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Commit07e4d03

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Improve LISTEN startup time when there are many unread notifications.
If some existing listener is far behind, incoming new listener sessionswould start from that session's read pointer and then need to advance overmany already-committed notification messages, which they have no interestin. This was expensive in itself and also thrashed the pg_notify SLRUbuffers a lot more than necessary. We can improve matters considerablyin typical scenarios, without much added cost, by starting from thefurthest-ahead read pointer, not the furthest-behind one. We do have toconsider only sessions in our own database when doing this, which requiresan extra field in the data structure, but that's a pretty small cost.Back-patch to 9.0 where the current LISTEN/NOTIFY logic was introduced.Matt Newell, slightly adjusted by me
1 parent3bd909b commit07e4d03

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‎src/backend/commands/async.c

Lines changed: 44 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -202,12 +202,19 @@ typedef struct QueuePosition
202202
(x).page != (y).page ? (y) : \
203203
(x).offset < (y).offset ? (x) : (y))
204204

205+
/* choose logically larger QueuePosition */
206+
#defineQUEUE_POS_MAX(x,y) \
207+
(asyncQueuePagePrecedes((x).page, (y).page) ? (y) : \
208+
(x).page != (y).page ? (x) : \
209+
(x).offset > (y).offset ? (x) : (y))
210+
205211
/*
206212
* Struct describing a listening backend's status
207213
*/
208214
typedefstructQueueBackendStatus
209215
{
210216
int32pid;/* either a PID or InvalidPid */
217+
Oiddboid;/* backend's database OID, or InvalidOid */
211218
QueuePositionpos;/* backend has read queue up to here */
212219
}QueueBackendStatus;
213220

@@ -235,8 +242,8 @@ typedef struct QueueBackendStatus
235242
typedefstructAsyncQueueControl
236243
{
237244
QueuePositionhead;/* head points to the next free location */
238-
QueuePositiontail;/* the global tail is equivalent to the pos
239-
*ofthe "slowest" backend */
245+
QueuePositiontail;/* the global tail is equivalent to the pos of
246+
* the "slowest" backend */
240247
TimestampTzlastQueueFillWarn;/* time of last queue-full msg */
241248
QueueBackendStatusbackend[FLEXIBLE_ARRAY_MEMBER];
242249
/* backend[0] is not used; used entries are from [1] to [MaxBackends] */
@@ -247,6 +254,7 @@ static AsyncQueueControl *asyncQueueControl;
247254
#defineQUEUE_HEAD(asyncQueueControl->head)
248255
#defineQUEUE_TAIL(asyncQueueControl->tail)
249256
#defineQUEUE_BACKEND_PID(i)(asyncQueueControl->backend[i].pid)
257+
#defineQUEUE_BACKEND_DBOID(i)(asyncQueueControl->backend[i].dboid)
250258
#defineQUEUE_BACKEND_POS(i)(asyncQueueControl->backend[i].pos)
251259

252260
/*
@@ -461,6 +469,7 @@ AsyncShmemInit(void)
461469
for (i=0;i <=MaxBackends;i++)
462470
{
463471
QUEUE_BACKEND_PID(i)=InvalidPid;
472+
QUEUE_BACKEND_DBOID(i)=InvalidOid;
464473
SET_QUEUE_POS(QUEUE_BACKEND_POS(i),0,0);
465474
}
466475
}
@@ -907,6 +916,10 @@ AtCommit_Notify(void)
907916
staticvoid
908917
Exec_ListenPreCommit(void)
909918
{
919+
QueuePositionhead;
920+
QueuePositionmax;
921+
inti;
922+
910923
/*
911924
* Nothing to do if we are already listening to something, nor if we
912925
* already ran this routine in this transaction.
@@ -934,10 +947,34 @@ Exec_ListenPreCommit(void)
934947
* over already-committed notifications. This ensures we cannot miss any
935948
* not-yet-committed notifications. We might get a few more but that
936949
* doesn't hurt.
950+
*
951+
* In some scenarios there might be a lot of committed notifications that
952+
* have not yet been pruned away (because some backend is being lazy about
953+
* reading them). To reduce our startup time, we can look at other
954+
* backends and adopt the maximum "pos" pointer of any backend that's in
955+
* our database; any notifications it's already advanced over are surely
956+
* committed and need not be re-examined by us. (We must consider only
957+
* backends connected to our DB, because others will not have bothered to
958+
* check committed-ness of notifications in our DB.) But we only bother
959+
* with that if there's more than a page worth of notifications
960+
* outstanding, otherwise scanning all the other backends isn't worth it.
961+
*
962+
* We need exclusive lock here so we can look at other backends' entries.
937963
*/
938-
LWLockAcquire(AsyncQueueLock,LW_SHARED);
939-
QUEUE_BACKEND_POS(MyBackendId)=QUEUE_TAIL;
964+
LWLockAcquire(AsyncQueueLock,LW_EXCLUSIVE);
965+
head=QUEUE_HEAD;
966+
max=QUEUE_TAIL;
967+
if (QUEUE_POS_PAGE(max)!=QUEUE_POS_PAGE(head))
968+
{
969+
for (i=1;i <=MaxBackends;i++)
970+
{
971+
if (QUEUE_BACKEND_DBOID(i)==MyDatabaseId)
972+
max=QUEUE_POS_MAX(max,QUEUE_BACKEND_POS(i));
973+
}
974+
}
975+
QUEUE_BACKEND_POS(MyBackendId)=max;
940976
QUEUE_BACKEND_PID(MyBackendId)=MyProcPid;
977+
QUEUE_BACKEND_DBOID(MyBackendId)=MyDatabaseId;
941978
LWLockRelease(AsyncQueueLock);
942979

943980
/* Now we are listed in the global array, so remember we're listening */
@@ -953,7 +990,8 @@ Exec_ListenPreCommit(void)
953990
*
954991
* This will also advance the global tail pointer if possible.
955992
*/
956-
asyncQueueReadAllNotifications();
993+
if (!QUEUE_POS_EQUAL(max,head))
994+
asyncQueueReadAllNotifications();
957995
}
958996

959997
/*
@@ -1156,6 +1194,7 @@ asyncQueueUnregister(void)
11561194
QUEUE_POS_EQUAL(QUEUE_BACKEND_POS(MyBackendId),QUEUE_TAIL);
11571195
/* ... then mark it invalid */
11581196
QUEUE_BACKEND_PID(MyBackendId)=InvalidPid;
1197+
QUEUE_BACKEND_DBOID(MyBackendId)=InvalidOid;
11591198
LWLockRelease(AsyncQueueLock);
11601199

11611200
/* mark ourselves as no longer listed in the global array */

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