Use group updates when setting transaction status in clog.

Commit 0e141c0f 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 ccce90b3
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

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/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,

src/include/storage/proc.h

@@ -14,6 +14,7 @@
 #ifndef _PROC_H_
 #define _PROC_H_
 
+#include "access/clog.h"
 #include "access/xlogdefs.h"
 #include "lib/ilist.h"
 #include "storage/latch.h"
@@ -171,6 +172,17 @@ struct PGPROC
 
 	uint32		wait_event_info;	/* proc's wait information */
 
+	/* Support for group transaction status update. */
+	bool		clogGroupMember;	/* true, if member of clog group */
+	pg_atomic_uint32 clogGroupNext; /* next clog group member */
+	TransactionId clogGroupMemberXid;	/* transaction id of clog group member */
+	XidStatus	clogGroupMemberXidStatus;	/* transaction status of clog
+											 * group member */
+	int			clogGroupMemberPage;	/* clog page corresponding to
+										 * transaction id of clog group member */
+	XLogRecPtr	clogGroupMemberLsn; /* WAL location of commit record for clog
+									 * group member */
+
 	/* Per-backend LWLock.  Protects fields below (but not group fields). */
 	LWLock		backendLock;
 
@@ -242,6 +254,8 @@ typedef struct PROC_HDR
 	PGPROC	   *bgworkerFreeProcs;
 	/* First pgproc waiting for group XID clear */
 	pg_atomic_uint32 procArrayGroupFirst;
+	/* First pgproc waiting for group transaction status update */
+	pg_atomic_uint32 clogGroupFirst;
 	/* WALWriter process's latch */
 	Latch	   *walwriterLatch;
 	/* Checkpointer process's latch */