From 6cf9d273b3b2c2845e94ace6e8332b5fd1b4e308 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Sat, 22 Oct 2022 22:54:31 +0300
Subject: [PATCH] Extra spin waits for the lock to be freed by concurrent
 callers

---
 src/backend/storage/lmgr/lwlock.c | 50 ++++++++++++++++++++++++-------
 1 file changed, 39 insertions(+), 11 deletions(-)

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index f5a149b4f42..0917b5874db 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -840,6 +840,19 @@ LWLockAttemptLock(LWLock *lock, LWLockMode mode, LWLockMode waitMode,
 				  bool queue_ok, bool wakeup)
 {
 	uint64		old_state;
+	uint64		mask;
+	uint64		increment;
+
+	if (mode == LW_EXCLUSIVE)
+	{
+		mask = LW_LOCK_MASK;
+		increment = LW_VAL_EXCLUSIVE;
+	}
+	else
+	{
+		mask = LW_VAL_EXCLUSIVE;
+		increment = LW_VAL_SHARED;
+	}
 
 	/*
 	 * Read once outside the loop, later iterations will get the newer value
@@ -853,21 +866,36 @@ LWLockAttemptLock(LWLock *lock, LWLockMode mode, LWLockMode waitMode,
 		uint64		desired_state;
 		bool		lock_free;
 
-		desired_state = old_state;
+		lock_free = (old_state & mask) == 0;
 
-		if (mode == LW_EXCLUSIVE)
-		{
-			lock_free = (old_state & LW_LOCK_MASK) == 0;
-			if (lock_free)
-				desired_state += LW_VAL_EXCLUSIVE;
-		}
-		else
+		/*
+		 * The retries on spin-wait helps to avoid going to relatively expensive
+		 * OS semaphore when lock is freed fast enough by a concurrent process.
+		 * XXX The initial delay and number of repeats are optimized based on
+		 * heavy lock concurrency tests and the values are to be discussed.
+		 */
+		if (!lock_free && queue_ok)
 		{
-			lock_free = (old_state & LW_VAL_EXCLUSIVE) == 0;
-			if (lock_free)
-				desired_state += LW_VAL_SHARED;
+			int		i = 0;
+			SpinDelayStatus delayStatus;
+
+			init_local_spin_delay(&delayStatus);
+			delayStatus.cur_delay = 500L;
+
+			while (!lock_free && i < 2)
+			{
+				perform_spin_delay(&delayStatus);
+				old_state = pg_atomic_read_u64(&lock->state);
+				lock_free = (old_state & mask) == 0;
+				i++;
+			}
 		}
 
+		desired_state = old_state;
+
+		if (lock_free)
+			desired_state += increment;
+
 		if (wakeup)
 			desired_state |= LW_FLAG_RELEASE_OK;
 
-- 
2.24.3 (Apple Git-128)