Subject: [PATCH v13 5/6] snapshot scalability: Move subxact info to ProcGlobal, remove PGXACT.

From: Andres Freund <[email-masked]>
Date: Wed, 15 Jul 2020 15:35:07 -0700

Similar to the previous changes this increases the chance that data
frequently needed by GetSnapshotData() stays in l2 cache. In many
workloads subtransactions are very rare, and this makes the check for
that considerably cheaper.

As this removes the last member of PGXACT, there is no need to keep it
around anymore.

Author: Andres Freund
Reviewed-By: Robert Haas, Thomas Munro, David Rowley
Discussion: https://postgr.es/m/[email-masked]
---
 src/include/storage/proc.h            |  34 ++++---
 src/backend/access/transam/clog.c     |   7 +-
 src/backend/access/transam/twophase.c |  17 ++--
 src/backend/access/transam/varsup.c   |  15 ++-
 src/backend/storage/ipc/procarray.c   | 128 ++++++++++++++------------
 src/backend/storage/lmgr/proc.c       |  24 +----
 src/tools/pgindent/typedefs.list      |   1 -
 7 files changed, 113 insertions(+), 113 deletions(-)

diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index ea95cf92402..43aa234709e 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -35,6 +35,14 @@
  */
 #define PGPROC_MAX_CACHED_SUBXIDS 64	/* XXX guessed-at value */
 
+typedef struct XidCacheStatus
+{
+	/* number of cached subxids, never more than PGPROC_MAX_CACHED_SUBXIDS */
+	uint8	count;
+	/* has PGPROC->subxids overflowed */
+	bool	overflowed;
+} XidCacheStatus;
+
 struct XidCache
 {
 	TransactionId xids[PGPROC_MAX_CACHED_SUBXIDS];
@@ -187,6 +195,8 @@ struct PGPROC
 	 */
 	SHM_QUEUE	myProcLocks[NUM_LOCK_PARTITIONS];
 
+	XidCacheStatus subxidStatus; /* mirrored with
+								  * ProcGlobal->subxidStates[i] */
 	struct XidCache subxids;	/* cache for subtransaction XIDs */
 
 	/* Support for group XID clearing. */
@@ -235,22 +245,6 @@ struct PGPROC

 extern PGDLLIMPORT PGPROC *MyProc;
-extern PGDLLIMPORT struct PGXACT *MyPgXact;
-
-/*
- * Prior to PostgreSQL 9.2, the fields below were stored as part of the
- * PGPROC.  However, benchmarking revealed that packing these particular
- * members into a separate array as tightly as possible sped up GetSnapshotData
- * considerably on systems with many CPU cores, by reducing the number of
- * cache lines needing to be fetched.  Thus, think very carefully before adding
- * anything else here.
- */
-typedef struct PGXACT
-{
-	bool		overflowed;
-
-	uint8		nxids;
-} PGXACT;
 
 /*
  * There is one ProcGlobal struct for the whole database cluster.
@@ -310,12 +304,16 @@ typedef struct PROC_HDR
 {
 	/* Array of PGPROC structures (not including dummies for prepared txns) */
 	PGPROC	   *allProcs;
-	/* Array of PGXACT structures (not including dummies for prepared txns) */
-	PGXACT	   *allPgXact;
 
 	/* Array mirroring PGPROC.xid for each PGPROC currently in the procarray */
 	TransactionId *xids;
 
+	/*
+	 * Array mirroring PGPROC.subxidStatus for each PGPROC currently in the
+	 * procarray.
+	 */
+	XidCacheStatus *subxidStates;
+
 	/*
 	 * Array mirroring PGPROC.vacuumFlags for each PGPROC currently in the
 	 * procarray.
diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index a4599e96610..65aa8841f7c 100644
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -295,7 +295,7 @@ TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
 	 */
 	if (all_xact_same_page && xid == MyProc->xid &&
 		nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
-		nsubxids == MyPgXact->nxids &&
+		nsubxids == MyProc->subxidStatus.count &&
 		memcmp(subxids, MyProc->subxids.xids,
 			   nsubxids * sizeof(TransactionId)) == 0)
 	{
@@ -510,16 +510,15 @@ TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
 	while (nextidx != INVALID_PGPROCNO)
 	{
 		PGPROC	   *proc = &ProcGlobal->allProcs[nextidx];
-		PGXACT	   *pgxact = &ProcGlobal->allPgXact[nextidx];
 
 		/*
 		 * Transactions with more than THRESHOLD_SUBTRANS_CLOG_OPT sub-XIDs
 		 * should not use group XID status update mechanism.
 		 */
-		Assert(pgxact->nxids <= THRESHOLD_SUBTRANS_CLOG_OPT);
+		Assert(proc->subxidStatus.count <= THRESHOLD_SUBTRANS_CLOG_OPT);
 
 		TransactionIdSetPageStatusInternal(proc->clogGroupMemberXid,
-										   pgxact->nxids,
+										   proc->subxidStatus.count,
 										   proc->subxids.xids,
 										   proc->clogGroupMemberXidStatus,
 										   proc->clogGroupMemberLsn,
diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 744b8a7f393..76465ad2c8b 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -21,9 +21,9 @@
  *		GIDs and aborts the transaction if there already is a global
  *		transaction in prepared state with the same GID.
  *
- *		A global transaction (gxact) also has dummy PGXACT and PGPROC; this is
- *		what keeps the XID considered running by TransactionIdIsInProgress.
- *		It is also convenient as a PGPROC to hook the gxact's locks to.
+ *		A global transaction (gxact) also has dummy PGPROC; this is what keeps
+ *		the XID considered running by TransactionIdIsInProgress.  It is also
+ *		convenient as a PGPROC to hook the gxact's locks to.
  *
  *		Information to recover prepared transactions in case of crash is
  *		now stored in WAL for the common case. In some cases there will be
@@ -447,14 +447,12 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
 					TimestampTz prepared_at, Oid owner, Oid databaseid)
 {
 	PGPROC	   *proc;
-	PGXACT	   *pgxact;
 	int			i;
 
 	Assert(LWLockHeldByMeInMode(TwoPhaseStateLock, LW_EXCLUSIVE));
 
 	Assert(gxact != NULL);
 	proc = &ProcGlobal->allProcs[gxact->pgprocno];
-	pgxact = &ProcGlobal->allPgXact[gxact->pgprocno];
 
 	/* Initialize the PGPROC entry */
 	MemSet(proc, 0, sizeof(PGPROC));
@@ -480,8 +478,8 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
 	for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
 		SHMQueueInit(&(proc->myProcLocks[i]));
 	/* subxid data must be filled later by GXactLoadSubxactData */
-	pgxact->overflowed = false;
-	pgxact->nxids = 0;
+	proc->subxidStatus.count = 0;
+	proc->subxidStatus.overflowed = 0;
 
 	gxact->prepared_at = prepared_at;
 	gxact->xid = xid;
@@ -510,19 +508,18 @@ GXactLoadSubxactData(GlobalTransaction gxact, int nsubxacts,
 					 TransactionId *children)
 {
 	PGPROC	   *proc = &ProcGlobal->allProcs[gxact->pgprocno];
-	PGXACT	   *pgxact = &ProcGlobal->allPgXact[gxact->pgprocno];
 
 	/* We need no extra lock since the GXACT isn't valid yet */
 	if (nsubxacts > PGPROC_MAX_CACHED_SUBXIDS)
 	{
-		pgxact->overflowed = true;
+		proc->subxidStatus.overflowed = true;
 		nsubxacts = PGPROC_MAX_CACHED_SUBXIDS;
 	}
 	if (nsubxacts > 0)
 	{
 		memcpy(proc->subxids.xids, children,
 			   nsubxacts * sizeof(TransactionId));
-		pgxact->nxids = nsubxacts;
+		proc->subxidStatus.count = PGPROC_MAX_CACHED_SUBXIDS;
 	}
 }
 
diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index 4c91b343ecd..2d2b05be36c 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -222,22 +222,31 @@ GetNewTransactionId(bool isSubXact)
 	 */
 	if (!isSubXact)
 	{
+		Assert(ProcGlobal->subxidStates[MyProc->pgxactoff].count == 0);
+		Assert(!ProcGlobal->subxidStates[MyProc->pgxactoff].overflowed);
+		Assert(MyProc->subxidStatus.count == 0);
+		Assert(!MyProc->subxidStatus.overflowed);
+
 		/* LWLockRelease acts as barrier */
 		MyProc->xid = xid;
 		ProcGlobal->xids[MyProc->pgxactoff] = xid;
 	}
 	else
 	{
-		int			nxids = MyPgXact->nxids;
+		XidCacheStatus *substat = &ProcGlobal->subxidStates[MyProc->pgxactoff];
+		int			nxids = MyProc->subxidStatus.count;
+
+		Assert(substat->count == MyProc->subxidStatus.count);
+		Assert(substat->overflowed == MyProc->subxidStatus.overflowed);
 
 		if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
 		{
 			MyProc->subxids.xids[nxids] = xid;
 			pg_write_barrier();
-			MyPgXact->nxids = nxids + 1;
+			MyProc->subxidStatus.count = substat->count = nxids + 1;
 		}
 		else
-			MyPgXact->overflowed = true;
+			MyProc->subxidStatus.overflowed = substat->overflowed = true;
 	}
 
 	LWLockRelease(XidGenLock);
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index e77d4e44fb8..8e8049d9715 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -4,9 +4,10 @@
  *	  POSTGRES process array code.
  *
  *
- * This module maintains arrays of the PGPROC and PGXACT structures for all
- * active backends.  Although there are several uses for this, the principal
- * one is as a means of determining the set of currently running transactions.
+ * This module maintains arrays of PGPROC substructures, as well as associated
+ * arrays in ProcGlobal, for all active backends.  Although there are several
+ * uses for this, the principal one is as a means of determining the set of
+ * currently running transactions.
  *
  * Because of various subtle race conditions it is critical that a backend
  * hold the correct locks while setting or clearing its xid (in
@@ -85,7 +86,7 @@ typedef struct ProcArrayStruct
 	/*
 	 * Highest subxid that has been removed from KnownAssignedXids array to
 	 * prevent overflow; or InvalidTransactionId if none.  We track this for
-	 * similar reasons to tracking overflowing cached subxids in PGXACT
+	 * similar reasons to tracking overflowing cached subxids in PGPROC
 	 * entries.  Must hold exclusive ProcArrayLock to change this, and shared
 	 * lock to read it.
 	 */
@@ -96,7 +97,7 @@ typedef struct ProcArrayStruct
 	/* oldest catalog xmin of any replication slot */
 	TransactionId replication_slot_catalog_xmin;
 
-	/* indexes into allPgXact[], has PROCARRAY_MAXPROCS entries */
+	/* indexes into allProcs[], has PROCARRAY_MAXPROCS entries */
 	int			pgprocnos[FLEXIBLE_ARRAY_MEMBER];
 } ProcArrayStruct;
 
@@ -239,7 +240,6 @@ typedef struct ComputeXidHorizonsResult
 static ProcArrayStruct *procArray;
 
 static PGPROC *allProcs;
-static PGXACT *allPgXact;
 
 /*
  * Bookkeeping for tracking emulated transactions in recovery
@@ -325,8 +325,7 @@ static int	KnownAssignedXidsGetAndSetXmin(TransactionId *xarray,
 static TransactionId KnownAssignedXidsGetOldestXmin(void);
 static void KnownAssignedXidsDisplay(int trace_level);
 static void KnownAssignedXidsReset(void);
-static inline void ProcArrayEndTransactionInternal(PGPROC *proc,
-												   PGXACT *pgxact, TransactionId latestXid);
+static inline void ProcArrayEndTransactionInternal(PGPROC *proc, TransactionId latestXid);
 static void ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid);
 static void MaintainLatestCompletedXid(TransactionId latestXid);
 static void MaintainLatestCompletedXidRecovery(TransactionId latestXid);
@@ -411,7 +410,6 @@ CreateSharedProcArray(void)
 	}
 
 	allProcs = ProcGlobal->allProcs;
-	allPgXact = ProcGlobal->allPgXact;
 
 	/* Create or attach to the KnownAssignedXids arrays too, if needed */
 	if (EnableHotStandby)
@@ -476,11 +474,14 @@ ProcArrayAdd(PGPROC *proc)
 			(arrayP->numProcs - index) * sizeof(*arrayP->pgprocnos));
 	memmove(&ProcGlobal->xids[index + 1], &ProcGlobal->xids[index],
 			(arrayP->numProcs - index) * sizeof(*ProcGlobal->xids));
+	memmove(&ProcGlobal->subxidStates[index + 1], &ProcGlobal->subxidStates[index],
+			(arrayP->numProcs - index) * sizeof(*ProcGlobal->subxidStates));
 	memmove(&ProcGlobal->vacuumFlags[index + 1], &ProcGlobal->vacuumFlags[index],
 			(arrayP->numProcs - index) * sizeof(*ProcGlobal->vacuumFlags));
 
 	arrayP->pgprocnos[index] = proc->pgprocno;
 	ProcGlobal->xids[index] = proc->xid;
+	ProcGlobal->subxidStates[index] = proc->subxidStatus;
 	ProcGlobal->vacuumFlags[index] = proc->vacuumFlags;
 
 	arrayP->numProcs++;
@@ -534,6 +535,8 @@ ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
 		MaintainLatestCompletedXid(latestXid);
 
 		ProcGlobal->xids[proc->pgxactoff] = 0;
+		ProcGlobal->subxidStates[proc->pgxactoff].overflowed = false;
+		ProcGlobal->subxidStates[proc->pgxactoff].count = 0;
 	}
 	else
 	{
@@ -542,6 +545,8 @@ ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
 	}
 
 	Assert(TransactionIdIsValid(ProcGlobal->xids[proc->pgxactoff] == 0));
+	Assert(TransactionIdIsValid(ProcGlobal->subxidStates[proc->pgxactoff].count == 0));
+	Assert(TransactionIdIsValid(ProcGlobal->subxidStates[proc->pgxactoff].overflowed == false));
 	ProcGlobal->vacuumFlags[proc->pgxactoff] = 0;
 
 	for (index = 0; index < arrayP->numProcs; index++)
@@ -553,6 +558,8 @@ ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
 					(arrayP->numProcs - index - 1) * sizeof(*arrayP->pgprocnos));
 			memmove(&ProcGlobal->xids[index], &ProcGlobal->xids[index + 1],
 					(arrayP->numProcs - index - 1) * sizeof(*ProcGlobal->xids));
+			memmove(&ProcGlobal->subxidStates[index], &ProcGlobal->subxidStates[index + 1],
+					(arrayP->numProcs - index - 1) * sizeof(*ProcGlobal->subxidStates));
 			memmove(&ProcGlobal->vacuumFlags[index], &ProcGlobal->vacuumFlags[index + 1],
 					(arrayP->numProcs - index - 1) * sizeof(*ProcGlobal->vacuumFlags));
 
@@ -598,8 +605,6 @@ ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
 void
 ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
 {
-	PGXACT	   *pgxact = &allPgXact[proc->pgprocno];
-
 	if (TransactionIdIsValid(latestXid))
 	{
 		/*
@@ -617,7 +622,7 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
 		 */
 		if (LWLockConditionalAcquire(ProcArrayLock, LW_EXCLUSIVE))
 		{
-			ProcArrayEndTransactionInternal(proc, pgxact, latestXid);
+			ProcArrayEndTransactionInternal(proc, latestXid);
 			LWLockRelease(ProcArrayLock);
 		}
 		else
@@ -631,15 +636,14 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
 		 * estimate of global xmin, but that's OK.
 		 */
 		Assert(!TransactionIdIsValid(proc->xid));
+		Assert(proc->subxidStatus.count == 0);
+		Assert(!proc->subxidStatus.overflowed);
 
 		proc->lxid = InvalidLocalTransactionId;
 		proc->xmin = InvalidTransactionId;
 		proc->delayChkpt = false;	/* be sure this is cleared in abort */
 		proc->recoveryConflictPending = false;
 
-		Assert(pgxact->nxids == 0);
-		Assert(pgxact->overflowed == false);
-
 		/* must be cleared with xid/xmin: */
 		/* avoid unnecessarily dirtying shared cachelines */
 		if (proc->vacuumFlags & PROC_VACUUM_STATE_MASK)
@@ -660,8 +664,7 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
  * We don't do any locking here; caller must handle that.
  */
 static inline void
-ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
-								TransactionId latestXid)
+ProcArrayEndTransactionInternal(PGPROC *proc, TransactionId latestXid)
 {
 	size_t		pgxactoff = proc->pgxactoff;
 
@@ -684,8 +687,15 @@ ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
 	}
 
 	/* Clear the subtransaction-XID cache too while holding the lock */
-	pgxact->nxids = 0;
-	pgxact->overflowed = false;
+	Assert(ProcGlobal->subxidStates[pgxactoff].count == proc->subxidStatus.count &&
+		   ProcGlobal->subxidStates[pgxactoff].overflowed == proc->subxidStatus.overflowed);
+	if (proc->subxidStatus.count > 0 || proc->subxidStatus.overflowed)
+	{
+		ProcGlobal->subxidStates[pgxactoff].count = 0;
+		ProcGlobal->subxidStates[pgxactoff].overflowed = false;
+		proc->subxidStatus.count = 0;
+		proc->subxidStatus.overflowed = false;
+	}
 
 	/* Also advance global latestCompletedXid while holding the lock */
 	MaintainLatestCompletedXid(latestXid);
@@ -775,9 +785,8 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	while (nextidx != INVALID_PGPROCNO)
 	{
 		PGPROC	   *proc = &allProcs[nextidx];
-		PGXACT	   *pgxact = &allPgXact[nextidx];
 
-		ProcArrayEndTransactionInternal(proc, pgxact, proc->procArrayGroupMemberXid);
+		ProcArrayEndTransactionInternal(proc, proc->procArrayGroupMemberXid);
 
 		/* Move to next proc in list. */
 		nextidx = pg_atomic_read_u32(&proc->procArrayGroupNext);
@@ -821,7 +830,6 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 void
 ProcArrayClearTransaction(PGPROC *proc)
 {
-	PGXACT	   *pgxact = &allPgXact[proc->pgprocno];
 	size_t		pgxactoff;
 
 	/*
@@ -846,8 +854,15 @@ ProcArrayClearTransaction(PGPROC *proc)
 	Assert(!proc->delayChkpt);
 
 	/* Clear the subtransaction-XID cache too */
-	pgxact->nxids = 0;
-	pgxact->overflowed = false;
+	Assert(ProcGlobal->subxidStates[pgxactoff].count == proc->subxidStatus.count &&
+		   ProcGlobal->subxidStates[pgxactoff].overflowed == proc->subxidStatus.overflowed);
+	if (proc->subxidStatus.count > 0 || proc->subxidStatus.overflowed)
+	{
+		ProcGlobal->subxidStates[pgxactoff].count = 0;
+		ProcGlobal->subxidStates[pgxactoff].overflowed = false;
+		proc->subxidStatus.count = 0;
+		proc->subxidStatus.overflowed = false;
+	}
 
 	LWLockRelease(ProcArrayLock);
 }
@@ -1268,6 +1283,7 @@ TransactionIdIsInProgress(TransactionId xid)
 {
 	static TransactionId *xids = NULL;
 	static TransactionId *other_xids;
+	XidCacheStatus *other_subxidstates;
 	int			nxids = 0;
 	ProcArrayStruct *arrayP = procArray;
 	TransactionId topxid;
@@ -1330,6 +1346,7 @@ TransactionIdIsInProgress(TransactionId xid)
 	}
 
 	other_xids = ProcGlobal->xids;
+	other_subxidstates = ProcGlobal->subxidStates;
 
 	LWLockAcquire(ProcArrayLock, LW_SHARED);
 
@@ -1352,7 +1369,6 @@ TransactionIdIsInProgress(TransactionId xid)
 	for (size_t pgxactoff = 0; pgxactoff < numProcs; pgxactoff++)
 	{
 		int			pgprocno;
-		PGXACT	   *pgxact;
 		PGPROC	   *proc;
 		TransactionId pxid;
 		int			pxids;
@@ -1387,9 +1403,7 @@ TransactionIdIsInProgress(TransactionId xid)
 		/*
 		 * Step 2: check the cached child-Xids arrays
 		 */
-		pgprocno = arrayP->pgprocnos[pgxactoff];
-		pgxact = &allPgXact[pgprocno];
-		pxids = pgxact->nxids;
+		pxids = other_subxidstates[pgxactoff].count;
 		pg_read_barrier();		/* pairs with barrier in GetNewTransactionId() */
 		pgprocno = arrayP->pgprocnos[pgxactoff];
 		proc = &allProcs[pgprocno];
@@ -1413,7 +1427,7 @@ TransactionIdIsInProgress(TransactionId xid)
 		 * we hold ProcArrayLock.  So we can't miss an Xid that we need to
 		 * worry about.)
 		 */
-		if (pgxact->overflowed)
+		if (other_subxidstates[pgxactoff].overflowed)
 			xids[nxids++] = pxid;
 	}
 
@@ -2017,6 +2031,7 @@ GetSnapshotData(Snapshot snapshot)
 		size_t		numProcs = arrayP->numProcs;
 		TransactionId *xip = snapshot->xip;
 		int		   *pgprocnos = arrayP->pgprocnos;
+		XidCacheStatus *subxidStates = ProcGlobal->subxidStates;
 		uint8	   *allVacuumFlags = ProcGlobal->vacuumFlags;
 
 		/*
@@ -2093,17 +2108,16 @@ GetSnapshotData(Snapshot snapshot)
 			 */
 			if (!suboverflowed)
 			{
-				int			pgprocno = pgprocnos[pgxactoff];
-				PGXACT	   *pgxact = &allPgXact[pgprocno];
 
-				if (pgxact->overflowed)
+				if (subxidStates[pgxactoff].overflowed)
 					suboverflowed = true;
 				else
 				{
-					int			nsubxids = pgxact->nxids;
+					int			nsubxids = subxidStates[pgxactoff].count;
 
 					if (nsubxids > 0)
 					{
+						int			pgprocno = pgprocnos[pgxactoff];
 						PGPROC	   *proc = &allProcs[pgprocno];
 
 						pg_read_barrier();	/* pairs with GetNewTransactionId */
@@ -2496,8 +2510,6 @@ GetRunningTransactionData(void)
 	 */
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
-		int			pgprocno = arrayP->pgprocnos[index];
-		PGXACT	   *pgxact = &allPgXact[pgprocno];
 		TransactionId xid;
 
 		/* Fetch xid just once - see GetNewTransactionId */
@@ -2518,7 +2530,7 @@ GetRunningTransactionData(void)
 		if (TransactionIdPrecedes(xid, oldestRunningXid))
 			oldestRunningXid = xid;
 
-		if (pgxact->overflowed)
+		if (ProcGlobal->subxidStates[index].overflowed)
 			suboverflowed = true;
 
 		/*
@@ -2538,27 +2550,28 @@ GetRunningTransactionData(void)
 	 */
 	if (!suboverflowed)
 	{
+		XidCacheStatus *other_subxidstates = ProcGlobal->subxidStates;
+
 		for (index = 0; index < arrayP->numProcs; index++)
 		{
 			int			pgprocno = arrayP->pgprocnos[index];
 			PGPROC	   *proc = &allProcs[pgprocno];
-			PGXACT	   *pgxact = &allPgXact[pgprocno];
-			int			nxids;
+			int			nsubxids;
 
 			/*
 			 * Save subtransaction XIDs. Other backends can't add or remove
 			 * entries while we're holding XidGenLock.
 			 */
-			nxids = pgxact->nxids;
-			if (nxids > 0)
+			nsubxids = other_subxidstates[index].count;
+			if (nsubxids > 0)
 			{
 				/* barrier not really required, as XidGenLock is held, but ... */
 				pg_read_barrier();	/* pairs with GetNewTransactionId */
 
 				memcpy(&xids[count], (void *) proc->subxids.xids,
-					   nxids * sizeof(TransactionId));
-				count += nxids;
-				subcount += nxids;
+					   nsubxids * sizeof(TransactionId));
+				count += nsubxids;
+				subcount += nsubxids;
 
 				/*
 				 * Top-level XID of a transaction is always less than any of
@@ -3625,14 +3638,6 @@ ProcArrayGetReplicationSlotXmin(TransactionId *xmin,
 	LWLockRelease(ProcArrayLock);
 }
 
-
-#define XidCacheRemove(i) \
-	do { \
-		MyProc->subxids.xids[i] = MyProc->subxids.xids[MyPgXact->nxids - 1]; \
-		pg_write_barrier(); \
-		MyPgXact->nxids--; \
-	} while (0)
-
 /*
  * XidCacheRemoveRunningXids
  *
@@ -3648,6 +3653,7 @@ XidCacheRemoveRunningXids(TransactionId xid,
 {
 	int			i,
 				j;
+	XidCacheStatus *mysubxidstat;
 
 	Assert(TransactionIdIsValid(xid));
 
@@ -3665,6 +3671,8 @@ XidCacheRemoveRunningXids(TransactionId xid,
 	 */
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
+	mysubxidstat = &ProcGlobal->subxidStates[MyProc->pgxactoff];
+
 	/*
 	 * Under normal circumstances xid and xids[] will be in increasing order,
 	 * as will be the entries in subxids.  Scan backwards to avoid O(N^2)
@@ -3674,11 +3682,14 @@ XidCacheRemoveRunningXids(TransactionId xid,
 	{
 		TransactionId anxid = xids[i];
 
-		for (j = MyPgXact->nxids - 1; j >= 0; j--)
+		for (j = MyProc->subxidStatus.count - 1; j >= 0; j--)
 		{
 			if (TransactionIdEquals(MyProc->subxids.xids[j], anxid))
 			{
-				XidCacheRemove(j);
+				MyProc->subxids.xids[j] = MyProc->subxids.xids[MyProc->subxidStatus.count - 1];
+				pg_write_barrier();
+				mysubxidstat->count--;
+				MyProc->subxidStatus.count--;
 				break;
 			}
 		}
@@ -3690,20 +3701,23 @@ XidCacheRemoveRunningXids(TransactionId xid,
 		 * error during AbortSubTransaction.  So instead of Assert, emit a
 		 * debug warning.
 		 */
-		if (j < 0 && !MyPgXact->overflowed)
+		if (j < 0 && !MyProc->subxidStatus.overflowed)
 			elog(WARNING, "did not find subXID %u in MyProc", anxid);
 	}
 
-	for (j = MyPgXact->nxids - 1; j >= 0; j--)
+	for (j = MyProc->subxidStatus.count - 1; j >= 0; j--)
 	{
 		if (TransactionIdEquals(MyProc->subxids.xids[j], xid))
 		{
-			XidCacheRemove(j);
+			MyProc->subxids.xids[j] = MyProc->subxids.xids[MyProc->subxidStatus.count - 1];
+			pg_write_barrier();
+			mysubxidstat->count--;
+			MyProc->subxidStatus.count--;
 			break;
 		}
 	}
 	/* Ordinarily we should have found it, unless the cache has overflowed */
-	if (j < 0 && !MyPgXact->overflowed)
+	if (j < 0 && !MyProc->subxidStatus.overflowed)
 		elog(WARNING, "did not find subXID %u in MyProc", xid);
 
 	/* Also advance global latestCompletedXid while holding the lock */
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index f6113b2d243..aa9fbd80545 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -63,9 +63,8 @@ int			LockTimeout = 0;
 int			IdleInTransactionSessionTimeout = 0;
 bool		log_lock_waits = false;
 
-/* Pointer to this process's PGPROC and PGXACT structs, if any */
+/* Pointer to this process's PGPROC struct, if any */
 PGPROC	   *MyProc = NULL;
-PGXACT	   *MyPgXact = NULL;
 
 /*
  * This spinlock protects the freelist of recycled PGPROC structures.
@@ -110,10 +109,8 @@ ProcGlobalShmemSize(void)
 	size = add_size(size, mul_size(TotalProcs, sizeof(PGPROC)));
 	size = add_size(size, sizeof(slock_t));
 
-	size = add_size(size, mul_size(MaxBackends, sizeof(PGXACT)));
-	size = add_size(size, mul_size(NUM_AUXILIARY_PROCS, sizeof(PGXACT)));
-	size = add_size(size, mul_size(max_prepared_xacts, sizeof(PGXACT)));
 	size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->xids)));
+	size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->subxidStates)));
 	size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->vacuumFlags)));
 
 	return size;
@@ -161,7 +158,6 @@ void
 InitProcGlobal(void)
 {
 	PGPROC	   *procs;
-	PGXACT	   *pgxacts;
 	int			i,
 				j;
 	bool		found;
@@ -202,18 +198,6 @@ InitProcGlobal(void)
 	/* XXX allProcCount isn't really all of them; it excludes prepared xacts */
 	ProcGlobal->allProcCount = MaxBackends + NUM_AUXILIARY_PROCS;
 
-	/*
-	 * Also allocate a separate array of PGXACT structures.  This is separate
-	 * from the main PGPROC array so that the most heavily accessed data is
-	 * stored contiguously in memory in as few cache lines as possible. This
-	 * provides significant performance benefits, especially on a
-	 * multiprocessor system.  There is one PGXACT structure for every PGPROC
-	 * structure.
-	 */
-	pgxacts = (PGXACT *) ShmemAlloc(TotalProcs * sizeof(PGXACT));
-	MemSet(pgxacts, 0, TotalProcs * sizeof(PGXACT));
-	ProcGlobal->allPgXact = pgxacts;
-
 	/*
 	 * Allocate arrays mirroring PGPROC fields in a dense manner. See
 	 * PROC_HDR.
@@ -224,6 +208,8 @@ InitProcGlobal(void)
 	ProcGlobal->xids =
 		(TransactionId *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->xids));
 	MemSet(ProcGlobal->xids, 0, TotalProcs * sizeof(*ProcGlobal->xids));
+	ProcGlobal->subxidStates = (XidCacheStatus *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->subxidStates));
+	MemSet(ProcGlobal->subxidStates, 0, TotalProcs * sizeof(*ProcGlobal->subxidStates));
 	ProcGlobal->vacuumFlags = (uint8 *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->vacuumFlags));
 	MemSet(ProcGlobal->vacuumFlags, 0, TotalProcs * sizeof(*ProcGlobal->vacuumFlags));
 
@@ -372,7 +358,6 @@ InitProcess(void)
 				(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
 				 errmsg("sorry, too many clients already")));
 	}
-	MyPgXact = &ProcGlobal->allPgXact[MyProc->pgprocno];
 
 	/*
 	 * Cross-check that the PGPROC is of the type we expect; if this were not
@@ -569,7 +554,6 @@ InitAuxiliaryProcess(void)
 	((volatile PGPROC *) auxproc)->pid = MyProcPid;
 
 	MyProc = auxproc;
-	MyPgXact = &ProcGlobal->allPgXact[auxproc->pgprocno];
 
 	SpinLockRelease(ProcStructLock);
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index b4948ac675f..3d990463ce9 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1536,7 +1536,6 @@ PGSetenvStatusType
 PGShmemHeader
 PGTransactionStatusType
 PGVerbosity
-PGXACT
 PG_Locale_Strategy
 PG_Lock_Status
 PG_init_t
-- 
2.25.0.114.g5b0ca878e0

--4il4azyv6rmablac
Content-Type: text/x-diff; charset=us-ascii
Content-Disposition: attachment;
	filename="v13-0006-snapshot-scalability-cache-snapshots-using-a-xac.patch"