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Reduce ProcArrayLock contention by removing backends in batches.

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When a write transaction commits, it must clear its XID advertised via
the ProcArray, which requires that we hold ProcArrayLock in exclusive
mode in order to prevent concurrent processes running GetSnapshotData
from seeing inconsistent results.  When many processes try to commit
at once, ProcArrayLock must change hands repeatedly, with each
concurrent process trying to commit waking up to acquire the lock in
turn.  To make things more efficient, when more than one backend is
trying to commit a write transaction at the same time, have just one
of them acquire ProcArrayLock in exclusive mode and clear the XIDs of
all processes in the group.  Benchmarking reveals that this is much
more efficient at very high client counts.

Amit Kapila, heavily revised by me, with some review also from Pavan
Deolasee.
This commit is contained in:
Robert Haas
2015-08-06 11:52:51 -04:00
parent 253de7e1eb
commit 0e141c0fbb
4 changed files with 166 additions and 20 deletions
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166
src/backend/storage/ipc/procarray.c
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@@ -167,6 +167,9 @@ 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 void ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid);
/*
* Report shared-memory space needed by CreateSharedProcArray.
@@ -399,26 +402,18 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
*/
Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
pgxact->xid = InvalidTransactionId;
proc->lxid = InvalidLocalTransactionId;
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->recoveryConflictPending = false;
/* Clear the subtransaction-XID cache too while holding the lock */
pgxact->nxids = 0;
pgxact->overflowed = false;
/* Also advance global latestCompletedXid while holding the lock */
if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid,
latestXid))
ShmemVariableCache->latestCompletedXid = latestXid;
LWLockRelease(ProcArrayLock);
/*
* If we can immediately acquire ProcArrayLock, we clear our own XID
* and release the lock. If not, use group XID clearing to improve
* efficiency.
*/
if (LWLockConditionalAcquire(ProcArrayLock, LW_EXCLUSIVE))
{
ProcArrayEndTransactionInternal(proc, pgxact, latestXid);
LWLockRelease(ProcArrayLock);
}
else
ProcArrayGroupClearXid(proc, latestXid);
}
else
{
@@ -441,6 +436,137 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
}
}
/*
* Mark a write transaction as no longer running.
*
* We don't do any locking here; caller must handle that.
*/
static inline void
ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
TransactionId latestXid)
{
pgxact->xid = InvalidTransactionId;
proc->lxid = InvalidLocalTransactionId;
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->recoveryConflictPending = false;
/* Clear the subtransaction-XID cache too while holding the lock */
pgxact->nxids = 0;
pgxact->overflowed = false;
/* Also advance global latestCompletedXid while holding the lock */
if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid,
latestXid))
ShmemVariableCache->latestCompletedXid = latestXid;
}
/*
* ProcArrayGroupClearXid -- group XID clearing
*
* When we cannot immediately acquire ProcArrayLock in exclusive mode at
* commit time, add ourselves to a list of processes that need their XIDs
* cleared. The first process to add itself to the list will acquire
* ProcArrayLock in exclusive mode and perform ProcArrayEndTransactionInternal
* on behalf of all group members. This avoids a great deal of context
* switching when many processes are trying to commit at once, since the lock
* only needs to be handed from the last share-locker to one process waiting
* for the exclusive lock, rather than to each one in turn.
*/
static void
ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
{
volatile PROC_HDR *procglobal = ProcGlobal;
uint32 nextidx;
uint32 wakeidx;
int extraWaits = -1;
/* We should definitely have an XID to clear. */
Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
/* Add ourselves to the list of processes needing a group XID clear. */
proc->backendLatestXid = latestXid;
while (true)
{
nextidx = pg_atomic_read_u32(&procglobal->nextClearXidElem);
pg_atomic_write_u32(&proc->nextClearXidElem, nextidx);
if (pg_atomic_compare_exchange_u32(&procglobal->nextClearXidElem,
&nextidx,
(uint32) proc->pgprocno))
break;
}
/* If the list was not empty, the leader will clear our XID. */
if (nextidx != INVALID_PGPROCNO)
{
/* Sleep until the leader clears our XID. */
while (pg_atomic_read_u32(&proc->nextClearXidElem) != INVALID_PGPROCNO)
{
extraWaits++;
PGSemaphoreLock(&proc->sem);
}
/* Fix semaphore count for any absorbed wakeups */
while (extraWaits-- > 0)
PGSemaphoreUnlock(&proc->sem);
return;
}
/* We are the leader. Acquire the lock on behalf of everyone. */
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
/*
* Now that we've got the lock, clear the list of processes waiting for
* group XID clearing, saving a pointer to the head of the list.
*/
while (true)
{
nextidx = pg_atomic_read_u32(&procglobal->nextClearXidElem);
if (pg_atomic_compare_exchange_u32(&procglobal->nextClearXidElem,
&nextidx,
INVALID_PGPROCNO))
break;
}
/* Remember head of list so we can perform wakeups after dropping lock. */
wakeidx = nextidx;
/* Walk the list and clear all XIDs. */
while (nextidx != INVALID_PGPROCNO)
{
PGPROC *proc = &allProcs[nextidx];
PGXACT *pgxact = &allPgXact[nextidx];
ProcArrayEndTransactionInternal(proc, pgxact, proc->backendLatestXid);
/* Move to next proc in list. */
nextidx = pg_atomic_read_u32(&proc->nextClearXidElem);
}
/* We're done with the lock now. */
LWLockRelease(ProcArrayLock);
/*
* Now that we've released the lock, go back and wake everybody up. We
* don't do this under the lock so as to keep lock hold times to a
* minimum. The system calls we need to perform to wake other processes
* up are probably much slower than the simple memory writes we did while
* holding the lock.
*/
while (wakeidx != INVALID_PGPROCNO)
{
PGPROC *proc = &allProcs[wakeidx];
wakeidx = pg_atomic_read_u32(&proc->nextClearXidElem);
pg_atomic_write_u32(&proc->nextClearXidElem, INVALID_PGPROCNO);
if (proc != MyProc)
PGSemaphoreUnlock(&proc->sem);
}
}
/*
* ProcArrayClearTransaction -- clear the transaction fields