Avoid SnapshotResetXmin() during AtEOXact_Snapshot()

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

src/backend/access/transam/xact.c

@@ -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)

src/backend/utils/time/snapmgr.c

@@ -1051,7 +1051,7 @@ 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 +1136,17 @@ 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(isPrepare || MyPgXact->xmin == 0);
 }
 
 

src/include/utils/snapmgr.h

@@ -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);