snapshot scalability: Move delayChkpt from PGXACT to PGPROC.

The goal of separating hotly accessed per-backend data from PGPROC
into PGXACT is to make accesses fast (GetSnapshotData() in
particular). But delayChkpt is not actually accessed frequently; only
when starting a checkpoint. As it is frequently modified (multiple
times in the course of a single transaction), storing it in the same
cacheline as hotly accessed data unnecessarily dirties a contended
cacheline.

Therefore move delayChkpt to PGPROC.

This is part of a larger series of patches intending to improve
GetSnapshotData() scalability. It is committed and pushed separately,
as it is independently beneficial (small but measurable win, limited
by the other frequent modifications of PGXACT).

Author: Andres Freund
Reviewed-By: Robert Haas, Thomas Munro, David Rowley
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de

src/backend/access/transam/multixact.c

@@ -3058,8 +3058,8 @@ TruncateMultiXact(MultiXactId newOldestMulti, Oid newOldestMultiDB)
 	 * crash/basebackup, even though the state of the data directory would
 	 * require it.
 	 */
-	Assert(!MyPgXact->delayChkpt);
-	MyPgXact->delayChkpt = true;
+	Assert(!MyProc->delayChkpt);
+	MyProc->delayChkpt = true;
 
 	/* WAL log truncation */
 	WriteMTruncateXlogRec(newOldestMultiDB,
@@ -3085,7 +3085,7 @@ TruncateMultiXact(MultiXactId newOldestMulti, Oid newOldestMultiDB)
 	/* Then offsets */
 	PerformOffsetsTruncation(oldestMulti, newOldestMulti);
 
-	MyPgXact->delayChkpt = false;
+	MyProc->delayChkpt = false;
 
 	END_CRIT_SECTION();
 	LWLockRelease(MultiXactTruncationLock);

src/backend/access/transam/twophase.c

@@ -465,7 +465,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
 	proc->lxid = (LocalTransactionId) xid;
 	pgxact->xid = xid;
 	pgxact->xmin = InvalidTransactionId;
-	pgxact->delayChkpt = false;
+	proc->delayChkpt = false;
 	pgxact->vacuumFlags = 0;
 	proc->pid = 0;
 	proc->backendId = InvalidBackendId;
@@ -1114,7 +1114,7 @@ EndPrepare(GlobalTransaction gxact)
 
 	START_CRIT_SECTION();
 
-	MyPgXact->delayChkpt = true;
+	MyProc->delayChkpt = true;
 
 	XLogBeginInsert();
 	for (record = records.head; record != NULL; record = record->next)
@@ -1157,7 +1157,7 @@ EndPrepare(GlobalTransaction gxact)
 	 * checkpoint starting after this will certainly see the gxact as a
 	 * candidate for fsyncing.
 	 */
-	MyPgXact->delayChkpt = false;
+	MyProc->delayChkpt = false;
 
 	/*
 	 * Remember that we have this GlobalTransaction entry locked for us.  If
@@ -2204,7 +2204,7 @@ RecordTransactionCommitPrepared(TransactionId xid,
 	START_CRIT_SECTION();
 
 	/* See notes in RecordTransactionCommit */
-	MyPgXact->delayChkpt = true;
+	MyProc->delayChkpt = true;
 
 	/*
 	 * Emit the XLOG commit record. Note that we mark 2PC commits as
@@ -2252,7 +2252,7 @@ RecordTransactionCommitPrepared(TransactionId xid,
 	TransactionIdCommitTree(xid, nchildren, children);
 
 	/* Checkpoint can proceed now */
-	MyPgXact->delayChkpt = false;
+	MyProc->delayChkpt = false;
 
 	END_CRIT_SECTION();
 

src/backend/access/transam/xact.c

@@ -1307,7 +1307,7 @@ RecordTransactionCommit(void)
 		 * a bit fuzzy, but it doesn't matter.
 		 */
 		START_CRIT_SECTION();
-		MyPgXact->delayChkpt = true;
+		MyProc->delayChkpt = true;
 
 		SetCurrentTransactionStopTimestamp();
 
@@ -1408,7 +1408,7 @@ RecordTransactionCommit(void)
 	 */
 	if (markXidCommitted)
 	{
-		MyPgXact->delayChkpt = false;
+		MyProc->delayChkpt = false;
 		END_CRIT_SECTION();
 	}
 

src/backend/access/transam/xloginsert.c

@@ -904,7 +904,7 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 	/*
 	 * Ensure no checkpoint can change our view of RedoRecPtr.
 	 */
-	Assert(MyPgXact->delayChkpt);
+	Assert(MyProc->delayChkpt);
 
 	/*
 	 * Update RedoRecPtr so that we can make the right decision

src/backend/storage/buffer/bufmgr.c

@@ -3587,7 +3587,7 @@ MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
 			 * essential that CreateCheckpoint waits for virtual transactions
 			 * rather than full transactionids.
 			 */
-			MyPgXact->delayChkpt = delayChkpt = true;
+			MyProc->delayChkpt = delayChkpt = true;
 			lsn = XLogSaveBufferForHint(buffer, buffer_std);
 		}
 
@@ -3620,7 +3620,7 @@ MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
 		UnlockBufHdr(bufHdr, buf_state);
 
 		if (delayChkpt)
-			MyPgXact->delayChkpt = false;
+			MyProc->delayChkpt = false;
 
 		if (dirtied)
 		{

src/backend/storage/ipc/procarray.c

@@ -436,7 +436,7 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
 		pgxact->xmin = InvalidTransactionId;
 		/* must be cleared with xid/xmin: */
 		pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
-		pgxact->delayChkpt = false; /* be sure this is cleared in abort */
+		proc->delayChkpt = false; /* be sure this is cleared in abort */
 		proc->recoveryConflictPending = false;
 
 		Assert(pgxact->nxids == 0);
@@ -458,7 +458,7 @@ ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
 	pgxact->xmin = InvalidTransactionId;
 	/* must be cleared with xid/xmin: */
 	pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
-	pgxact->delayChkpt = false; /* be sure this is cleared in abort */
+	proc->delayChkpt = false; /* be sure this is cleared in abort */
 	proc->recoveryConflictPending = false;
 
 	/* Clear the subtransaction-XID cache too while holding the lock */
@@ -616,7 +616,7 @@ ProcArrayClearTransaction(PGPROC *proc)
 
 	/* redundant, but just in case */
 	pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
-	pgxact->delayChkpt = false;
+	proc->delayChkpt = false;
 
 	/* Clear the subtransaction-XID cache too */
 	pgxact->nxids = 0;
@@ -2257,7 +2257,7 @@ GetOldestSafeDecodingTransactionId(bool catalogOnly)
  * delaying checkpoint because they have critical actions in progress.
  *
  * Constructs an array of VXIDs of transactions that are currently in commit
- * critical sections, as shown by having delayChkpt set in their PGXACT.
+ * critical sections, as shown by having delayChkpt set in their PGPROC.
  *
  * Returns a palloc'd array that should be freed by the caller.
  * *nvxids is the number of valid entries.
@@ -2288,9 +2288,8 @@ GetVirtualXIDsDelayingChkpt(int *nvxids)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
 		PGPROC	   *proc = &allProcs[pgprocno];
-		PGXACT	   *pgxact = &allPgXact[pgprocno];
 
-		if (pgxact->delayChkpt)
+		if (proc->delayChkpt)
 		{
 			VirtualTransactionId vxid;
 
@@ -2328,12 +2327,11 @@ HaveVirtualXIDsDelayingChkpt(VirtualTransactionId *vxids, int nvxids)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
 		PGPROC	   *proc = &allProcs[pgprocno];
-		PGXACT	   *pgxact = &allPgXact[pgprocno];
 		VirtualTransactionId vxid;
 
 		GET_VXID_FROM_PGPROC(vxid, *proc);
 
-		if (pgxact->delayChkpt && VirtualTransactionIdIsValid(vxid))
+		if (proc->delayChkpt && VirtualTransactionIdIsValid(vxid))
 		{
 			int			i;
 

src/backend/storage/lmgr/proc.c

@@ -397,7 +397,7 @@ InitProcess(void)
 	MyProc->roleId = InvalidOid;
 	MyProc->tempNamespaceId = InvalidOid;
 	MyProc->isBackgroundWorker = IsBackgroundWorker;
-	MyPgXact->delayChkpt = false;
+	MyProc->delayChkpt = false;
 	MyPgXact->vacuumFlags = 0;
 	/* NB -- autovac launcher intentionally does not set IS_AUTOVACUUM */
 	if (IsAutoVacuumWorkerProcess())
@@ -579,7 +579,7 @@ InitAuxiliaryProcess(void)
 	MyProc->roleId = InvalidOid;
 	MyProc->tempNamespaceId = InvalidOid;
 	MyProc->isBackgroundWorker = IsBackgroundWorker;
-	MyPgXact->delayChkpt = false;
+	MyProc->delayChkpt = false;
 	MyPgXact->vacuumFlags = 0;
 	MyProc->lwWaiting = false;
 	MyProc->lwWaitMode = 0;

src/include/storage/proc.h

@@ -142,6 +142,8 @@ struct PGPROC
 	LOCKMASK	heldLocks;		/* bitmask for lock types already held on this
 								 * lock object by this backend */
 
+	bool		delayChkpt;		/* true if this proc delays checkpoint start */
+
 	/*
 	 * Info to allow us to wait for synchronous replication, if needed.
 	 * waitLSN is InvalidXLogRecPtr if not waiting; set only by user backend.
@@ -232,8 +234,6 @@ typedef struct PGXACT
 
 	uint8		vacuumFlags;	/* vacuum-related flags, see above */
 	bool		overflowed;
-	bool		delayChkpt;		/* true if this proc delays checkpoint start;
-								 * previously called InCommit */
 
 	uint8		nxids;
 } PGXACT;