From: Andres Freund <andres@anarazel.de>
Date: Thu, 31 Dec 2020 04:04:10 +0000 (-0800)
Subject: xlog: Use LWLockAcquireOrWait() in AdvanceXLInsertBuffer.
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=559f13e6162b40b7c8327ed916e37e5437e567bc;p=users%2Fandresfreund%2Fpostgres.git

xlog: Use LWLockAcquireOrWait() in AdvanceXLInsertBuffer.

Otherwise a lot of backends may unnecessarily acquire the lock
exclusively one-by-one.

Author:
Reviewed-By:
Discussion: https://postgr.es/m/
Backpatch:
---

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 56d1d29fbe..dec645c416 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2284,14 +2284,33 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
 	XLogPageHeader NewPage;
 	int			npages = 0;
 
-	LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
+retry:
+	if (opportunistic)
+	{
+		if (!LWLockConditionalAcquire(WALBufMappingLock, LW_EXCLUSIVE))
+			return;
+	}
+	else if (!LWLockAcquireOrWait(WALBufMappingLock, LW_EXCLUSIVE))
+	{
+		if (upto < XLogCtl->InitializedUpTo)
+			return;
+		goto retry;
+	}
 
 	/*
 	 * Now that we have the lock, check if someone initialized the page
 	 * already.
 	 */
-	while (upto >= XLogCtl->InitializedUpTo || opportunistic)
+	while (true)
 	{
+		if (!opportunistic && upto < XLogCtl->InitializedUpTo)
+		{
+			ereport(DEBUG3, errmsg("moving to opportunistic"),
+					errhidestmt(true),
+					errhidecontext(true));
+			opportunistic = true;
+		}
+
 		nextidx = XLogRecPtrToBufIdx(XLogCtl->InitializedUpTo);
 
 		/*
@@ -2341,9 +2360,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
 				pgWalUsage.wal_buffers_full++;
 				TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
 
-				/* Re-acquire WALBufMappingLock and retry */
-				LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
-				continue;
+				goto retry;
 			}
 		}
 
@@ -2361,6 +2378,10 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
 		/*
 		 * Be sure to re-zero the buffer so that bytes beyond what we've
 		 * written will look like zeroes and not valid XLOG records...
+		 *
+		 * FIXME: We should avoid doing this under the lock - that's pretty
+		 * slow. It's also not helpful to cause unnecessary cacheline pingpong
+		 * for the page(s) in concurrent workloads.
 		 */
 		MemSet((char *) NewPage, 0, XLOG_BLCKSZ);