From 4e9b159484ca088ad9ce6527a2a635f83f308555 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 7 Oct 2002 17:04:30 +0000
Subject: [PATCH] Change order of operations during XLogFlush so that we try to
 include in our write/flush operation any WAL entries that got queued while we
 were waiting to get the WALWriteLock.  This improves throughput when
 transactions are small enough that several can be committed per WAL write
 (ie, per disk revolution).

---
 src/backend/access/transam/xlog.c | 41 +++++++++++++++++--------------
 1 file changed, 23 insertions(+), 18 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 1944e557708..6a3732c914b 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.107 2002/09/26 22:58:33 tgl Exp $
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.108 2002/10/07 17:04:30 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1252,28 +1252,33 @@ XLogFlush(XLogRecPtr record)
 	/* done already? */
 	if (!XLByteLE(record, LogwrtResult.Flush))
 	{
-		/* if something was added to log cache then try to flush this too */
-		if (LWLockConditionalAcquire(WALInsertLock, LW_EXCLUSIVE))
-		{
-			XLogCtlInsert *Insert = &XLogCtl->Insert;
-			uint32		freespace = INSERT_FREESPACE(Insert);
-
-			if (freespace < SizeOfXLogRecord)	/* buffer is full */
-				WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
-			else
-			{
-				WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
-				WriteRqstPtr.xrecoff -= freespace;
-			}
-			LWLockRelease(WALInsertLock);
-		}
 		/* now wait for the write lock */
 		LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
 		LogwrtResult = XLogCtl->Write.LogwrtResult;
 		if (!XLByteLE(record, LogwrtResult.Flush))
 		{
-			WriteRqst.Write = WriteRqstPtr;
-			WriteRqst.Flush = record;
+			/* try to write/flush later additions to XLOG as well */
+			if (LWLockConditionalAcquire(WALInsertLock, LW_EXCLUSIVE))
+			{
+				XLogCtlInsert *Insert = &XLogCtl->Insert;
+				uint32		freespace = INSERT_FREESPACE(Insert);
+
+				if (freespace < SizeOfXLogRecord)	/* buffer is full */
+					WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
+				else
+				{
+					WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
+					WriteRqstPtr.xrecoff -= freespace;
+				}
+				LWLockRelease(WALInsertLock);
+				WriteRqst.Write = WriteRqstPtr;
+				WriteRqst.Flush = WriteRqstPtr;
+			}
+			else
+			{
+				WriteRqst.Write = WriteRqstPtr;
+				WriteRqst.Flush = record;
+			}
 			XLogWrite(WriteRqst);
 		}
 		LWLockRelease(WALWriteLock);
-- 
2.39.5