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

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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.
This commit is contained in:
Tom Lane
2007-09-08 20:31:15 +00:00
parent 0a51e7073c
commit 6bd4f401b0
14 changed files with 331 additions and 213 deletions
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6
src/backend/access/transam/xlog.c
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@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.280 2007/09/05 18:10:47 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.281 2007/09/08 20:31:14 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -5196,6 +5196,10 @@ StartupXLOG(void)
XLogCtl->ckptXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
XLogCtl->ckptXid = ControlFile->checkPointCopy.nextXid;
/* also initialize latestCompletedXid, to nextXid - 1 */
ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
TransactionIdRetreat(ShmemVariableCache->latestCompletedXid);
/* Start up the commit log and related stuff, too */
StartupCLOG();
StartupSUBTRANS(oldestActiveXID);