Use group updates when setting transaction status in clog.

Commit 0e141c0 introduced a mechanism
to reduce contention on ProcArrayLock by having a single process clear
XIDs in the procArray on behalf of multiple processes, reducing the
need to hand the lock around.  A previous attempt to introduce a similar
mechanism for CLogControlLock in ccce90b
crashed and burned, but the design problem which resulted in those
failures is believed to have been corrected in this version.

Amit Kapila, with some cosmetic changes by me.  See the previous commit
message for additional credits.

Discussion: http://postgr.es/m/CAA4eK1KudxzgWhuywY_X=yeSAhJMT4DwCjroV5Ay60xaeB2Eew@mail.gmail.com

Author: robertmhaas

doc/src/sgml/monitoring.sgml

@@ -1250,7 +1250,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry>Waiting in an extension.</entry>
         </row>
         <row>
-         <entry morerows="16"><literal>IPC</></entry>
+         <entry morerows="17"><literal>IPC</></entry>
          <entry><literal>BgWorkerShutdown</></entry>
          <entry>Waiting for background worker to shut down.</entry>
         </row>
@@ -1302,6 +1302,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry><literal>ProcArrayGroupUpdate</></entry>
          <entry>Waiting for group leader to clear transaction id at transaction end.</entry>
         </row>
+        <row>
+         <entry><literal>ClogGroupUpdate</></entry>
+         <entry>Waiting for group leader to update transaction status at transaction end.</entry>
+        </row>
         <row>
          <entry><literal>ReplicationOriginDrop</></entry>
          <entry>Waiting for a replication origin to become inactive to be dropped.</entry>

src/backend/access/transam/clog.c

@@ -39,7 +39,9 @@
 #include "access/xloginsert.h"
 #include "access/xlogutils.h"
 #include "miscadmin.h"
+#include "pgstat.h"
 #include "pg_trace.h"
+#include "storage/proc.h"
 
 /*
  * Defines for CLOG page sizes.  A page is the same BLCKSZ as is used
@@ -71,6 +73,12 @@
 #define GetLSNIndex(slotno, xid)	((slotno) * CLOG_LSNS_PER_PAGE + \
 	((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP)
 
+/*
+ * The number of subtransactions below which we consider to apply clog group
+ * update optimization.  Testing reveals that the number higher than this can
+ * hurt performance.
+ */
+#define THRESHOLD_SUBTRANS_CLOG_OPT	5
 
 /*
  * Link to shared-memory data structures for CLOG control
@@ -87,11 +95,17 @@ static void WriteTruncateXlogRec(int pageno, TransactionId oldestXact,
 					 Oid oldestXidDb);
 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);
 
 
 /*
@@ -174,7 +188,7 @@ TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
 		 * Set the parent and all subtransactions in a single call
 		 */
 		TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
-								   pageno);
+								   pageno, true);
 	}
 	else
 	{
@@ -201,7 +215,7 @@ TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
 		 */
 		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,
@@ -239,31 +253,100 @@ set_status_by_pages(int nsubxids, TransactionId *subxids,
 
 		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()
+ * Record the final state of transaction entries in the commit log for all
+ * entries on a single page.  Atomic only on this page.
  */
 static void
 TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
 						   TransactionId *subxids, XidStatus status,
-						   XLogRecPtr lsn, int pageno)
+						   XLogRecPtr lsn, int pageno,
+						   bool all_xact_same_page)
+{
+	/* Can't use group update when PGPROC overflows. */
+	StaticAssertStmt(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS,
+					 "group clog threshold less than PGPROC cached subxids");
+
+	/*
+	 * 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.
+	 *
+	 * For this optimization to be safe, the XID in MyPgXact and the subxids
+	 * in MyProc must be the same as the ones for which we're setting the
+	 * status.  Check that this is the case.
+	 *
+	 * For this optimization to be efficient, we shouldn't have too many
+	 * sub-XIDs and all of the XIDs for which we're adjusting clog should be
+	 * on the same page.  Check those conditions, too.
+	 */
+	if (all_xact_same_page && xid == MyPgXact->xid &&
+		nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
+		nsubxids == MyPgXact->nxids &&
+		memcmp(subxids, MyProc->subxids.xids,
+			   nsubxids * sizeof(TransactionId)) == 0)
+	{
+		/*
+		 * We don't try to do group update optimization 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.
+		 */
+		Assert(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS);
+
+		/*
+		 * 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))
+		{
+			/* Got the lock without waiting!  Do the update. */
+			TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
+											   lsn, pageno);
+			LWLockRelease(CLogControlLock);
+			return;
+		}
+		else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
+		{
+			/* Group update mechanism has done the work. */
+			return;
+		}
+
+		/* Fall through only if update isn't done yet. */
+	}
+
+	/* Group update not applicable, or couldn't accept this page number. */
+	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
@@ -311,8 +394,167 @@ TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
 	}
 
 	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. */
+		pgstat_report_wait_start(WAIT_EVENT_CLOG_GROUP_UPDATE);
+		for (;;)
+		{
+			/* acts as a read barrier */
+			PGSemaphoreLock(proc->sem);
+			if (!proc->clogGroupMember)
+				break;
+			extraWaits++;
+		}
+		pgstat_report_wait_end();
+
+		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.
+	 */
+	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;
 }
 
 /*

src/backend/postmaster/pgstat.c

@@ -3609,6 +3609,9 @@ pgstat_get_wait_ipc(WaitEventIPC w)
 		case WAIT_EVENT_PROCARRAY_GROUP_UPDATE:
 			event_name = "ProcArrayGroupUpdate";
 			break;
+		case WAIT_EVENT_CLOG_GROUP_UPDATE:
+			event_name = "ClogGroupUpdate";
+			break;
 		case WAIT_EVENT_REPLICATION_ORIGIN_DROP:
 			event_name = "ReplicationOriginDrop";
 			break;

src/backend/storage/lmgr/proc.c

@@ -186,6 +186,7 @@ InitProcGlobal(void)
 	ProcGlobal->walwriterLatch = NULL;
 	ProcGlobal->checkpointerLatch = NULL;
 	pg_atomic_init_u32(&ProcGlobal->procArrayGroupFirst, INVALID_PGPROCNO);
+	pg_atomic_init_u32(&ProcGlobal->clogGroupFirst, INVALID_PGPROCNO);
 
 	/*
 	 * Create and initialize all the PGPROC structures we'll need.  There are
@@ -408,6 +409,14 @@ InitProcess(void)
 	/* Initialize wait event information. */
 	MyProc->wait_event_info = 0;
 
+	/* Initialize fields for group transaction status update. */
+	MyProc->clogGroupMember = false;
+	MyProc->clogGroupMemberXid = InvalidTransactionId;
+	MyProc->clogGroupMemberXidStatus = TRANSACTION_STATUS_IN_PROGRESS;
+	MyProc->clogGroupMemberPage = -1;
+	MyProc->clogGroupMemberLsn = InvalidXLogRecPtr;
+	pg_atomic_init_u32(&MyProc->clogGroupNext, INVALID_PGPROCNO);
+
 	/*
 	 * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch
 	 * on it.  That allows us to repoint the process latch, which so far

src/include/pgstat.h

@@ -812,6 +812,7 @@ typedef enum
 	WAIT_EVENT_PARALLEL_FINISH,
 	WAIT_EVENT_PARALLEL_BITMAP_SCAN,
 	WAIT_EVENT_PROCARRAY_GROUP_UPDATE,
+	WAIT_EVENT_CLOG_GROUP_UPDATE,
 	WAIT_EVENT_REPLICATION_ORIGIN_DROP,
 	WAIT_EVENT_REPLICATION_SLOT_DROP,
 	WAIT_EVENT_SAFE_SNAPSHOT,