Reduce sinval synchronization overhead.

Testing shows that the overhead of acquiring and releasing
SInvalReadLock and msgNumLock on high-core count boxes can waste a lot
of CPU time and hurt performance.  This patch adds a per-backend flag
that allows us to skip all that locking in most cases.  Further
testing shows that this improves performance even when sinval traffic
is very high.

Patch by me.  Review and testing by Noah Misch.

src/backend/storage/ipc/sinvaladt.c

@@ -143,6 +143,7 @@ typedef struct ProcState
 	int			nextMsgNum;		/* next message number to read */
 	bool		resetState;		/* backend needs to reset its state */
 	bool		signaled;		/* backend has been sent catchup signal */
+	bool		hasMessages;	/* backend has unread messages */
 
 	/*
 	 * Backend only sends invalidations, never receives them. This only makes
@@ -248,6 +249,7 @@ CreateSharedInvalidationState(void)
 		shmInvalBuffer->procState[i].nextMsgNum = 0;	/* meaningless */
 		shmInvalBuffer->procState[i].resetState = false;
 		shmInvalBuffer->procState[i].signaled = false;
+		shmInvalBuffer->procState[i].hasMessages = false;
 		shmInvalBuffer->procState[i].nextLXID = InvalidLocalTransactionId;
 	}
 }
@@ -264,11 +266,9 @@ SharedInvalBackendInit(bool sendOnly)
 	SISeg	   *segP = shmInvalBuffer;
 
 	/*
-	 * This can run in parallel with read operations, and for that matter with
-	 * write operations; but not in parallel with additions and removals of
-	 * backends, nor in parallel with SICleanupQueue.  It doesn't seem worth
-	 * having a third lock, so we choose to use SInvalWriteLock to serialize
-	 * additions/removals.
+	 * This can run in parallel with read operations, but not with write
+	 * operations, since SIInsertDataEntries relies on lastBackend to set
+	 * hasMessages appropriately.
 	 */
 	LWLockAcquire(SInvalWriteLock, LW_EXCLUSIVE);
 
@@ -316,6 +316,7 @@ SharedInvalBackendInit(bool sendOnly)
 	stateP->nextMsgNum = segP->maxMsgNum;
 	stateP->resetState = false;
 	stateP->signaled = false;
+	stateP->hasMessages = false;
 	stateP->sendOnly = sendOnly;
 
 	LWLockRelease(SInvalWriteLock);
@@ -417,6 +418,7 @@ SIInsertDataEntries(const SharedInvalidationMessage *data, int n)
 		int			nthistime = Min(n, WRITE_QUANTUM);
 		int			numMsgs;
 		int			max;
+		int			i;
 
 		n -= nthistime;
 
@@ -459,6 +461,19 @@ SIInsertDataEntries(const SharedInvalidationMessage *data, int n)
 			SpinLockRelease(&vsegP->msgnumLock);
 		}
 
+		/*
+		 * Now that the maxMsgNum change is globally visible, we give
+		 * everyone a swift kick to make sure they read the newly added
+		 * messages.  Releasing SInvalWriteLock will enforce a full memory
+		 * barrier, so these (unlocked) changes will be committed to memory
+		 * before we exit the function.
+		 */
+		for (i = 0; i < segP->lastBackend; i++)
+		{
+			ProcState  *stateP = &segP->procState[i];
+			stateP->hasMessages = TRUE;
+		}
+
 		LWLockRelease(SInvalWriteLock);
 	}
 }
@@ -499,11 +514,36 @@ SIGetDataEntries(SharedInvalidationMessage *data, int datasize)
 	int			max;
 	int			n;
 
-	LWLockAcquire(SInvalReadLock, LW_SHARED);
-
 	segP = shmInvalBuffer;
 	stateP = &segP->procState[MyBackendId - 1];
 
+	/*
+	 * Before starting to take locks, do a quick, unlocked test to see whether
+	 * there can possibly be anything to read.  On a multiprocessor system,
+	 * it's possible that this load could migrate backwards and occur before we
+	 * actually enter this function, so we might miss a sinval message that
+	 * was just added by some other processor.  But they can't migrate
+	 * backwards over a preceding lock acquisition, so it should be OK.  If
+	 * we haven't acquired a lock preventing against further relevant
+	 * invalidations, any such occurrence is not much different than if the
+	 * invalidation had arrived slightly later in the first place.
+	 */
+	if (!stateP->hasMessages)
+		return 0;
+
+	LWLockAcquire(SInvalReadLock, LW_SHARED);
+
+	/*
+	 * We must reset hasMessages before determining how many messages we're
+	 * going to read.  That way, if new messages arrive after we have
+	 * determined how many we're reading, the flag will get reset and we'll
+	 * notice those messages part-way through.
+	 *
+	 * Note that, if we don't end up reading all of the messages, we had
+	 * better be certain to reset this flag before exiting!
+	 */
+	stateP->hasMessages = FALSE;
+
 	/* Fetch current value of maxMsgNum using spinlock */
 	{
 		/* use volatile pointer to prevent code rearrangement */
@@ -544,10 +584,16 @@ SIGetDataEntries(SharedInvalidationMessage *data, int datasize)
 	}
 
 	/*
-	 * Reset our "signaled" flag whenever we have caught up completely.
+	 * If we have caught up completely, reset our "signaled" flag so that
+	 * we'll get another signal if we fall behind again.
+	 *
+	 * If we haven't catch up completely, reset the hasMessages flag so that
+	 * we see the remaining messages next time.
 	 */
 	if (stateP->nextMsgNum >= max)
 		stateP->signaled = false;
+	else
+		stateP->hasMessages = TRUE;
 
 	LWLockRelease(SInvalReadLock);
 	return n;