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Avoid SnapshotResetXmin() during AtEOXact_Snapshot() 

For normal commits and aborts we already reset PgXact->xmin
Avoiding touching highly contented shmem improves concurrent
performance.

Simon Riggs

Discussion: CANP8+jJdXE9b+b9F8CQT-LuxxO0PBCB-SZFfMVAdp+akqo4zfg@mail.gmail.com
This commit is contained in:
Simon Riggs 2017-03-24 14:20:59 +00:00
parent 8398c83689
commit 42b4b0b241
3 changed files with 22 additions and 7 deletions
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6
src/backend/access/transam/xact.c
View File

@ -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)

21
src/backend/utils/time/snapmgr.c
View File

@ -954,7 +954,12 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
*
* If there are no more snapshots, we can reset our PGXACT->xmin to InvalidXid.
* Note we can do this without locking because we assume that storing an Xid
* is atomic.
* is atomic. We do this because it will allow multi-statement transactions to
* reset their xmin and prevent us from holding back removal of dead rows;
* this has little purpose when we are dealing with very fast statements in
* read committed mode since the xmin will advance quickly anyway. It has no
* use at all when we are running single statement transactions since the xmin
* is reset as part of end of transaction anyway.
*
* Even if there are some remaining snapshots, we may be able to advance our
* PGXACT->xmin to some degree. This typically happens when a portal is
@ -1051,7 +1056,7 @@ AtSubAbort_Snapshot(int level)
* Snapshot manager's cleanup function for end of transaction
*/
void
AtEOXact_Snapshot(bool isCommit)
AtEOXact_Snapshot(bool isCommit, bool isPrepare)
{
/*
* In transaction-snapshot mode we must release our privately-managed
@ -1136,7 +1141,17 @@ AtEOXact_Snapshot(bool isCommit)
FirstSnapshotSet = false;
SnapshotResetXmin();
/*
* During normal commit and abort processing, we call
* ProcArrayEndTransaction() or ProcArrayClearTransaction() to
* reset the PgXact->xmin. That call happens prior to the call to
* AtEOXact_Snapshot(), so we need not touch xmin here at all,
* accept when we are preparing a transaction.
*/
if (isPrepare)
SnapshotResetXmin();
Assert(MyPgXact->xmin == 0);
}

2
src/include/utils/snapmgr.h
View File

@ -85,7 +85,7 @@ extern void UnregisterSnapshotFromOwner(Snapshot snapshot, ResourceOwner owner);
extern void AtSubCommit_Snapshot(int level);
extern void AtSubAbort_Snapshot(int level);
extern void AtEOXact_Snapshot(bool isCommit);
extern void AtEOXact_Snapshot(bool isCommit, bool isPrepare);
extern void ImportSnapshot(const char *idstr);
extern bool XactHasExportedSnapshots(void);