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

Browse Source 
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 dc7420c2c9, 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
This commit is contained in:
Andres Freund
2020-08-13 16:25:21 -07:00
parent a811ea5bde
commit 1f51c17c68
12 changed files with 52 additions and 56 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
src/backend/access/gist/gistxlog.c
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@ -389,7 +389,7 @@ gistRedoPageReuse(XLogReaderState *record)
* *
* latestRemovedXid was the page's deleteXid. The * latestRemovedXid was the page's deleteXid. The
* GlobalVisIsRemovableFullXid(deleteXid) test in gistPageRecyclable() * 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 * GetConflictingVirtualXIDs(). Consequently, one XID value achieves the
* same exclusion effect on primary and standby. * same exclusion effect on primary and standby.
*/ */

2
src/backend/access/nbtree/nbtpage.c
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@ -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 * we're in VACUUM and would not otherwise have an XID. Having already
* updated links to the target, ReadNewTransactionId() suffices as an * updated links to the target, ReadNewTransactionId() suffices as an
* upper bound. Any scan having retained a now-stale link is advertising * 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 * will continue to do so, holding back the xmin horizon, for the duration
* of that scan. * of that scan.
*/ */

4
src/backend/access/transam/README
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@ -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 Another important activity that uses the shared ProcArray is
ComputeXidHorizons, which must determine a lower bound for the oldest xmin ComputeXidHorizons, which must determine a lower bound for the oldest xmin
of any active MVCC snapshot, system-wide. Each individual backend 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 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 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 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 Note that while it is certain that two concurrent executions of
GetSnapshotData will compute the same xmin for their own snapshots, there is GetSnapshotData will compute the same xmin for their own snapshots, there is
no such guarantee for the horizons computed by ComputeXidHorizons. This 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 asynchronously (without taking ProcArrayLock), so one execution might see
what had been the oldest xmin, and another not. This is OK since the 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 thresholds need only be a valid lower bound. As noted above, we are already

2
src/backend/access/transam/twophase.c
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@ -464,7 +464,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
/* We set up the gxact's VXID as InvalidBackendId/XID */ /* We set up the gxact's VXID as InvalidBackendId/XID */
proc->lxid = (LocalTransactionId) xid; proc->lxid = (LocalTransactionId) xid;
pgxact->xid = xid; pgxact->xid = xid;
pgxact->xmin = InvalidTransactionId; Assert(proc->xmin == InvalidTransactionId);
proc->delayChkpt = false; proc->delayChkpt = false;
pgxact->vacuumFlags = 0; pgxact->vacuumFlags = 0;
proc->pid = 0; proc->pid = 0;

2
src/backend/commands/indexcmds.c
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@ -1535,7 +1535,7 @@ DefineIndex(Oid relationId,
StartTransactionCommand(); StartTransactionCommand();
/* We should now definitely not be advertising any xmin. */ /* 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 * The index is now valid in the sense that it contains all currently

6
src/backend/replication/logical/snapbuild.c
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@ -553,8 +553,8 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
elog(ERROR, "cannot build an initial slot snapshot, not all transactions are monitored anymore"); elog(ERROR, "cannot build an initial slot snapshot, not all transactions are monitored anymore");
/* so we don't overwrite the existing value */ /* so we don't overwrite the existing value */
if (TransactionIdIsValid(MyPgXact->xmin)) if (TransactionIdIsValid(MyProc->xmin))
elog(ERROR, "cannot build an initial slot snapshot when MyPgXact->xmin already is valid"); elog(ERROR, "cannot build an initial slot snapshot when MyProc->xmin already is valid");
snap = SnapBuildBuildSnapshot(builder); snap = SnapBuildBuildSnapshot(builder);
@ -575,7 +575,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
} }
#endif #endif
MyPgXact->xmin = snap->xmin; MyProc->xmin = snap->xmin;
/* allocate in transaction context */ /* allocate in transaction context */
newxip = (TransactionId *) newxip = (TransactionId *)

10
src/backend/replication/walsender.c
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@ -1964,7 +1964,7 @@ PhysicalReplicationSlotNewXmin(TransactionId feedbackXmin, TransactionId feedbac
ReplicationSlot *slot = MyReplicationSlot; ReplicationSlot *slot = MyReplicationSlot;
SpinLockAcquire(&slot->mutex); SpinLockAcquire(&slot->mutex);
MyPgXact->xmin = InvalidTransactionId; MyProc->xmin = InvalidTransactionId;
/* /*
* For physical replication we don't need the interlock provided by xmin * For physical replication we don't need the interlock provided by xmin
@ -2093,7 +2093,7 @@ ProcessStandbyHSFeedbackMessage(void)
if (!TransactionIdIsNormal(feedbackXmin) if (!TransactionIdIsNormal(feedbackXmin)
&& !TransactionIdIsNormal(feedbackCatalogXmin)) && !TransactionIdIsNormal(feedbackCatalogXmin))
{ {
MyPgXact->xmin = InvalidTransactionId; MyProc->xmin = InvalidTransactionId;
if (MyReplicationSlot != NULL) if (MyReplicationSlot != NULL)
PhysicalReplicationSlotNewXmin(feedbackXmin, feedbackCatalogXmin); PhysicalReplicationSlotNewXmin(feedbackXmin, feedbackCatalogXmin);
return; return;
@ -2135,7 +2135,7 @@ ProcessStandbyHSFeedbackMessage(void)
* risk already since a VACUUM could already have determined the horizon.) * risk already since a VACUUM could already have determined the horizon.)
* *
* If we're using a replication slot we reserve the xmin via that, * 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 * catalog xmin separately when using a slot, so we store the least of the
* two provided when not using a slot. * two provided when not using a slot.
* *
@ -2148,9 +2148,9 @@ ProcessStandbyHSFeedbackMessage(void)
{ {
if (TransactionIdIsNormal(feedbackCatalogXmin) if (TransactionIdIsNormal(feedbackCatalogXmin)
&& TransactionIdPrecedes(feedbackCatalogXmin, feedbackXmin)) && TransactionIdPrecedes(feedbackCatalogXmin, feedbackXmin))
MyPgXact->xmin = feedbackCatalogXmin; MyProc->xmin = feedbackCatalogXmin;
else else
MyPgXact->xmin = feedbackXmin; MyProc->xmin = feedbackXmin;
} }
} }

36
src/backend/storage/ipc/procarray.c
View File

@ -587,9 +587,9 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
Assert(!TransactionIdIsValid(allPgXact[proc->pgprocno].xid)); Assert(!TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
proc->lxid = InvalidLocalTransactionId; proc->lxid = InvalidLocalTransactionId;
pgxact->xmin = InvalidTransactionId;
/* must be cleared with xid/xmin: */ /* must be cleared with xid/xmin: */
pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK; pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
proc->xmin = InvalidTransactionId;
proc->delayChkpt = false; /* be sure this is cleared in abort */ proc->delayChkpt = false; /* be sure this is cleared in abort */
proc->recoveryConflictPending = false; proc->recoveryConflictPending = false;
@ -609,9 +609,9 @@ ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
{ {
pgxact->xid = InvalidTransactionId; pgxact->xid = InvalidTransactionId;
proc->lxid = InvalidLocalTransactionId; proc->lxid = InvalidLocalTransactionId;
pgxact->xmin = InvalidTransactionId;
/* must be cleared with xid/xmin: */ /* must be cleared with xid/xmin: */
pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK; pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
proc->xmin = InvalidTransactionId;
proc->delayChkpt = false; /* be sure this is cleared in abort */ proc->delayChkpt = false; /* be sure this is cleared in abort */
proc->recoveryConflictPending = false; proc->recoveryConflictPending = false;
@ -763,7 +763,7 @@ ProcArrayClearTransaction(PGPROC *proc)
*/ */
pgxact->xid = InvalidTransactionId; pgxact->xid = InvalidTransactionId;
proc->lxid = InvalidLocalTransactionId; proc->lxid = InvalidLocalTransactionId;
pgxact->xmin = InvalidTransactionId; proc->xmin = InvalidTransactionId;
proc->recoveryConflictPending = false; proc->recoveryConflictPending = false;
/* redundant, but just in case */ /* redundant, but just in case */
@ -1563,7 +1563,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
/* Fetch xid just once - see GetNewTransactionId */ /* Fetch xid just once - see GetNewTransactionId */
xid = UINT32_ACCESS_ONCE(pgxact->xid); 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. * Consider both the transaction's Xmin, and its Xid.
@ -1838,7 +1838,7 @@ GetMaxSnapshotSubxidCount(void)
* *
* We also update the following backend-global variables: * We also update the following backend-global variables:
* TransactionXmin: the oldest xmin of any snapshot in use in the * 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 * RecentXmin: the xmin computed for the most recent snapshot. XIDs
* older than this are known not running any more. * 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 * 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); LWLockAcquire(ProcArrayLock, LW_SHARED);
@ -2051,8 +2051,8 @@ GetSnapshotData(Snapshot snapshot)
replication_slot_xmin = procArray->replication_slot_xmin; replication_slot_xmin = procArray->replication_slot_xmin;
replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin; replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;
if (!TransactionIdIsValid(MyPgXact->xmin)) if (!TransactionIdIsValid(MyProc->xmin))
MyPgXact->xmin = TransactionXmin = xmin; MyProc->xmin = TransactionXmin = xmin;
LWLockRelease(ProcArrayLock); 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 * This is called when installing a snapshot imported from another
* transaction. To ensure that OldestXmin doesn't go backwards, we must * 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. * 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) || if (!TransactionIdIsNormal(xid) ||
!TransactionIdPrecedesOrEquals(xid, xmin)) !TransactionIdPrecedesOrEquals(xid, xmin))
continue; continue;
@ -2236,7 +2236,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
* GetSnapshotData first, we'll be overwriting a valid xmin here, so * GetSnapshotData first, we'll be overwriting a valid xmin here, so
* we don't check that.) * we don't check that.)
*/ */
MyPgXact->xmin = TransactionXmin = xmin; MyProc->xmin = TransactionXmin = xmin;
result = true; result = true;
break; 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 * This is like ProcArrayInstallImportedXmin, but we have a pointer to the
* PGPROC of the transaction from which we imported the snapshot, rather than * PGPROC of the transaction from which we imported the snapshot, rather than
@ -2261,7 +2261,6 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
{ {
bool result = false; bool result = false;
TransactionId xid; TransactionId xid;
PGXACT *pgxact;
Assert(TransactionIdIsNormal(xmin)); Assert(TransactionIdIsNormal(xmin));
Assert(proc != NULL); 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 */ /* Get lock so source xact can't end while we're doing this */
LWLockAcquire(ProcArrayLock, LW_SHARED); LWLockAcquire(ProcArrayLock, LW_SHARED);
pgxact = &allPgXact[proc->pgprocno];
/* /*
* Be certain that the referenced PGPROC has an advertised xmin which is * 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 * 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, * can't go backwards. Also, make sure it's running in the same database,
* so that the per-database xmin cannot go backwards. * 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 && if (proc->databaseId == MyDatabaseId &&
TransactionIdIsNormal(xid) && TransactionIdIsNormal(xid) &&
TransactionIdPrecedesOrEquals(xid, xmin)) TransactionIdPrecedesOrEquals(xid, xmin))
{ {
MyPgXact->xmin = TransactionXmin = xmin; MyProc->xmin = TransactionXmin = xmin;
result = true; result = true;
} }
@ -2908,7 +2905,7 @@ GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0,
if (allDbs || proc->databaseId == MyDatabaseId) if (allDbs || proc->databaseId == MyDatabaseId)
{ {
/* Fetch xmin just once - might change on us */ /* 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)) if (excludeXmin0 && !TransactionIdIsValid(pxmin))
continue; continue;
@ -2994,7 +2991,6 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
{ {
int pgprocno = arrayP->pgprocnos[index]; int pgprocno = arrayP->pgprocnos[index];
PGPROC *proc = &allProcs[pgprocno]; PGPROC *proc = &allProcs[pgprocno];
PGXACT *pgxact = &allPgXact[pgprocno];
/* Exclude prepared transactions */ /* Exclude prepared transactions */
if (proc->pid == 0) if (proc->pid == 0)
@ -3004,7 +3000,7 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
proc->databaseId == dbOid) proc->databaseId == dbOid)
{ {
/* Fetch xmin just once - can't change on us, but good coding */ /* 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 * We ignore an invalid pxmin because this means that backend has

2
src/backend/storage/ipc/sinvaladt.c
View File

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

4
src/backend/storage/lmgr/proc.c
View File

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

28
src/backend/utils/time/snapmgr.c
View File

@ -27,11 +27,11 @@
* their lifetime is managed separately (as they live longer than one xact.c * their lifetime is managed separately (as they live longer than one xact.c
* transaction). * 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 * referenced by this transaction, and advance it when the one with oldest
* Xmin is no longer referenced. For simplicity however, only registered * Xmin is no longer referenced. For simplicity however, only registered
* snapshots not active snapshots participate in tracking which one is oldest; * 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. * stack is empty.
* *
* *
@ -187,7 +187,7 @@ static ActiveSnapshotElt *OldestActiveSnapshot = NULL;
/* /*
* Currently registered Snapshots. Ordered in a heap by xmin, so that we can * 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, static int xmin_cmp(const pairingheap_node *a, const pairingheap_node *b,
void *arg); void *arg);
@ -475,7 +475,7 @@ GetNonHistoricCatalogSnapshot(Oid relid)
/* /*
* Make sure the catalog snapshot will be accounted for in decisions * 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 * that would result in making a physical copy, which is overkill; and
* it would also create a dependency on some resource owner, which we * 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 * 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 */ /* 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 * 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 * causing the global xmin to go backwards, we have to test that the
* source transaction is still running, and that has to be done * 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 * 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 * Note we can do this without locking because we assume that storing an Xid
* is atomic. * is atomic.
* *
* Even if there are some remaining snapshots, we may be able to advance our * 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 * PGPROC->xmin to some degree. This typically happens when a portal is
* dropped. For efficiency, we only consider recomputing PGXACT->xmin when * dropped. For efficiency, we only consider recomputing PGPROC->xmin when
* the active snapshot stack is empty; this allows us not to need to track * the active snapshot stack is empty; this allows us not to need to track
* which active snapshot is oldest. * which active snapshot is oldest.
* *
@ -977,15 +977,15 @@ SnapshotResetXmin(void)
if (pairingheap_is_empty(&RegisteredSnapshots)) if (pairingheap_is_empty(&RegisteredSnapshots))
{ {
MyPgXact->xmin = InvalidTransactionId; MyProc->xmin = InvalidTransactionId;
return; return;
} }
minSnapshot = pairingheap_container(SnapshotData, ph_node, minSnapshot = pairingheap_container(SnapshotData, ph_node,
pairingheap_first(&RegisteredSnapshots)); pairingheap_first(&RegisteredSnapshots));
if (TransactionIdPrecedes(MyPgXact->xmin, minSnapshot->xmin)) if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
MyPgXact->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 * 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. * AtEOXact_Snapshot(), so we need not touch xmin here at all.
*/ */
if (resetXmin) if (resetXmin)
SnapshotResetXmin(); SnapshotResetXmin();
Assert(resetXmin || MyPgXact->xmin == 0); Assert(resetXmin || MyProc->xmin == 0);
} }
@ -1830,7 +1830,7 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
*/ */
if (old_snapshot_threshold == 0) if (old_snapshot_threshold == 0)
{ {
if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin) if (TransactionIdPrecedes(latest_xmin, MyProc->xmin)
&& TransactionIdFollows(latest_xmin, xlimit)) && TransactionIdFollows(latest_xmin, xlimit))
xlimit = latest_xmin; xlimit = latest_xmin;

10
src/include/storage/proc.h
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@ -101,6 +101,11 @@ struct PGPROC
Latch procLatch; /* generic latch for process */ 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 LocalTransactionId lxid; /* local id of top-level transaction currently
* being executed by this proc, if running; * being executed by this proc, if running;
* else InvalidLocalTransactionId */ * else InvalidLocalTransactionId */
@ -223,11 +228,6 @@ typedef struct PGXACT
* executed by this proc, if running and XID * executed by this proc, if running and XID
* is assigned; else InvalidTransactionId */ * 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 */ uint8 vacuumFlags; /* vacuum-related flags, see above */
bool overflowed; bool overflowed;