2 files changed, 30 insertions, 8 deletions

diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 69af75c6b64..6e84cd0a216 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -327,7 +327,7 @@ MarkAsPreparing(TransactionId xid, const char *gid,
 	proc->databaseId = databaseid;
 	proc->roleId = owner;
 	proc->lwWaiting = false;
-	proc->lwExclusive = false;
+	proc->lwWaitMode = 0;
 	proc->lwWaitLink = NULL;
 	proc->waitLock = NULL;
 	proc->waitProcLock = NULL;
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 4b273a8318f..cce87a3cd30 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2118,23 +2118,43 @@ XLogFlush(XLogRecPtr record)
 	/* initialize to given target; may increase below */
 	WriteRqstPtr = record;
 
-	/* read LogwrtResult and update local state */
+	/*
+	 * Now wait until we get the write lock, or someone else does the
+	 * flush for us.
+	 */
+	for (;;)
 	{
 		/* use volatile pointer to prevent code rearrangement */
 		volatile XLogCtlData *xlogctl = XLogCtl;
 
+		/* read LogwrtResult and update local state */
 		SpinLockAcquire(&xlogctl->info_lck);
 		if (XLByteLT(WriteRqstPtr, xlogctl->LogwrtRqst.Write))
 			WriteRqstPtr = xlogctl->LogwrtRqst.Write;
 		LogwrtResult = xlogctl->LogwrtResult;
 		SpinLockRelease(&xlogctl->info_lck);
-	}
 
-	/* done already? */
-	if (!XLByteLE(record, LogwrtResult.Flush))
-	{
-		/* now wait for the write lock */
-		LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
+		/* done already? */
+		if (XLByteLE(record, LogwrtResult.Flush))
+			break;
+
+		/*
+		 * Try to get the write lock. If we can't get it immediately, wait
+		 * until it's released, and recheck if we still need to do the flush
+		 * or if the backend that held the lock did it for us already. This
+		 * helps to maintain a good rate of group committing when the system
+		 * is bottlenecked by the speed of fsyncing.
+		 */
+		if (!LWLockWaitUntilFree(WALWriteLock, LW_EXCLUSIVE))
+		{
+			/*
+			 * The lock is now free, but we didn't acquire it yet. Before we
+			 * do, loop back to check if someone else flushed the record for
+			 * us already.
+			 */
+			continue;
+		}
+		/* Got the lock */
 		LogwrtResult = XLogCtl->Write.LogwrtResult;
 		if (!XLByteLE(record, LogwrtResult.Flush))
 		{
@@ -2163,6 +2183,8 @@ XLogFlush(XLogRecPtr record)
 			XLogWrite(WriteRqst, false, false);
 		}
 		LWLockRelease(WALWriteLock);
+		/* done */
+		break;
 	}
 
 	END_CRIT_SECTION();