#include "access/clog.h"
#include "access/slru.h"
#include "access/transam.h"
+#include "access/twophase.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "pg_trace.h"
+#include "storage/proc.h"
/*
* Defines for CLOG page sizes. A page is the same BLCKSZ as is used
static void WriteTruncateXlogRec(int pageno);
static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status,
- XLogRecPtr lsn, int pageno);
+ XLogRecPtr lsn, int pageno,
+ bool all_xact_same_page);
static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status,
XLogRecPtr lsn, int slotno);
static void set_status_by_pages(int nsubxids, TransactionId *subxids,
XidStatus status, XLogRecPtr lsn);
+static bool TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
+ XLogRecPtr lsn, int pageno);
+static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
+ TransactionId *subxids, XidStatus status,
+ XLogRecPtr lsn, int pageno);
/*
* Set the parent and all subtransactions in a single call
*/
TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
- pageno);
+ pageno, true);
}
else
{
*/
pageno = TransactionIdToPage(xid);
TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status,
- lsn, pageno);
+ lsn, pageno, false);
/*
* Now work through the rest of the subxids one clog page at a time,
TransactionIdSetPageStatus(InvalidTransactionId,
num_on_page, subxids + offset,
- status, lsn, pageno);
+ status, lsn, pageno, false);
offset = i;
pageno = TransactionIdToPage(subxids[offset]);
}
* Record the final state of transaction entries in the commit log for
* all entries on a single page. Atomic only on this page.
*
- * Otherwise API is same as TransactionIdSetTreeStatus()
+ * When there is contention on CLogControlLock, we try to group multiple
+ * updates; a single leader process will perform transaction status updates
+ * for multiple backends so that the number of times CLogControlLock needs
+ * to be acquired is reduced. We don't try to do this if a process has
+ * overflowed the subxids array in its PGPROC, since in that case we
+ * don't have a complete list of XIDs for it. We also skip it if a process
+ * has XIDs on more than one CLOG page, or on a different CLOG page than
+ * processes already waiting for a group update. This latter condition
+ * has a race condition (see TransactionGroupUpdateXidStatus) but the
+ * worst thing that happens if we mess up is a small loss of efficiency;
+ * the intent is to avoid having the leader access pages it wouldn't
+ * otherwise need to touch. Finally, we skip it for prepared transactions,
+ * which don't have the semaphore we would need for this optimization,
+ * and which are anyway probably not all that common.
*/
static void
TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status,
- XLogRecPtr lsn, int pageno)
+ XLogRecPtr lsn, int pageno,
+ bool all_xact_same_page)
+{
+ if (all_xact_same_page &&
+ nsubxids < PGPROC_MAX_CACHED_SUBXIDS &&
+ !IsGXactActive())
+ {
+ /*
+ * If we can immediately acquire CLogControlLock, we update the status
+ * of our own XID and release the lock. If not, try use group XID
+ * update. If that doesn't work out, fall back to waiting for the
+ * lock to perform an update for this transaction only.
+ */
+ if (LWLockConditionalAcquire(CLogControlLock, LW_EXCLUSIVE))
+ {
+ TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status, lsn, pageno);
+ LWLockRelease(CLogControlLock);
+ }
+ else if (!TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
+ {
+ LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+
+ TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status, lsn, pageno);
+
+ LWLockRelease(CLogControlLock);
+ }
+ }
+ else
+ {
+ LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+
+ TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status, lsn, pageno);
+
+ LWLockRelease(CLogControlLock);
+ }
+}
+
+/*
+ * Record the final state of transaction entry in the commit log
+ *
+ * We don't do any locking here; caller must handle that.
+ */
+static void
+TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
+ TransactionId *subxids, XidStatus status,
+ XLogRecPtr lsn, int pageno)
{
int slotno;
int i;
Assert(status == TRANSACTION_STATUS_COMMITTED ||
status == TRANSACTION_STATUS_ABORTED ||
(status == TRANSACTION_STATUS_SUB_COMMITTED && !TransactionIdIsValid(xid)));
-
- LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+ Assert(LWLockHeldByMeInMode(CLogControlLock, LW_EXCLUSIVE));
/*
* If we're doing an async commit (ie, lsn is valid), then we must wait
}
ClogCtl->shared->page_dirty[slotno] = true;
+}
+/*
+ * When we cannot immediately acquire CLogControlLock in exclusive mode at
+ * commit time, add ourselves to a list of processes that need their XIDs
+ * status update. The first process to add itself to the list will acquire
+ * CLogControlLock in exclusive mode and set transaction status as required
+ * on behalf of all group members. This avoids a great deal of contention
+ * around CLogControlLock when many processes are trying to commit at once,
+ * since the lock need not be repeatedly handed off from one committing
+ * process to the next.
+ *
+ * Returns true when transaction status has been updated in clog; returns
+ * false if we decided against applying the optimization because the page
+ * number we need to update differs from those processes already waiting.
+ */
+static bool
+TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
+ XLogRecPtr lsn, int pageno)
+{
+ volatile PROC_HDR *procglobal = ProcGlobal;
+ PGPROC *proc = MyProc;
+ uint32 nextidx;
+ uint32 wakeidx;
+
+ /* We should definitely have an XID whose status needs to be updated. */
+ Assert(TransactionIdIsValid(xid));
+
+ /*
+ * Add ourselves to the list of processes needing a group XID status
+ * update.
+ */
+ proc->clogGroupMember = true;
+ proc->clogGroupMemberXid = xid;
+ proc->clogGroupMemberXidStatus = status;
+ proc->clogGroupMemberPage = pageno;
+ proc->clogGroupMemberLsn = lsn;
+
+ nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
+
+ while (true)
+ {
+ /*
+ * Add the proc to list, if the clog page where we need to update the
+ * current transaction status is same as group leader's clog page.
+ *
+ * There is a race condition here, which is that after doing the below
+ * check and before adding this proc's clog update to a group, the
+ * group leader might have already finished the group update for this
+ * page and becomes group leader of another group. This will lead to a
+ * situation where a single group can have different clog page
+ * updates. This isn't likely and will still work, just maybe a bit
+ * less efficiently.
+ */
+ if (nextidx != INVALID_PGPROCNO &&
+ ProcGlobal->allProcs[nextidx].clogGroupMemberPage != proc->clogGroupMemberPage)
+ {
+ proc->clogGroupMember = false;
+ return false;
+ }
+
+ pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
+
+ if (pg_atomic_compare_exchange_u32(&procglobal->clogGroupFirst,
+ &nextidx,
+ (uint32) proc->pgprocno))
+ break;
+ }
+
+ /*
+ * If the list was not empty, the leader will update the status of our
+ * XID. It is impossible to have followers without a leader because the
+ * first process that has added itself to the list will always have
+ * nextidx as INVALID_PGPROCNO.
+ */
+ if (nextidx != INVALID_PGPROCNO)
+ {
+ int extraWaits = 0;
+
+ /* Sleep until the leader updates our XID status. */
+ for (;;)
+ {
+ /* acts as a read barrier */
+ PGSemaphoreLock(proc->sem);
+ if (!proc->clogGroupMember)
+ break;
+ extraWaits++;
+ }
+
+ Assert(pg_atomic_read_u32(&proc->clogGroupNext) == INVALID_PGPROCNO);
+
+ /* Fix semaphore count for any absorbed wakeups */
+ while (extraWaits-- > 0)
+ PGSemaphoreUnlock(proc->sem);
+ return true;
+ }
+
+ /* We are the leader. Acquire the lock on behalf of everyone. */
+ LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+
+ /*
+ * Now that we've got the lock, clear the list of processes waiting for
+ * group XID status update, saving a pointer to the head of the list.
+ * Trying to pop elements one at a time could lead to an ABA problem.
+ */
+ nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst, INVALID_PGPROCNO);
+
+ /* Remember head of list so we can perform wakeups after dropping lock. */
+ wakeidx = nextidx;
+
+ /* Walk the list and update the status of all XIDs. */
+ while (nextidx != INVALID_PGPROCNO)
+ {
+ PGPROC *proc = &ProcGlobal->allProcs[nextidx];
+ PGXACT *pgxact = &ProcGlobal->allPgXact[nextidx];
+
+ /*
+ * Overflowed transactions should not use group XID status update
+ * mechanism.
+ */
+ Assert(!pgxact->overflowed);
+
+ TransactionIdSetPageStatusInternal(proc->clogGroupMemberXid,
+ pgxact->nxids,
+ proc->subxids.xids,
+ proc->clogGroupMemberXidStatus,
+ proc->clogGroupMemberLsn,
+ proc->clogGroupMemberPage);
+
+ /* Move to next proc in list. */
+ nextidx = pg_atomic_read_u32(&proc->clogGroupNext);
+ }
+
+ /* We're done with the lock now. */
LWLockRelease(CLogControlLock);
+
+ /*
+ * 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 slower and can cause performance slowdown if done under
+ * lock.
+ */
+ while (wakeidx != INVALID_PGPROCNO)
+ {
+ PGPROC *proc = &ProcGlobal->allProcs[wakeidx];
+
+ wakeidx = pg_atomic_read_u32(&proc->clogGroupNext);
+ pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PGPROCNO);
+
+ /* ensure all previous writes are visible before follower continues. */
+ pg_write_barrier();
+
+ proc->clogGroupMember = false;
+
+ if (proc != MyProc)
+ PGSemaphoreUnlock(proc->sem);
+ }
+
+ return true;
}
/*
projects / postgresql / commitdiff