Optimize shared memory usage for WaitLSNProcInfo

We need separate pairing heaps for different WaitLSNType's, because there
might be waiters for different LSN's at the same time.  However, one process
can wait only for one type of LSN at a time.  So, no need for inHeap
and heapNode fields to be arrays.

Discussion: https://postgr.es/m/CAPpHfdsBR-7sDtXFJ1qpJtKiohfGoj%3DvqzKVjWxtWsWidx7G_A%40mail.gmail.com
Author: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>

src/backend/access/transam/xlogwait.c

@@ -90,7 +90,7 @@ WaitLSNShmemInit(void)
 		for (i = 0; i < WAIT_LSN_TYPE_COUNT; i++)
 		{
 			pg_atomic_init_u64(&waitLSNState->minWaitedLSN[i], PG_UINT64_MAX);
-			pairingheap_initialize(&waitLSNState->waitersHeap[i], waitlsn_cmp, (void *) (uintptr_t) i);
+			pairingheap_initialize(&waitLSNState->waitersHeap[i], waitlsn_cmp, NULL);
 		}
 
 		/* Initialize process info array */
@@ -106,9 +106,8 @@ WaitLSNShmemInit(void)
 static int
 waitlsn_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
 {
-	int			i = (uintptr_t) arg;
-	const WaitLSNProcInfo *aproc = pairingheap_const_container(WaitLSNProcInfo, heapNode[i], a);
-	const WaitLSNProcInfo *bproc = pairingheap_const_container(WaitLSNProcInfo, heapNode[i], b);
+	const WaitLSNProcInfo *aproc = pairingheap_const_container(WaitLSNProcInfo, heapNode, a);
+	const WaitLSNProcInfo *bproc = pairingheap_const_container(WaitLSNProcInfo, heapNode, b);
 
 	if (aproc->waitLSN < bproc->waitLSN)
 		return 1;
@@ -132,7 +131,7 @@ updateMinWaitedLSN(WaitLSNType lsnType)
 	if (!pairingheap_is_empty(&waitLSNState->waitersHeap[i]))
 	{
 		pairingheap_node *node = pairingheap_first(&waitLSNState->waitersHeap[i]);
-		WaitLSNProcInfo *procInfo = pairingheap_container(WaitLSNProcInfo, heapNode[i], node);
+		WaitLSNProcInfo *procInfo = pairingheap_container(WaitLSNProcInfo, heapNode, node);
 
 		minWaitedLSN = procInfo->waitLSN;
 	}
@@ -154,10 +153,11 @@ addLSNWaiter(XLogRecPtr lsn, WaitLSNType lsnType)
 
 	procInfo->procno = MyProcNumber;
 	procInfo->waitLSN = lsn;
+	procInfo->lsnType = lsnType;
 
-	Assert(!procInfo->inHeap[i]);
-	pairingheap_add(&waitLSNState->waitersHeap[i], &procInfo->heapNode[i]);
-	procInfo->inHeap[i] = true;
+	Assert(!procInfo->inHeap);
+	pairingheap_add(&waitLSNState->waitersHeap[i], &procInfo->heapNode);
+	procInfo->inHeap = true;
 	updateMinWaitedLSN(lsnType);
 
 	LWLockRelease(WaitLSNLock);
@@ -176,10 +176,12 @@ deleteLSNWaiter(WaitLSNType lsnType)
 
 	LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
 
-	if (procInfo->inHeap[i])
+	Assert(procInfo->lsnType == lsnType);
+
+	if (procInfo->inHeap)
 	{
-		pairingheap_remove(&waitLSNState->waitersHeap[i], &procInfo->heapNode[i]);
-		procInfo->inHeap[i] = false;
+		pairingheap_remove(&waitLSNState->waitersHeap[i], &procInfo->heapNode);
+		procInfo->inHeap = false;
 		updateMinWaitedLSN(lsnType);
 	}
 
@@ -228,7 +230,7 @@ wakeupWaiters(WaitLSNType lsnType, XLogRecPtr currentLSN)
 			WaitLSNProcInfo *procInfo;
 
 			/* Get procInfo using appropriate heap node */
-			procInfo = pairingheap_container(WaitLSNProcInfo, heapNode[i], node);
+			procInfo = pairingheap_container(WaitLSNProcInfo, heapNode, node);
 
 			if (XLogRecPtrIsValid(currentLSN) && procInfo->waitLSN > currentLSN)
 				break;
@@ -238,7 +240,7 @@ wakeupWaiters(WaitLSNType lsnType, XLogRecPtr currentLSN)
 			(void) pairingheap_remove_first(&waitLSNState->waitersHeap[i]);
 
 			/* Update appropriate flag */
-			procInfo->inHeap[i] = false;
+			procInfo->inHeap = false;
 
 			if (numWakeUpProcs == WAKEUP_PROC_STATIC_ARRAY_SIZE)
 				break;
@@ -289,20 +291,14 @@ WaitLSNCleanup(void)
 {
 	if (waitLSNState)
 	{
-		int			i;
-
 		/*
-		 * We do a fast-path check of the heap flags without the lock.  These
-		 * flags are set to true only by the process itself.  So, it's only
+		 * We do a fast-path check of the inHeap flag without the lock.  This
+		 * flag is set to true only by the process itself.  So, it's only
 		 * possible to get a false positive.  But that will be eliminated by a
 		 * recheck inside deleteLSNWaiter().
 		 */
-
-		for (i = 0; i < (int) WAIT_LSN_TYPE_COUNT; i++)
-		{
-			if (waitLSNState->procInfos[MyProcNumber].inHeap[i])
-				deleteLSNWaiter((WaitLSNType) i);
-		}
+		if (waitLSNState->procInfos[MyProcNumber].inHeap)
+			deleteLSNWaiter(waitLSNState->procInfos[MyProcNumber].lsnType);
 	}
 }
 

src/include/access/xlogwait.h

@@ -50,14 +50,20 @@ typedef struct WaitLSNProcInfo
 	/* LSN, which this process is waiting for */
 	XLogRecPtr	waitLSN;
 
+	/* The type of LSN to wait */
+	WaitLSNType lsnType;
+
 	/* Process to wake up once the waitLSN is reached */
 	ProcNumber	procno;
 
-	/* Heap membership flags for LSN types */
-	bool		inHeap[WAIT_LSN_TYPE_COUNT];
+	/*
+	 * Heap membership flag.  A process can wait for only one LSN type at a
+	 * time, so a single flag suffices (tracked by the lsnType field).
+	 */
+	bool		inHeap;
 
-	/* Heap nodes for LSN types */
-	pairingheap_node heapNode[WAIT_LSN_TYPE_COUNT];
+	/* Pairing heap node for the waiters' heap (one per process) */
+	pairingheap_node heapNode;
 } WaitLSNProcInfo;
 
 /*