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

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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

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

@@ -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

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];
*xid = xact->xid;
*xmin = xact->xmin;
*xmin = proc->xmin;
}
}