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Avoid SnapshotResetXmin() during AtEOXact_Snapshot()

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For normal commits and aborts we already reset PgXact->xmin,
so we can simply avoid running SnapshotResetXmin() twice.

During performance tests by Alexander Korotkov, diagnosis
by Andres Freund showed PgXact array as a bottleneck. After
manual analysis by me of the code paths that touch those
memory locations, I was able to identify extraneous code
in the main transaction commit path.

Avoiding touching highly contented shmem improves concurrent
performance slightly on all workloads, confirmed by tests
run by Ashutosh Sharma and Alexander Korotkov.

Simon Riggs

Discussion: CANP8+jJdXE9b+b9F8CQT-LuxxO0PBCB-SZFfMVAdp+akqo4zfg@mail.gmail.com
This commit is contained in:
Simon Riggs
2017-04-06 08:31:52 -04:00
parent fd01983594
commit 6bad580d9e
3 changed files with 16 additions and 6 deletions
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6
src/backend/access/transam/xact.c
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@ -2137,7 +2137,7 @@ CommitTransaction(void)
AtEOXact_ComboCid();
AtEOXact_HashTables(true);
AtEOXact_PgStat(true);
AtEOXact_Snapshot(true);
AtEOXact_Snapshot(true, false);
AtCommit_ApplyLauncher();
pgstat_report_xact_timestamp(0);
@ -2409,7 +2409,7 @@ PrepareTransaction(void)
AtEOXact_ComboCid();
AtEOXact_HashTables(true);
/* don't call AtEOXact_PgStat here; we fixed pgstat state above */
AtEOXact_Snapshot(true);
AtEOXact_Snapshot(true, true);
pgstat_report_xact_timestamp(0);
CurrentResourceOwner = NULL;
@ -2640,7 +2640,7 @@ CleanupTransaction(void)
* do abort cleanup processing
*/
AtCleanup_Portals(); /* now safe to release portal memory */
AtEOXact_Snapshot(false); /* and release the transaction's snapshots */
AtEOXact_Snapshot(false, false); /* and release the transaction's snapshots */
CurrentResourceOwner = NULL; /* and resource owner */
if (TopTransactionResourceOwner)