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Commit38f7831

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Avoid holding AutovacuumScheduleLock while rechecking table statistics.
In databases with many tables, re-fetching the statistics takes some time,so that this behavior seriously decreases the available concurrency formultiple autovac workers. There's discussion afoot about more completefixes, but a simple and back-patchable amelioration is to claim the tableand release the lock before rechecking stats. If we find out there's nolonger a reason to process the table, re-taking the lock to un-claim thetable is cheap enough.(This patch is quite old, but got lost amongst a discussion of moreaggressive fixes. It's not clear when or if such a fix will beaccepted, but in any case it'd be unlikely to get back-patched.Let's do this now so we have some improvement for the back branches.)In passing, make the normal un-claim step take AutovacuumScheduleLocknot AutovacuumLock, since that is what is documented to protect thewi_tableoid field. This wasn't an actual bug in view of the fact thatreaders of that field hold both locks, but it creates some concurrencypenalty against operations that need only AutovacuumLock.Back-patch to all supported versions.Jeff JanesDiscussion:https://postgr.es/m/26118.1520865816@sss.pgh.pa.us
1 parentb32fad5 commit38f7831

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‎src/backend/postmaster/autovacuum.c

Lines changed: 37 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -212,9 +212,9 @@ typedef struct autovac_table
212212
* wi_launchtime Time at which this worker was launched
213213
* wi_cost_*Vacuum cost-based delay parameters current in this worker
214214
*
215-
* All fields are protected by AutovacuumLock, except for wi_tableoidwhich is
216-
* protected by AutovacuumScheduleLock (which is read-only for everyone except
217-
* that worker itself).
215+
* All fields are protected by AutovacuumLock, except for wi_tableoidand
216+
*wi_sharedrel which areprotected by AutovacuumScheduleLock (note these
217+
*two fields are read-only for everyone exceptthat worker itself).
218218
*-------------
219219
*/
220220
typedefstructWorkerInfoData
@@ -2317,7 +2317,9 @@ do_autovacuum(void)
23172317
foreach(cell,table_oids)
23182318
{
23192319
Oidrelid=lfirst_oid(cell);
2320+
HeapTupleclassTup;
23202321
autovac_table*tab;
2322+
boolisshared;
23212323
boolskipit;
23222324
intstdVacuumCostDelay;
23232325
intstdVacuumCostLimit;
@@ -2342,9 +2344,23 @@ do_autovacuum(void)
23422344
}
23432345

23442346
/*
2345-
* hold schedule lock from here until we're sure that this table still
2346-
* needs vacuuming. We also need the AutovacuumLock to walk the
2347-
* worker array, but we'll let go of that one quickly.
2347+
* Find out whether the table is shared or not. (It's slightly
2348+
* annoying to fetch the syscache entry just for this, but in typical
2349+
* cases it adds little cost because table_recheck_autovac would
2350+
* refetch the entry anyway. We could buy that back by copying the
2351+
* tuple here and passing it to table_recheck_autovac, but that
2352+
* increases the odds of that function working with stale data.)
2353+
*/
2354+
classTup=SearchSysCache1(RELOID,ObjectIdGetDatum(relid));
2355+
if (!HeapTupleIsValid(classTup))
2356+
continue;/* somebody deleted the rel, forget it */
2357+
isshared= ((Form_pg_class)GETSTRUCT(classTup))->relisshared;
2358+
ReleaseSysCache(classTup);
2359+
2360+
/*
2361+
* Hold schedule lock from here until we've claimed the table. We
2362+
* also need the AutovacuumLock to walk the worker array, but that one
2363+
* can just be a shared lock.
23482364
*/
23492365
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
23502366
LWLockAcquire(AutovacuumLock,LW_SHARED);
@@ -2380,6 +2396,16 @@ do_autovacuum(void)
23802396
continue;
23812397
}
23822398

2399+
/*
2400+
* Store the table's OID in shared memory before releasing the
2401+
* schedule lock, so that other workers don't try to vacuum it
2402+
* concurrently. (We claim it here so as not to hold
2403+
* AutovacuumScheduleLock while rechecking the stats.)
2404+
*/
2405+
MyWorkerInfo->wi_tableoid=relid;
2406+
MyWorkerInfo->wi_sharedrel=isshared;
2407+
LWLockRelease(AutovacuumScheduleLock);
2408+
23832409
/*
23842410
* Check whether pgstat data still says we need to vacuum this table.
23852411
* It could have changed if something else processed the table while
@@ -2396,18 +2422,13 @@ do_autovacuum(void)
23962422
if (tab==NULL)
23972423
{
23982424
/* someone else vacuumed the table, or it went away */
2425+
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
2426+
MyWorkerInfo->wi_tableoid=InvalidOid;
2427+
MyWorkerInfo->wi_sharedrel= false;
23992428
LWLockRelease(AutovacuumScheduleLock);
24002429
continue;
24012430
}
24022431

2403-
/*
2404-
* Ok, good to go. Store the table in shared memory before releasing
2405-
* the lock so that other workers don't vacuum it concurrently.
2406-
*/
2407-
MyWorkerInfo->wi_tableoid=relid;
2408-
MyWorkerInfo->wi_sharedrel=tab->at_sharedrel;
2409-
LWLockRelease(AutovacuumScheduleLock);
2410-
24112432
/*
24122433
* Remember the prevailing values of the vacuum cost GUCs. We have to
24132434
* restore these at the bottom of the loop, else we'll compute wrong
@@ -2522,10 +2543,10 @@ do_autovacuum(void)
25222543
* settings, so we don't want to give up our share of I/O for a very
25232544
* short interval and thereby thrash the global balance.
25242545
*/
2525-
LWLockAcquire(AutovacuumLock,LW_EXCLUSIVE);
2546+
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
25262547
MyWorkerInfo->wi_tableoid=InvalidOid;
25272548
MyWorkerInfo->wi_sharedrel= false;
2528-
LWLockRelease(AutovacuumLock);
2549+
LWLockRelease(AutovacuumScheduleLock);
25292550

25302551
/* restore vacuum cost GUCs for the next iteration */
25312552
VacuumCostDelay=stdVacuumCostDelay;

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