 			ok_buffers = test_buffs;
 			break;
@@ -1167,6 +1177,13 @@ test_config_settings(void)
 
 	printf("%d\n", n_connections);
 
+	/*
+	 * We chose the default for autovacuum_worker_slots during the
+	 * max_connections tests above, but we print a progress message anyway.
+	 */
+	printf(_("selecting default \"autovacuum_worker_slots\" ... %d\n"),
+		   n_av_slots);
+
 	printf(_("selecting default \"shared_buffers\" ... "));
 	fflush(stdout);
 
@@ -1180,7 +1197,7 @@ test_config_settings(void)
 			break;
 		}
 
-		if (test_specific_config_settings(n_connections, test_buffs))
+		if (test_specific_config_settings(n_connections, n_av_slots, test_buffs))
 			break;
 	}
 	n_buffers = test_buffs;
@@ -1200,7 +1217,7 @@ test_config_settings(void)
  * Test a specific combination of configuration settings.
  */
 static bool
-test_specific_config_settings(int test_conns, int test_buffs)
+test_specific_config_settings(int test_conns, int test_av_slots, int test_buffs)
 {
 	PQExpBufferData cmd;
 	_stringlist *gnames,
@@ -1213,10 +1230,11 @@ test_specific_config_settings(int test_conns, int test_buffs)
 	printfPQExpBuffer(&cmd,
 					  "\"%s\" --check %s %s "
 					  "-c max_connections=%d "
+					  "-c autovacuum_worker_slots=%d "
 					  "-c shared_buffers=%d "
 					  "-c dynamic_shared_memory_type=%s",
 					  backend_exec, boot_options, extra_options,
-					  test_conns, test_buffs,
+					  test_conns, test_av_slots, test_buffs,
 					  dynamic_shared_memory_type);
 
 	/* Add any user-given setting overrides */
@@ -1280,6 +1298,10 @@ setup_config(void)
 	conflines = replace_guc_value(conflines, "max_connections",
 								  repltok, false);
 
+	snprintf(repltok, sizeof(repltok), "%d", n_av_slots);
+	conflines = replace_guc_value(conflines, "autovacuum_worker_slots",
+								  repltok, false);
+
 	if ((n_buffers * (BLCKSZ / 1024)) % 1024 == 0)
 		snprintf(repltok, sizeof(repltok), "%dMB",
 				 (n_buffers * (BLCKSZ / 1024)) / 1024);
-- 
2.39.5 (Apple Git-154)

From d8b9fe6d3c166c14d9a8d17f43418be33c4e0784 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 7 Jan 2025 11:15:22 -0600
Subject: [PATCH v3 1/1] Lower default value of autovacuum_worker_slots in
 initdb as needed.

TODO

Reported-by: Tom Lane
Suggested-by: Andres Freund
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/1346002.1736198977%40sss.pgh.pa.us
---
 doc/src/sgml/config.sgml |  5 +++--
 src/bin/initdb/initdb.c  | 40 ++++++++++++++++++++++++++++++++++------
 2 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 740ff5d5044..8683f0bdf53 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8639,8 +8639,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       <listitem>
        <para>
         Specifies the number of backend slots to reserve for autovacuum worker
-        processes.  The default is 16.  This parameter can only be set at server
-        start.
+        processes.  The default is typically 16 slots, but might be less if
+        your kernel settings will not support it (as determined during initdb).
+        This parameter can only be set at server start.
        </para>
        <para>
         When changing this value, consider also adjusting
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index 4e4b7ede190..f2b9d50e9b3 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -196,6 +196,7 @@ static char *pgdata_native;
 
 /* defaults */
 static int	n_connections = 10;
+static int	n_av_slots = 16;
 static int	n_buffers = 50;
 static const char *dynamic_shared_memory_type = NULL;
 static const char *default_timezone = NULL;
@@ -273,7 +274,8 @@ static void check_input(char *path);
 static void write_version_file(const char *extrapath);
 static void set_null_conf(void);
 static void test_config_settings(void);
-static bool test_specific_config_settings(int test_conns, int test_buffs);
+static bool test_specific_config_settings(int test_conns, int test_av_slots,
+										  int test_buffs);
 static void setup_config(void);
 static void bootstrap_template1(void);
 static void setup_auth(FILE *cmdfd);
@@ -1118,6 +1120,18 @@ test_config_settings(void)
 	 */
 #define MIN_BUFS_FOR_CONNS(nconns)	((nconns) * 10)
 
+	/*
+	 * This macro defines the default value of autovacuum_worker_slots we want
+	 * for a given max_connections value.  Note that it has been carefully
+	 * crafted to provide specific values for the associated values in
+	 * trial_conns.  We want it to return autovacuum_worker_slot's initial
+	 * default value (16) for the maximum value in trial_conns (100), and we
+	 * want it to return close to the minimum value we'd consider (3, which is
+	 * the default of autovacuum_max_workers) for the minimum value in
+	 * trial_conns (25).
+	 */
+#define AV_SLOTS_FOR_CONNS(nconns)	((nconns) / 6)
+
 	static const int trial_conns[] = {
 		100, 50, 40, 30, 25
 	};
@@ -1145,7 +1159,8 @@ test_config_settings(void)
 
 	/*
 	 * Probe for max_connections before shared_buffers, since it is subject to
-	 * more constraints than shared_buffers.
+	 * more constraints than shared_buffers.  We also choose the default
+	 * autovacuum_worker_slots here.
 	 */
 	printf(_("selecting default \"max_connections\" ... "));
 	fflush(stdout);
@@ -1153,9 +1168,10 @@ test_config_settings(void)
 	for (i = 0; i < connslen; i++)
 	{
 		test_conns = trial_conns[i];
+		n_av_slots = AV_SLOTS_FOR_CONNS(test_conns);
 		test_buffs = MIN_BUFS_FOR_CONNS(test_conns);
 
-		if (test_specific_config_settings(test_conns, test_buffs))
+		if (test_specific_config_settings(test_conns, n_av_slots, test_buffs))
 		{
 			ok_buffers = test_buffs;
 			break;
@@ -1167,6 +1183,13 @@ test_config_settings(void)
 
 	printf("%d\n", n_connections);
 
+	/*
+	 * We chose the default for autovacuum_worker_slots during the
+	 * max_connections tests above, but we print a progress message anyway.
+	 */
+	printf(_("selecting default \"autovacuum_worker_slots\" ... %d\n"),
+		   n_av_slots);
+
 	printf(_("selecting default \"shared_buffers\" ... "));
 	fflush(stdout);
 
@@ -1180,7 +1203,7 @@ test_config_settings(void)
 			break;
 		}
 
-		if (test_specific_config_settings(n_connections, test_buffs))
+		if (test_specific_config_settings(n_connections, n_av_slots, test_buffs))
 			break;
 	}
 	n_buffers = test_buffs;
@@ -1200,7 +1223,7 @@ test_config_settings(void)
  * Test a specific combination of configuration settings.
  */
 static bool
-test_specific_config_settings(int test_conns, int test_buffs)
+test_specific_config_settings(int test_conns, int test_av_slots, int test_buffs)
 {
 	PQExpBufferData cmd;
 	_stringlist *gnames,
@@ -1213,10 +1236,11 @@ test_specific_config_settings(int test_conns, int test_buffs)
 	printfPQExpBuffer(&cmd,
 					  "\"%s\" --check %s %s "
 					  "-c max_connections=%d "
+					  "-c autovacuum_worker_slots=%d "
 					  "-c shared_buffers=%d "
 					  "-c dynamic_shared_memory_type=%s",
 					  backend_exec, boot_options, extra_options,
-					  test_conns, test_buffs,
+					  test_conns, test_av_slots, test_buffs,
 					  dynamic_shared_memory_type);
 
 	/* Add any user-given setting overrides */
@@ -1280,6 +1304,10 @@ setup_config(void)
 	conflines = replace_guc_value(conflines, "max_connections",
 								  repltok, false);
 
+	snprintf(repltok, sizeof(repltok), "%d", n_av_slots);
+	conflines = replace_guc_value(conflines, "autovacuum_worker_slots",
+								  repltok, false);
+
 	if ((n_buffers * (BLCKSZ / 1024)) % 1024 == 0)
 		snprintf(repltok, sizeof(repltok), "%dMB",
 				 (n_buffers * (BLCKSZ / 1024)) / 1024);
-- 
2.39.5 (Apple Git-154)