Replace the former method of determining snapshot xmax --- to wit, calling

ReadNewTransactionId from GetSnapshotData --- with a "latestCompletedXid"
variable that is updated during transaction commit or abort.  Since
latestCompletedXid is written only in places that had to lock ProcArrayLock
exclusively anyway, and is read only in places that had to lock ProcArrayLock
shared anyway, it adds no new locking requirements to the system despite being
cluster-wide.  Moreover, removing ReadNewTransactionId from snapshot
acquisition eliminates the need to take both XidGenLock and ProcArrayLock at
the same time.  Since XidGenLock is sometimes held across I/O this can be a
significant win.  Some preliminary benchmarking suggested that this patch has
no effect on average throughput but can significantly improve the worst-case
transaction times seen in pgbench.  Concept by Florian Pflug, implementation
by Tom Lane.

1 files changed, 28 insertions, 1 deletions

diff --git a/src/backend/access/transam/transam.c b/src/backend/access/transam/transam.c
index 3466b50ef24..e53b05e04d5 100644
--- a/src/backend/access/transam/transam.c
+++ b/src/backend/access/transam/transam.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.70 2007/08/01 22:45:07 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.71 2007/09/08 20:31:14 tgl Exp $
  *
  * NOTES
  *	  This file contains the high level access-method interface to the
@@ -432,6 +432,33 @@ TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2)
 	return (diff >= 0);
 }
 
+
+/*
+ * TransactionIdLatest --- get latest XID among a main xact and its children
+ */
+TransactionId
+TransactionIdLatest(TransactionId mainxid,
+					int nxids, const TransactionId *xids)
+{
+	TransactionId	result;
+
+	/*
+	 * In practice it is highly likely that the xids[] array is sorted, and
+	 * so we could save some cycles by just taking the last child XID, but
+	 * this probably isn't so performance-critical that it's worth depending
+	 * on that assumption.  But just to show we're not totally stupid, scan
+	 * the array back-to-front to avoid useless assignments.
+	 */
+	result = mainxid;
+	while (--nxids >= 0)
+	{
+		if (TransactionIdPrecedes(result, xids[nxids]))
+			result = xids[nxids];
+	}
+	return result;
+}
+
+
 /*
  * TransactionIdGetCommitLSN
  *