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.

This has been originally fixed for 16~ with a4adc31f6902 without a
backpatch, and we have heard complaints from users impacted by this
quadratic behavior in older versions as well.

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
Backpatch-through: 12

diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 5293c6920b38c5bc0419cabcd978e133b05e2282..ca7037eb2f987a52f93a8dc9dc898c62bffd4951 100644 (file)
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -486,7 +486,7 @@ 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;

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 3bb448627cd0ea0e4b36e22f890b078990f0196e..ea34ee785cba5406b43482c223e0ebf59507b0b0 100644 (file)
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -987,6 +987,15 @@ 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.
@@ -1044,7 +1053,7 @@ LWLockWakeup(LWLock *lock)
                 * another lock.
                 */
                pg_write_barrier();
-               waiter->lwWaiting = false;
+               waiter->lwWaiting = LW_WS_NOT_WAITING;
                PGSemaphoreUnlock(waiter->sem);
        }
 }
@@ -1065,7 +1074,7 @@ 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);
@@ -1073,7 +1082,7 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
        /* 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 */
@@ -1100,8 +1109,7 @@ 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;
@@ -1114,18 +1122,13 @@ 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)
@@ -1137,8 +1140,8 @@ 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;
@@ -1162,7 +1165,7 @@ LWLockDequeueSelf(LWLock *lock)
                for (;;)
                {
                        PGSemaphoreLock(MyProc->sem);
-                       if (!MyProc->lwWaiting)
+                       if (MyProc->lwWaiting == LW_WS_NOT_WAITING)
                                break;
                        extraWaits++;
                }
@@ -1313,7 +1316,7 @@ LWLockAcquire(LWLock *lock, LWLockMode mode)
                for (;;)
                {
                        PGSemaphoreLock(proc->sem);
-                       if (!proc->lwWaiting)
+                       if (proc->lwWaiting == LW_WS_NOT_WAITING)
                                break;
                        extraWaits++;
                }
@@ -1478,7 +1481,7 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
                        for (;;)
                        {
                                PGSemaphoreLock(proc->sem);
-                               if (!proc->lwWaiting)
+                               if (proc->lwWaiting == LW_WS_NOT_WAITING)
                                        break;
                                extraWaits++;
                        }
@@ -1694,7 +1697,7 @@ 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++;
                }
@@ -1772,6 +1775,10 @@ 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. */
@@ -1787,7 +1794,7 @@ 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);
        }
 }

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1b514f44e3349d802735a649729b72ee454d5978..c60707b9dbf9ba662454170330c99ea33ba1d97e 100644 (file)
--- a/src/backend/storage/lmgr/proc.c
+++ b/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;

diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h
index e03d317eeac744a5ca2fe18992b3572619e59bad..d88fa4b4ad985acdfd86a0a7b19cd183c7729897 100644 (file)
--- a/src/include/storage/lwlock.h
+++ b/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

diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 2579e619ebcdd7d72e10218cd4fab1b4ead35a58..2ea773098b1817a6680e244f2f74df86a77cd44f 100644 (file)
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -211,7 +211,7 @@ 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 */