From 8dd1ae41ac3f2c8b378dfad64bbfe94eb41d39ab Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Sat, 8 Aug 2020 15:08:09 +1200
Subject: [PATCH v2 1/4] Optimize latches to send fewer signals.

Don't send signals to processes that aren't currently sleeping.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJjxPDpzBE0a3hyUywBvaZuC89yx3jK9RFZgfv_KHU7gg@mail.gmail.com
---
 src/backend/storage/ipc/latch.c | 26 ++++++++++++++++++++++++++
 src/include/storage/latch.h     |  1 +
 2 files changed, 27 insertions(+)

diff --git a/src/backend/storage/ipc/latch.c b/src/backend/storage/ipc/latch.c
index 24d44c982d..f4768b79c2 100644
--- a/src/backend/storage/ipc/latch.c
+++ b/src/backend/storage/ipc/latch.c
@@ -274,6 +274,7 @@ void
 InitLatch(Latch *latch)
 {
 	latch->is_set = false;
+	latch->maybe_sleeping = false;
 	latch->owner_pid = MyProcPid;
 	latch->is_shared = false;
 
@@ -321,6 +322,7 @@ InitSharedLatch(Latch *latch)
 #endif
 
 	latch->is_set = false;
+	latch->maybe_sleeping = false;
 	latch->owner_pid = 0;
 	latch->is_shared = true;
 }
@@ -523,6 +525,10 @@ SetLatch(Latch *latch)
 
 	latch->is_set = true;
 
+	pg_memory_barrier();
+	if (!latch->maybe_sleeping)
+		return;
+
 #ifndef WIN32
 
 	/*
@@ -589,6 +595,7 @@ ResetLatch(Latch *latch)
 {
 	/* Only the owner should reset the latch */
 	Assert(latch->owner_pid == MyProcPid);
+	Assert(latch->maybe_sleeping == false);
 
 	latch->is_set = false;
 
@@ -1289,6 +1296,14 @@ WaitEventSetWait(WaitEventSet *set, long timeout,
 		 * ordering, so that we cannot miss seeing is_set if a notification
 		 * has already been queued.
 		 */
+		if (set->latch && !set->latch->is_set)
+		{
+			/* about to sleep on a latch */
+			set->latch->maybe_sleeping = true;
+			pg_memory_barrier();
+			/* and recheck */
+		}
+
 		if (set->latch && set->latch->is_set)
 		{
 			occurred_events->fd = PGINVALID_SOCKET;
@@ -1299,6 +1314,9 @@ WaitEventSetWait(WaitEventSet *set, long timeout,
 			occurred_events++;
 			returned_events++;
 
+			/* could have been set above */
+			set->latch->maybe_sleeping = false;
+
 			break;
 		}
 
@@ -1310,6 +1328,12 @@ WaitEventSetWait(WaitEventSet *set, long timeout,
 		rc = WaitEventSetWaitBlock(set, cur_timeout,
 								   occurred_events, nevents);
 
+		if (set->latch)
+		{
+			Assert(set->latch->maybe_sleeping);
+			set->latch->maybe_sleeping = false;
+		}
+
 		if (rc == -1)
 			break;				/* timeout occurred */
 		else
@@ -1399,6 +1423,8 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 		if (cur_event->events == WL_LATCH_SET &&
 			cur_epoll_event->events & (EPOLLIN | EPOLLERR | EPOLLHUP))
 		{
+			Assert(set->latch);
+
 			/* There's data in the self-pipe, clear it. */
 			drainSelfPipe();
 
diff --git a/src/include/storage/latch.h b/src/include/storage/latch.h
index 7c742021fb..d0da7e5d31 100644
--- a/src/include/storage/latch.h
+++ b/src/include/storage/latch.h
@@ -110,6 +110,7 @@
 typedef struct Latch
 {
 	sig_atomic_t is_set;
+	sig_atomic_t maybe_sleeping;
 	bool		is_shared;
 	int			owner_pid;
 #ifdef WIN32
-- 
2.20.1