snapshot scalability: Move PGXACT->xmin back to PGPROC.

Now that xmin isn't needed for GetSnapshotData() anymore, it leads to
unnecessary cacheline ping-pong to have it in PGXACT, as it is updated
considerably more frequently than the other PGXACT members.

After the changes in dc7420c2, this is a very straight-forward change.

For highly concurrent, snapshot acquisition heavy, workloads this change alone
can significantly increase scalability. E.g. plain pgbench on a smaller 2
socket machine gains 1.07x for read-only pgbench, 1.22x for read-only pgbench
when submitting queries in batches of 100, and 2.85x for batches of 100
'SELECT';.  The latter numbers are obviously not to be expected in the
real-world, but micro-benchmark the snapshot computation
scalability (previously spending ~80% of the time in GetSnapshotData()).

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de

src/backend/access/gist/gistxlog.c

@@ -389,7 +389,7 @@ gistRedoPageReuse(XLogReaderState *record)
 	 *
 	 * latestRemovedXid was the page's deleteXid.  The
 	 * GlobalVisIsRemovableFullXid(deleteXid) test in gistPageRecyclable()
-	 * conceptually mirrors the pgxact->xmin > limitXmin test in
+	 * conceptually mirrors the PGPROC->xmin > limitXmin test in
 	 * GetConflictingVirtualXIDs().  Consequently, one XID value achieves the
 	 * same exclusion effect on primary and standby.
 	 */

src/backend/access/nbtree/nbtpage.c

@@ -2317,7 +2317,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
 	 * we're in VACUUM and would not otherwise have an XID.  Having already
 	 * updated links to the target, ReadNewTransactionId() suffices as an
 	 * upper bound.  Any scan having retained a now-stale link is advertising
-	 * in its PGXACT an xmin less than or equal to the value we read here.  It
+	 * in its PGPROC an xmin less than or equal to the value we read here.  It
 	 * will continue to do so, holding back the xmin horizon, for the duration
 	 * of that scan.
 	 */

src/backend/access/transam/README

@@ -296,7 +296,7 @@ ensure that the C compiler does exactly what you tell it to.)
 Another important activity that uses the shared ProcArray is
 ComputeXidHorizons, which must determine a lower bound for the oldest xmin
 of any active MVCC snapshot, system-wide.  Each individual backend
-advertises the smallest xmin of its own snapshots in MyPgXact->xmin, or zero
+advertises the smallest xmin of its own snapshots in MyProc->xmin, or zero
 if it currently has no live snapshots (eg, if it's between transactions or
 hasn't yet set a snapshot for a new transaction).  ComputeXidHorizons takes
 the MIN() of the valid xmin fields.  It does this with only shared lock on
@@ -331,7 +331,7 @@ necessary.
 Note that while it is certain that two concurrent executions of
 GetSnapshotData will compute the same xmin for their own snapshots, there is
 no such guarantee for the horizons computed by ComputeXidHorizons.  This is
-because we allow XID-less transactions to clear their MyPgXact->xmin
+because we allow XID-less transactions to clear their MyProc->xmin
 asynchronously (without taking ProcArrayLock), so one execution might see
 what had been the oldest xmin, and another not.  This is OK since the
 thresholds need only be a valid lower bound.  As noted above, we are already

src/backend/access/transam/twophase.c

@@ -464,7 +464,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
 	/* We set up the gxact's VXID as InvalidBackendId/XID */
 	proc->lxid = (LocalTransactionId) xid;
 	pgxact->xid = xid;
-	pgxact->xmin = InvalidTransactionId;
+	Assert(proc->xmin == InvalidTransactionId);
 	proc->delayChkpt = false;
 	pgxact->vacuumFlags = 0;
 	proc->pid = 0;

src/backend/commands/indexcmds.c

@@ -1535,7 +1535,7 @@ DefineIndex(Oid relationId,
 	StartTransactionCommand();
 
 	/* We should now definitely not be advertising any xmin. */
-	Assert(MyPgXact->xmin == InvalidTransactionId);
+	Assert(MyProc->xmin == InvalidTransactionId);
 
 	/*
 	 * The index is now valid in the sense that it contains all currently

src/backend/replication/logical/snapbuild.c

@@ -553,8 +553,8 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
 		elog(ERROR, "cannot build an initial slot snapshot, not all transactions are monitored anymore");
 
 	/* so we don't overwrite the existing value */
-	if (TransactionIdIsValid(MyPgXact->xmin))
-		elog(ERROR, "cannot build an initial slot snapshot when MyPgXact->xmin already is valid");
+	if (TransactionIdIsValid(MyProc->xmin))
+		elog(ERROR, "cannot build an initial slot snapshot when MyProc->xmin already is valid");
 
 	snap = SnapBuildBuildSnapshot(builder);
 
@@ -575,7 +575,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
 	}
 #endif
 
-	MyPgXact->xmin = snap->xmin;
+	MyProc->xmin = snap->xmin;
 
 	/* allocate in transaction context */
 	newxip = (TransactionId *)

src/backend/replication/walsender.c

@@ -1964,7 +1964,7 @@ PhysicalReplicationSlotNewXmin(TransactionId feedbackXmin, TransactionId feedbac
 	ReplicationSlot *slot = MyReplicationSlot;
 
 	SpinLockAcquire(&slot->mutex);
-	MyPgXact->xmin = InvalidTransactionId;
+	MyProc->xmin = InvalidTransactionId;
 
 	/*
 	 * For physical replication we don't need the interlock provided by xmin
@@ -2093,7 +2093,7 @@ ProcessStandbyHSFeedbackMessage(void)
 	if (!TransactionIdIsNormal(feedbackXmin)
 		&& !TransactionIdIsNormal(feedbackCatalogXmin))
 	{
-		MyPgXact->xmin = InvalidTransactionId;
+		MyProc->xmin = InvalidTransactionId;
 		if (MyReplicationSlot != NULL)
 			PhysicalReplicationSlotNewXmin(feedbackXmin, feedbackCatalogXmin);
 		return;
@@ -2135,7 +2135,7 @@ ProcessStandbyHSFeedbackMessage(void)
 	 * risk already since a VACUUM could already have determined the horizon.)
 	 *
 	 * If we're using a replication slot we reserve the xmin via that,
-	 * otherwise via the walsender's PGXACT entry. We can only track the
+	 * otherwise via the walsender's PGPROC entry. We can only track the
 	 * catalog xmin separately when using a slot, so we store the least of the
 	 * two provided when not using a slot.
 	 *
@@ -2148,9 +2148,9 @@ ProcessStandbyHSFeedbackMessage(void)
 	{
 		if (TransactionIdIsNormal(feedbackCatalogXmin)
 			&& TransactionIdPrecedes(feedbackCatalogXmin, feedbackXmin))
-			MyPgXact->xmin = feedbackCatalogXmin;
+			MyProc->xmin = feedbackCatalogXmin;
 		else
-			MyPgXact->xmin = feedbackXmin;
+			MyProc->xmin = feedbackXmin;
 	}
 }
 

src/backend/storage/ipc/procarray.c

@@ -587,9 +587,9 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
 		Assert(!TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
 
 		proc->lxid = InvalidLocalTransactionId;
-		pgxact->xmin = InvalidTransactionId;
 		/* must be cleared with xid/xmin: */
 		pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
+		proc->xmin = InvalidTransactionId;
 		proc->delayChkpt = false;	/* be sure this is cleared in abort */
 		proc->recoveryConflictPending = false;
 
@@ -609,9 +609,9 @@ ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
 {
 	pgxact->xid = InvalidTransactionId;
 	proc->lxid = InvalidLocalTransactionId;
-	pgxact->xmin = InvalidTransactionId;
 	/* must be cleared with xid/xmin: */
 	pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
+	proc->xmin = InvalidTransactionId;
 	proc->delayChkpt = false;	/* be sure this is cleared in abort */
 	proc->recoveryConflictPending = false;
 
@@ -763,7 +763,7 @@ ProcArrayClearTransaction(PGPROC *proc)
 	 */
 	pgxact->xid = InvalidTransactionId;
 	proc->lxid = InvalidLocalTransactionId;
-	pgxact->xmin = InvalidTransactionId;
+	proc->xmin = InvalidTransactionId;
 	proc->recoveryConflictPending = false;
 
 	/* redundant, but just in case */
@@ -1563,7 +1563,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 
 		/* Fetch xid just once - see GetNewTransactionId */
 		xid = UINT32_ACCESS_ONCE(pgxact->xid);
-		xmin = UINT32_ACCESS_ONCE(pgxact->xmin);
+		xmin = UINT32_ACCESS_ONCE(proc->xmin);
 
 		/*
 		 * Consider both the transaction's Xmin, and its Xid.
@@ -1838,7 +1838,7 @@ GetMaxSnapshotSubxidCount(void)
  *
  * We also update the following backend-global variables:
  *		TransactionXmin: the oldest xmin of any snapshot in use in the
- *			current transaction (this is the same as MyPgXact->xmin).
+ *			current transaction (this is the same as MyProc->xmin).
  *		RecentXmin: the xmin computed for the most recent snapshot.  XIDs
  *			older than this are known not running any more.
  *
@@ -1899,7 +1899,7 @@ GetSnapshotData(Snapshot snapshot)
 
 	/*
 	 * It is sufficient to get shared lock on ProcArrayLock, even if we are
-	 * going to set MyPgXact->xmin.
+	 * going to set MyProc->xmin.
 	 */
 	LWLockAcquire(ProcArrayLock, LW_SHARED);
 
@@ -2051,8 +2051,8 @@ GetSnapshotData(Snapshot snapshot)
 	replication_slot_xmin = procArray->replication_slot_xmin;
 	replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;
 
-	if (!TransactionIdIsValid(MyPgXact->xmin))
-		MyPgXact->xmin = TransactionXmin = xmin;
+	if (!TransactionIdIsValid(MyProc->xmin))
+		MyProc->xmin = TransactionXmin = xmin;
 
 	LWLockRelease(ProcArrayLock);
 
@@ -2172,7 +2172,7 @@ GetSnapshotData(Snapshot snapshot)
 }
 
 /*
- * ProcArrayInstallImportedXmin -- install imported xmin into MyPgXact->xmin
+ * ProcArrayInstallImportedXmin -- install imported xmin into MyProc->xmin
  *
  * This is called when installing a snapshot imported from another
  * transaction.  To ensure that OldestXmin doesn't go backwards, we must
@@ -2225,7 +2225,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
 		/*
 		 * Likewise, let's just make real sure its xmin does cover us.
 		 */
-		xid = UINT32_ACCESS_ONCE(pgxact->xmin);
+		xid = UINT32_ACCESS_ONCE(proc->xmin);
 		if (!TransactionIdIsNormal(xid) ||
 			!TransactionIdPrecedesOrEquals(xid, xmin))
 			continue;
@@ -2236,7 +2236,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
 		 * GetSnapshotData first, we'll be overwriting a valid xmin here, so
 		 * we don't check that.)
 		 */
-		MyPgXact->xmin = TransactionXmin = xmin;
+		MyProc->xmin = TransactionXmin = xmin;
 
 		result = true;
 		break;
@@ -2248,7 +2248,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
 }
 
 /*
- * ProcArrayInstallRestoredXmin -- install restored xmin into MyPgXact->xmin
+ * ProcArrayInstallRestoredXmin -- install restored xmin into MyProc->xmin
  *
  * This is like ProcArrayInstallImportedXmin, but we have a pointer to the
  * PGPROC of the transaction from which we imported the snapshot, rather than
@@ -2261,7 +2261,6 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
 {
 	bool		result = false;
 	TransactionId xid;
-	PGXACT	   *pgxact;
 
 	Assert(TransactionIdIsNormal(xmin));
 	Assert(proc != NULL);
@@ -2269,20 +2268,18 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
 	/* Get lock so source xact can't end while we're doing this */
 	LWLockAcquire(ProcArrayLock, LW_SHARED);
 
-	pgxact = &allPgXact[proc->pgprocno];
-
 	/*
 	 * Be certain that the referenced PGPROC has an advertised xmin which is
 	 * no later than the one we're installing, so that the system-wide xmin
 	 * can't go backwards.  Also, make sure it's running in the same database,
 	 * so that the per-database xmin cannot go backwards.
 	 */
-	xid = UINT32_ACCESS_ONCE(pgxact->xmin);
+	xid = UINT32_ACCESS_ONCE(proc->xmin);
 	if (proc->databaseId == MyDatabaseId &&
 		TransactionIdIsNormal(xid) &&
 		TransactionIdPrecedesOrEquals(xid, xmin))
 	{
-		MyPgXact->xmin = TransactionXmin = xmin;
+		MyProc->xmin = TransactionXmin = xmin;
 		result = true;
 	}
 
@@ -2908,7 +2905,7 @@ GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0,
 		if (allDbs || proc->databaseId == MyDatabaseId)
 		{
 			/* Fetch xmin just once - might change on us */
-			TransactionId pxmin = UINT32_ACCESS_ONCE(pgxact->xmin);
+			TransactionId pxmin = UINT32_ACCESS_ONCE(proc->xmin);
 
 			if (excludeXmin0 && !TransactionIdIsValid(pxmin))
 				continue;
@@ -2994,7 +2991,6 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
 		PGPROC	   *proc = &allProcs[pgprocno];
-		PGXACT	   *pgxact = &allPgXact[pgprocno];
 
 		/* Exclude prepared transactions */
 		if (proc->pid == 0)
@@ -3004,7 +3000,7 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
 			proc->databaseId == dbOid)
 		{
 			/* Fetch xmin just once - can't change on us, but good coding */
-			TransactionId pxmin = UINT32_ACCESS_ONCE(pgxact->xmin);
+			TransactionId pxmin = UINT32_ACCESS_ONCE(proc->xmin);
 
 			/*
 			 * We ignore an invalid pxmin because this means that backend has

src/backend/storage/ipc/sinvaladt.c

@@ -420,7 +420,7 @@ BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmi
 			PGXACT	   *xact = &ProcGlobal->allPgXact[proc->pgprocno];
 
 			*xid = xact->xid;
-			*xmin = xact->xmin;
+			*xmin = proc->xmin;
 		}
 	}
 

src/backend/storage/lmgr/proc.c

@@ -388,7 +388,7 @@ InitProcess(void)
 	MyProc->fpVXIDLock = false;
 	MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
 	MyPgXact->xid = InvalidTransactionId;
-	MyPgXact->xmin = InvalidTransactionId;
+	MyProc->xmin = InvalidTransactionId;
 	MyProc->pid = MyProcPid;
 	/* backendId, databaseId and roleId will be filled in later */
 	MyProc->backendId = InvalidBackendId;
@@ -572,7 +572,7 @@ InitAuxiliaryProcess(void)
 	MyProc->fpVXIDLock = false;
 	MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
 	MyPgXact->xid = InvalidTransactionId;
-	MyPgXact->xmin = InvalidTransactionId;
+	MyProc->xmin = InvalidTransactionId;
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
 	MyProc->roleId = InvalidOid;

src/backend/utils/time/snapmgr.c

@@ -27,11 +27,11 @@
  * their lifetime is managed separately (as they live longer than one xact.c
  * transaction).
  *
- * These arrangements let us reset MyPgXact->xmin when there are no snapshots
+ * These arrangements let us reset MyProc->xmin when there are no snapshots
  * referenced by this transaction, and advance it when the one with oldest
  * Xmin is no longer referenced.  For simplicity however, only registered
  * snapshots not active snapshots participate in tracking which one is oldest;
- * we don't try to change MyPgXact->xmin except when the active-snapshot
+ * we don't try to change MyProc->xmin except when the active-snapshot
  * stack is empty.
  *
  *
@@ -187,7 +187,7 @@ static ActiveSnapshotElt *OldestActiveSnapshot = NULL;
 
 /*
  * Currently registered Snapshots.  Ordered in a heap by xmin, so that we can
- * quickly find the one with lowest xmin, to advance our MyPgXact->xmin.
+ * quickly find the one with lowest xmin, to advance our MyProc->xmin.
  */
 static int	xmin_cmp(const pairingheap_node *a, const pairingheap_node *b,
 					 void *arg);
@@ -475,7 +475,7 @@ GetNonHistoricCatalogSnapshot(Oid relid)
 
 		/*
 		 * Make sure the catalog snapshot will be accounted for in decisions
-		 * about advancing PGXACT->xmin.  We could apply RegisterSnapshot, but
+		 * about advancing PGPROC->xmin.  We could apply RegisterSnapshot, but
 		 * that would result in making a physical copy, which is overkill; and
 		 * it would also create a dependency on some resource owner, which we
 		 * do not want for reasons explained at the head of this file. Instead
@@ -596,7 +596,7 @@ SetTransactionSnapshot(Snapshot sourcesnap, VirtualTransactionId *sourcevxid,
 	/* NB: curcid should NOT be copied, it's a local matter */
 
 	/*
-	 * Now we have to fix what GetSnapshotData did with MyPgXact->xmin and
+	 * Now we have to fix what GetSnapshotData did with MyProc->xmin and
 	 * TransactionXmin.  There is a race condition: to make sure we are not
 	 * causing the global xmin to go backwards, we have to test that the
 	 * source transaction is still running, and that has to be done
@@ -950,13 +950,13 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
 /*
  * SnapshotResetXmin
  *
- * If there are no more snapshots, we can reset our PGXACT->xmin to InvalidXid.
+ * If there are no more snapshots, we can reset our PGPROC->xmin to InvalidXid.
  * Note we can do this without locking because we assume that storing an Xid
  * is atomic.
  *
  * 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
- * dropped.  For efficiency, we only consider recomputing PGXACT->xmin when
+ * PGPROC->xmin to some degree.  This typically happens when a portal is
+ * dropped.  For efficiency, we only consider recomputing PGPROC->xmin when
  * the active snapshot stack is empty; this allows us not to need to track
  * which active snapshot is oldest.
  *
@@ -977,15 +977,15 @@ SnapshotResetXmin(void)
 
 	if (pairingheap_is_empty(&RegisteredSnapshots))
 	{
-		MyPgXact->xmin = InvalidTransactionId;
+		MyProc->xmin = InvalidTransactionId;
 		return;
 	}
 
 	minSnapshot = pairingheap_container(SnapshotData, ph_node,
 										pairingheap_first(&RegisteredSnapshots));
 
-	if (TransactionIdPrecedes(MyPgXact->xmin, minSnapshot->xmin))
-		MyPgXact->xmin = minSnapshot->xmin;
+	if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
+		MyProc->xmin = minSnapshot->xmin;
 }
 
 /*
@@ -1132,13 +1132,13 @@ AtEOXact_Snapshot(bool isCommit, bool resetXmin)
 
 	/*
 	 * During normal commit processing, we call ProcArrayEndTransaction() to
-	 * reset the MyPgXact->xmin. That call happens prior to the call to
+	 * reset the MyProc->xmin. That call happens prior to the call to
 	 * AtEOXact_Snapshot(), so we need not touch xmin here at all.
 	 */
 	if (resetXmin)
 		SnapshotResetXmin();
 
-	Assert(resetXmin || MyPgXact->xmin == 0);
+	Assert(resetXmin || MyProc->xmin == 0);
 }
 
 
@@ -1830,7 +1830,7 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
 	 */
 	if (old_snapshot_threshold == 0)
 	{
-		if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
+		if (TransactionIdPrecedes(latest_xmin, MyProc->xmin)
 			&& TransactionIdFollows(latest_xmin, xlimit))
 			xlimit = latest_xmin;
 

src/include/storage/proc.h

@@ -101,6 +101,11 @@ struct PGPROC
 
 	Latch		procLatch;		/* generic latch for process */
 
+	TransactionId xmin;			/* minimal running XID as it was when we were
+								 * starting our xact, excluding LAZY VACUUM:
+								 * vacuum must not remove tuples deleted by
+								 * xid >= xmin ! */
+
 	LocalTransactionId lxid;	/* local id of top-level transaction currently
 								 * being executed by this proc, if running;
 								 * else InvalidLocalTransactionId */
@@ -223,11 +228,6 @@ typedef struct PGXACT
 								 * executed by this proc, if running and XID
 								 * is assigned; else InvalidTransactionId */
 
-	TransactionId xmin;			/* minimal running XID as it was when we were
-								 * starting our xact, excluding LAZY VACUUM:
-								 * vacuum must not remove tuples deleted by
-								 * xid >= xmin ! */
-
 	uint8		vacuumFlags;	/* vacuum-related flags, see above */
 	bool		overflowed;