lwlock: Fix quadratic behavior with very long wait lists

Until now LWLockDequeueSelf() sequentially searched the list of waiters to see
if the current proc is still is on the list of waiters, or has already been
removed. In extreme workloads, where the wait lists are very long, this leads
to a quadratic behavior. #backends iterating over a list #backends
long. Additionally, the likelihood of needing to call LWLockDequeueSelf() in
the first place also increases with the increased length of the wait queue, as
it becomes more likely that a lock is released while waiting for the wait list
lock, which is held for longer during lock release.

Due to the exponential back-off in perform_spin_delay() this is surprisingly
hard to detect. We should make that easier, e.g. by adding a wait event around
the pg_usleep() - but that's a separate patch.

The fix is simple - track whether a proc is currently waiting in the wait list
or already removed but waiting to be woken up in PGPROC->lwWaiting.

In some workloads with a lot of clients contending for a small number of
lwlocks (e.g. WALWriteLock), the fix can substantially increase throughput.

As the quadratic behavior arguably is a bug, we might want to decide to
backpatch this fix in the future.

Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/20221027165914.2hofzp4cvutj6gin@awork3.anarazel.de
Discussion: https://postgr.es/m/CALj2ACXktNbG=K8Xi7PSqbofTZozavhaxjatVc14iYaLu4Maag@mail.gmail.com

Author: anarazel

src/backend/access/transam/twophase.c

@@ -485,7 +485,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
 	proc->roleId = owner;
 	proc->tempNamespaceId = InvalidOid;
 	proc->isBackgroundWorker = false;
-	proc->lwWaiting = false;
+	proc->lwWaiting = LW_WS_NOT_WAITING;
 	proc->lwWaitMode = 0;
 	proc->waitLock = NULL;
 	proc->waitProcLock = NULL;

src/backend/storage/lmgr/lwlock.c

@@ -982,6 +982,15 @@ LWLockWakeup(LWLock *lock)
 			wokeup_somebody = true;
 		}
 
+		/*
+		 * Signal that the process isn't on the wait list anymore. This allows
+		 * LWLockDequeueSelf() to remove itself of the waitlist with a
+		 * proclist_delete(), rather than having to check if it has been
+		 * removed from the list.
+		 */
+		Assert(waiter->lwWaiting == LW_WS_WAITING);
+		waiter->lwWaiting = LW_WS_PENDING_WAKEUP;
+
 		/*
 		 * Once we've woken up an exclusive lock, there's no point in waking
 		 * up anybody else.
@@ -1039,7 +1048,7 @@ LWLockWakeup(LWLock *lock)
 		 * another lock.
 		 */
 		pg_write_barrier();
-		waiter->lwWaiting = false;
+		waiter->lwWaiting = LW_WS_NOT_WAITING;
 		PGSemaphoreUnlock(waiter->sem);
 	}
 }
@@ -1060,15 +1069,15 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
 	if (MyProc == NULL)
 		elog(PANIC, "cannot wait without a PGPROC structure");
 
-	if (MyProc->lwWaiting)
+	if (MyProc->lwWaiting != LW_WS_NOT_WAITING)
 		elog(PANIC, "queueing for lock while waiting on another one");
 
 	LWLockWaitListLock(lock);
 
 	/* setting the flag is protected by the spinlock */
 	pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_HAS_WAITERS);
 
-	MyProc->lwWaiting = true;
+	MyProc->lwWaiting = LW_WS_WAITING;
 	MyProc->lwWaitMode = mode;
 
 	/* LW_WAIT_UNTIL_FREE waiters are always at the front of the queue */
@@ -1095,8 +1104,7 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
 static void
 LWLockDequeueSelf(LWLock *lock)
 {
-	bool		found = false;
-	proclist_mutable_iter iter;
+	bool		on_waitlist;
 
 #ifdef LWLOCK_STATS
 	lwlock_stats *lwstats;
@@ -1109,18 +1117,13 @@ LWLockDequeueSelf(LWLock *lock)
 	LWLockWaitListLock(lock);
 
 	/*
-	 * Can't just remove ourselves from the list, but we need to iterate over
-	 * all entries as somebody else could have dequeued us.
+	 * Remove ourselves from the waitlist, unless we've already been
+	 * removed. The removal happens with the wait list lock held, so there's
+	 * no race in this check.
 	 */
-	proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
-	{
-		if (iter.cur == MyProc->pgprocno)
-		{
-			found = true;
-			proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
-			break;
-		}
-	}
+	on_waitlist = MyProc->lwWaiting == LW_WS_WAITING;
+	if (on_waitlist)
+		proclist_delete(&lock->waiters, MyProc->pgprocno, lwWaitLink);
 
 	if (proclist_is_empty(&lock->waiters) &&
 		(pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS) != 0)
@@ -1132,8 +1135,8 @@ LWLockDequeueSelf(LWLock *lock)
 	LWLockWaitListUnlock(lock);
 
 	/* clear waiting state again, nice for debugging */
-	if (found)
-		MyProc->lwWaiting = false;
+	if (on_waitlist)
+		MyProc->lwWaiting = LW_WS_NOT_WAITING;
 	else
 	{
 		int			extraWaits = 0;
@@ -1157,7 +1160,7 @@ LWLockDequeueSelf(LWLock *lock)
 		for (;;)
 		{
 			PGSemaphoreLock(MyProc->sem);
-			if (!MyProc->lwWaiting)
+			if (MyProc->lwWaiting == LW_WS_NOT_WAITING)
 				break;
 			extraWaits++;
 		}
@@ -1308,7 +1311,7 @@ LWLockAcquire(LWLock *lock, LWLockMode mode)
 		for (;;)
 		{
 			PGSemaphoreLock(proc->sem);
-			if (!proc->lwWaiting)
+			if (proc->lwWaiting == LW_WS_NOT_WAITING)
 				break;
 			extraWaits++;
 		}
@@ -1473,7 +1476,7 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
 			for (;;)
 			{
 				PGSemaphoreLock(proc->sem);
-				if (!proc->lwWaiting)
+				if (proc->lwWaiting == LW_WS_NOT_WAITING)
 					break;
 				extraWaits++;
 			}
@@ -1689,7 +1692,7 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
 		for (;;)
 		{
 			PGSemaphoreLock(proc->sem);
-			if (!proc->lwWaiting)
+			if (proc->lwWaiting == LW_WS_NOT_WAITING)
 				break;
 			extraWaits++;
 		}
@@ -1767,6 +1770,10 @@ LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
 
 		proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
 		proclist_push_tail(&wakeup, iter.cur, lwWaitLink);
+
+		/* see LWLockWakeup() */
+		Assert(waiter->lwWaiting == LW_WS_WAITING);
+		waiter->lwWaiting = LW_WS_PENDING_WAKEUP;
 	}
 
 	/* We are done updating shared state of the lock itself. */
@@ -1782,7 +1789,7 @@ LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
 		proclist_delete(&wakeup, iter.cur, lwWaitLink);
 		/* check comment in LWLockWakeup() about this barrier */
 		pg_write_barrier();
-		waiter->lwWaiting = false;
+		waiter->lwWaiting = LW_WS_NOT_WAITING;
 		PGSemaphoreUnlock(waiter->sem);
 	}
 }

src/backend/storage/lmgr/proc.c

@@ -397,7 +397,7 @@ InitProcess(void)
 	/* NB -- autovac launcher intentionally does not set IS_AUTOVACUUM */
 	if (IsAutoVacuumWorkerProcess())
 		MyProc->statusFlags |= PROC_IS_AUTOVACUUM;
-	MyProc->lwWaiting = false;
+	MyProc->lwWaiting = LW_WS_NOT_WAITING;
 	MyProc->lwWaitMode = 0;
 	MyProc->waitLock = NULL;
 	MyProc->waitProcLock = NULL;
@@ -579,7 +579,7 @@ InitAuxiliaryProcess(void)
 	MyProc->isBackgroundWorker = IsBackgroundWorker;
 	MyProc->delayChkptFlags = 0;
 	MyProc->statusFlags = 0;
-	MyProc->lwWaiting = false;
+	MyProc->lwWaiting = LW_WS_NOT_WAITING;
 	MyProc->lwWaitMode = 0;
 	MyProc->waitLock = NULL;
 	MyProc->waitProcLock = NULL;

src/include/storage/lwlock.h

@@ -23,6 +23,14 @@
 
 struct PGPROC;
 
+/* what state of the wait process is a backend in */
+typedef enum LWLockWaitState
+{
+	LW_WS_NOT_WAITING, /* not currently waiting / woken up */
+	LW_WS_WAITING, /* currently waiting */
+	LW_WS_PENDING_WAKEUP, /* removed from waitlist, but not yet signalled */
+} LWLockWaitState;
+
 /*
  * Code outside of lwlock.c should not manipulate the contents of this
  * structure directly, but we have to declare it here to allow LWLocks to be

src/include/storage/proc.h

@@ -217,7 +217,7 @@ struct PGPROC
 	bool		recoveryConflictPending;
 
 	/* Info about LWLock the process is currently waiting for, if any. */
-	bool		lwWaiting;		/* true if waiting for an LW lock */
+	uint8		lwWaiting;		/* see LWLockWaitState */
 	uint8		lwWaitMode;		/* lwlock mode being waited for */
 	proclist_node lwWaitLink;	/* position in LW lock wait list */