Use an LWLock instead of a spinlock in waitlsn.c

akorotkov · akorotkov · commit 25f42429e2ff · 2024-04-07T00:49:53.000+03:00
This should prevent busy-waiting when number of waiting processes is high. Discussion: https://postgr.es/m/202404030658.hhj3vfxeyhft%40alvherre.pgsql Author: Alvaro Herrera
diff --git a/src/backend/commands/waitlsn.c b/src/backend/commands/waitlsn.c
@@ -58,7 +58,6 @@ WaitLSNShmemInit(void)
 											   &found);
 	if (!found)
 	{
-		SpinLockInit(&waitLSN->waitersHeapMutex);
 		pg_atomic_init_u64(&waitLSN->minWaitedLSN, PG_UINT64_MAX);
 		pairingheap_initialize(&waitLSN->waitersHeap, lsn_cmp, NULL);
 		memset(&waitLSN->procInfos, 0, MaxBackends * sizeof(WaitLSNProcInfo));
@@ -115,13 +114,13 @@ addLSNWaiter(XLogRecPtr lsn)
 	procInfo->procnum = MyProcNumber;
 	procInfo->waitLSN = lsn;
 
-	SpinLockAcquire(&waitLSN->waitersHeapMutex);
+	LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
 
 	pairingheap_add(&waitLSN->waitersHeap, &procInfo->phNode);
 	procInfo->inHeap = true;
 	updateMinWaitedLSN();
 
-	SpinLockRelease(&waitLSN->waitersHeapMutex);
+	LWLockRelease(WaitLSNLock);
 }
 
 /*
@@ -132,19 +131,19 @@ deleteLSNWaiter(void)
 {
 	WaitLSNProcInfo *procInfo = &waitLSN->procInfos[MyProcNumber];
 
-	SpinLockAcquire(&waitLSN->waitersHeapMutex);
+	LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
 
 	if (!procInfo->inHeap)
 	{
-		SpinLockRelease(&waitLSN->waitersHeapMutex);
+		LWLockRelease(WaitLSNLock);
 		return;
 	}
 
 	pairingheap_remove(&waitLSN->waitersHeap, &procInfo->phNode);
 	procInfo->inHeap = false;
 	updateMinWaitedLSN();
 
-	SpinLockRelease(&waitLSN->waitersHeapMutex);
+	LWLockRelease(WaitLSNLock);
 }
 
 /*
@@ -160,7 +159,7 @@ WaitLSNSetLatches(XLogRecPtr currentLSN)
 
 	wakeUpProcNums = palloc(sizeof(int) * MaxBackends);
 
-	SpinLockAcquire(&waitLSN->waitersHeapMutex);
+	LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
 
 	/*
 	 * Iterate the pairing heap of waiting processes till we find LSN not yet
@@ -182,7 +181,7 @@ WaitLSNSetLatches(XLogRecPtr currentLSN)
 
 	updateMinWaitedLSN();
 
-	SpinLockRelease(&waitLSN->waitersHeapMutex);
+	LWLockRelease(WaitLSNLock);
 
 	/*
 	 * Set latches for processes, whose waited LSNs are already replayed. This
diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt
@@ -345,6 +345,7 @@ WALSummarizer	"Waiting to read or update WAL summarization state."
 DSMRegistry	"Waiting to read or update the dynamic shared memory registry."
 InjectionPoint	"Waiting to read or update information related to injection points."
 SerialControl	"Waiting to read or update shared <filename>pg_serial</filename> state."
+WaitLSN	"Waiting to read or update shared Wait-for-LSN state."
 
 #
 # END OF PREDEFINED LWLOCKS (DO NOT CHANGE THIS LINE)
diff --git a/src/include/commands/waitlsn.h b/src/include/commands/waitlsn.h
@@ -55,13 +55,10 @@ typedef struct WaitLSNState
 
 	/*
 	 * A pairing heap of waiting processes order by LSN values (least LSN is
-	 * on top).
+	 * on top).  Protected by WaitLSNLock.
 	 */
 	pairingheap waitersHeap;
 
-	/* A mutex protecting the pairing heap above */
-	slock_t		waitersHeapMutex;
-
 	/* An array with per-process information, indexed by the process number */
 	WaitLSNProcInfo procInfos[FLEXIBLE_ARRAY_MEMBER];
 } WaitLSNState;
diff --git a/src/include/storage/lwlocklist.h b/src/include/storage/lwlocklist.h
@@ -83,3 +83,4 @@ PG_LWLOCK(49, WALSummarizer)
 PG_LWLOCK(50, DSMRegistry)
 PG_LWLOCK(51, InjectionPoint)
 PG_LWLOCK(52, SerialControl)
+PG_LWLOCK(53, WaitLSN)

Original file line number	Diff line number	Diff line change
`@@ -345,6 +345,7 @@ WALSummarizer "Waiting to read or update WAL summarization state."`
`345`	`345`	`DSMRegistry "Waiting to read or update the dynamic shared memory registry."`
`346`	`346`	`InjectionPoint "Waiting to read or update information related to injection points."`
`347`	`347`	`SerialControl "Waiting to read or update shared <filename>pg_serial</filename> state."`
	`348`	`+WaitLSN "Waiting to read or update shared Wait-for-LSN state."`
`348`	`349`
`349`	`350`	`#`
`350`	`351`	`# END OF PREDEFINED LWLOCKS (DO NOT CHANGE THIS LINE)`