Remove num_xloginsert_locks GUC, replace with a #define

I left the GUC in place for the beta period, so that people could experiment
with different values. No-one's come up with any data that a different value
would be better under some circumstances, so rather than try to document to
users what the GUC, let's just hard-code the current value, 8.

src/backend/access/transam/xlog.c

@@ -91,12 +91,18 @@ int			sync_method = DEFAULT_SYNC_METHOD;
 int			wal_level = WAL_LEVEL_MINIMAL;
 int			CommitDelay = 0;	/* precommit delay in microseconds */
 int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
-int			num_xloginsert_locks = 8;
 
 #ifdef WAL_DEBUG
 bool		XLOG_DEBUG = false;
 #endif
 
+/*
+ * Number of WAL insertion locks to use. A higher value allows more insertions
+ * to happen concurrently, but adds some CPU overhead to flushing the WAL,
+ * which needs to iterate all the locks.
+ */
+#define NUM_XLOGINSERT_LOCKS  8
+
 /*
  * XLOGfileslop is the maximum number of preallocated future XLOG segments.
  * When we are done with an old XLOG segment file, we will recycle it as a
@@ -1095,9 +1101,9 @@ begin:;
 	 * inserter acquires an insertion lock. In addition to just indicating that
 	 * an insertion is in progress, the lock tells others how far the inserter
 	 * has progressed. There is a small fixed number of insertion locks,
-	 * determined by the num_xloginsert_locks GUC. When an inserter crosses a
-	 * page boundary, it updates the value stored in the lock to the how far it
-	 * has inserted, to allow the previous buffer to be flushed.
+	 * determined by NUM_XLOGINSERT_LOCKS. When an inserter crosses a page
+	 * boundary, it updates the value stored in the lock to the how far it has
+	 * inserted, to allow the previous buffer to be flushed.
 	 *
 	 * Holding onto an insertion lock also protects RedoRecPtr and
 	 * fullPageWrites from changing until the insertion is finished.
@@ -1578,7 +1584,7 @@ WALInsertLockAcquire(void)
 	static int	lockToTry = -1;
 
 	if (lockToTry == -1)
-		lockToTry = MyProc->pgprocno % num_xloginsert_locks;
+		lockToTry = MyProc->pgprocno % NUM_XLOGINSERT_LOCKS;
 	MyLockNo = lockToTry;
 
 	/*
@@ -1598,7 +1604,7 @@ WALInsertLockAcquire(void)
 		 * than locks, it still helps to distribute the inserters evenly
 		 * across the locks.
 		 */
-		lockToTry = (lockToTry + 1) % num_xloginsert_locks;
+		lockToTry = (lockToTry + 1) % NUM_XLOGINSERT_LOCKS;
 	}
 }
 
@@ -1617,7 +1623,7 @@ WALInsertLockAcquireExclusive(void)
 	 * than any real XLogRecPtr value, to make sure that no-one blocks waiting
 	 * on those.
 	 */
-	for (i = 0; i < num_xloginsert_locks - 1; i++)
+	for (i = 0; i < NUM_XLOGINSERT_LOCKS - 1; i++)
 	{
 		LWLockAcquireWithVar(&WALInsertLocks[i].l.lock,
 							 &WALInsertLocks[i].l.insertingAt,
@@ -1640,7 +1646,7 @@ WALInsertLockRelease(void)
 	{
 		int			i;
 
-		for (i = 0; i < num_xloginsert_locks; i++)
+		for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
 			LWLockRelease(&WALInsertLocks[i].l.lock);
 
 		holdingAllLocks = false;
@@ -1664,8 +1670,8 @@ WALInsertLockUpdateInsertingAt(XLogRecPtr insertingAt)
 		 * We use the last lock to mark our actual position, see comments in
 		 * WALInsertLockAcquireExclusive.
 		 */
-		LWLockUpdateVar(&WALInsertLocks[num_xloginsert_locks - 1].l.lock,
-					 &WALInsertLocks[num_xloginsert_locks - 1].l.insertingAt,
+		LWLockUpdateVar(&WALInsertLocks[NUM_XLOGINSERT_LOCKS - 1].l.lock,
+					 &WALInsertLocks[NUM_XLOGINSERT_LOCKS - 1].l.insertingAt,
 						insertingAt);
 	}
 	else
@@ -1732,7 +1738,7 @@ WaitXLogInsertionsToFinish(XLogRecPtr upto)
 	 * out for any insertion that's still in progress.
 	 */
 	finishedUpto = reservedUpto;
-	for (i = 0; i < num_xloginsert_locks; i++)
+	for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
 	{
 		XLogRecPtr	insertingat = InvalidXLogRecPtr;
 
@@ -4752,7 +4758,7 @@ XLOGShmemSize(void)
 	size = sizeof(XLogCtlData);
 
 	/* WAL insertion locks, plus alignment */
-	size = add_size(size, mul_size(sizeof(WALInsertLockPadded), num_xloginsert_locks + 1));
+	size = add_size(size, mul_size(sizeof(WALInsertLockPadded), NUM_XLOGINSERT_LOCKS + 1));
 	/* xlblocks array */
 	size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
 	/* extra alignment padding for XLOG I/O buffers */
@@ -4829,7 +4835,7 @@ XLOGShmemInit(void)
 		((uintptr_t) allocptr) %sizeof(WALInsertLockPadded);
 	WALInsertLocks = XLogCtl->Insert.WALInsertLocks =
 		(WALInsertLockPadded *) allocptr;
-	allocptr += sizeof(WALInsertLockPadded) * num_xloginsert_locks;
+	allocptr += sizeof(WALInsertLockPadded) * NUM_XLOGINSERT_LOCKS;
 
 	XLogCtl->Insert.WALInsertLockTrancheId = LWLockNewTrancheId();
 
@@ -4838,7 +4844,7 @@ XLOGShmemInit(void)
 	XLogCtl->Insert.WALInsertLockTranche.array_stride = sizeof(WALInsertLockPadded);
 
 	LWLockRegisterTranche(XLogCtl->Insert.WALInsertLockTrancheId, &XLogCtl->Insert.WALInsertLockTranche);
-	for (i = 0; i < num_xloginsert_locks; i++)
+	for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
 	{
 		LWLockInitialize(&WALInsertLocks[i].l.lock,
 						 XLogCtl->Insert.WALInsertLockTrancheId);

src/backend/utils/misc/guc.c

@@ -2143,17 +2143,6 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
-	{
-		{"xloginsert_locks", PGC_POSTMASTER, WAL_SETTINGS,
-			gettext_noop("Sets the number of locks used for concurrent xlog insertions."),
-			NULL,
-			GUC_NOT_IN_SAMPLE
-		},
-		&num_xloginsert_locks,
-		8, 1, 1000,
-		NULL, NULL, NULL
-	},
-
 	{
 		/* see max_connections */
 		{"max_wal_senders", PGC_POSTMASTER, REPLICATION_SENDING,

src/include/access/xlog.h

@@ -194,7 +194,6 @@ extern bool EnableHotStandby;
 extern bool fullPageWrites;
 extern bool wal_log_hints;
 extern bool log_checkpoints;
-extern int	num_xloginsert_locks;
 
 /* WAL levels */
 typedef enum WalLevel