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Commit4460964

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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 parentfe65f59 commit4460964

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‎src/backend/postmaster/autovacuum.c‎

Lines changed: 37 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -211,9 +211,9 @@ typedef struct autovac_table
211211
* wi_launchtime Time at which this worker was launched
212212
* wi_cost_*Vacuum cost-based delay parameters current in this worker
213213
*
214-
* All fields are protected by AutovacuumLock, except for wi_tableoidwhich is
215-
* protected by AutovacuumScheduleLock (which is read-only for everyone except
216-
* that worker itself).
214+
* All fields are protected by AutovacuumLock, except for wi_tableoidand
215+
*wi_sharedrel which areprotected by AutovacuumScheduleLock (note these
216+
*two fields are read-only for everyone exceptthat worker itself).
217217
*-------------
218218
*/
219219
typedefstructWorkerInfoData
@@ -2316,7 +2316,9 @@ do_autovacuum(void)
23162316
foreach(cell,table_oids)
23172317
{
23182318
Oidrelid=lfirst_oid(cell);
2319+
HeapTupleclassTup;
23192320
autovac_table*tab;
2321+
boolisshared;
23202322
boolskipit;
23212323
intstdVacuumCostDelay;
23222324
intstdVacuumCostLimit;
@@ -2341,9 +2343,23 @@ do_autovacuum(void)
23412343
}
23422344

23432345
/*
2344-
* hold schedule lock from here until we're sure that this table still
2345-
* needs vacuuming. We also need the AutovacuumLock to walk the
2346-
* worker array, but we'll let go of that one quickly.
2346+
* Find out whether the table is shared or not. (It's slightly
2347+
* annoying to fetch the syscache entry just for this, but in typical
2348+
* cases it adds little cost because table_recheck_autovac would
2349+
* refetch the entry anyway. We could buy that back by copying the
2350+
* tuple here and passing it to table_recheck_autovac, but that
2351+
* increases the odds of that function working with stale data.)
2352+
*/
2353+
classTup=SearchSysCache1(RELOID,ObjectIdGetDatum(relid));
2354+
if (!HeapTupleIsValid(classTup))
2355+
continue;/* somebody deleted the rel, forget it */
2356+
isshared= ((Form_pg_class)GETSTRUCT(classTup))->relisshared;
2357+
ReleaseSysCache(classTup);
2358+
2359+
/*
2360+
* Hold schedule lock from here until we've claimed the table. We
2361+
* also need the AutovacuumLock to walk the worker array, but that one
2362+
* can just be a shared lock.
23472363
*/
23482364
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
23492365
LWLockAcquire(AutovacuumLock,LW_SHARED);
@@ -2379,6 +2395,16 @@ do_autovacuum(void)
23792395
continue;
23802396
}
23812397

2398+
/*
2399+
* Store the table's OID in shared memory before releasing the
2400+
* schedule lock, so that other workers don't try to vacuum it
2401+
* concurrently. (We claim it here so as not to hold
2402+
* AutovacuumScheduleLock while rechecking the stats.)
2403+
*/
2404+
MyWorkerInfo->wi_tableoid=relid;
2405+
MyWorkerInfo->wi_sharedrel=isshared;
2406+
LWLockRelease(AutovacuumScheduleLock);
2407+
23822408
/*
23832409
* Check whether pgstat data still says we need to vacuum this table.
23842410
* It could have changed if something else processed the table while
@@ -2395,18 +2421,13 @@ do_autovacuum(void)
23952421
if (tab==NULL)
23962422
{
23972423
/* someone else vacuumed the table, or it went away */
2424+
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
2425+
MyWorkerInfo->wi_tableoid=InvalidOid;
2426+
MyWorkerInfo->wi_sharedrel= false;
23982427
LWLockRelease(AutovacuumScheduleLock);
23992428
continue;
24002429
}
24012430

2402-
/*
2403-
* Ok, good to go. Store the table in shared memory before releasing
2404-
* the lock so that other workers don't vacuum it concurrently.
2405-
*/
2406-
MyWorkerInfo->wi_tableoid=relid;
2407-
MyWorkerInfo->wi_sharedrel=tab->at_sharedrel;
2408-
LWLockRelease(AutovacuumScheduleLock);
2409-
24102431
/*
24112432
* Remember the prevailing values of the vacuum cost GUCs. We have to
24122433
* restore these at the bottom of the loop, else we'll compute wrong
@@ -2521,10 +2542,10 @@ do_autovacuum(void)
25212542
* settings, so we don't want to give up our share of I/O for a very
25222543
* short interval and thereby thrash the global balance.
25232544
*/
2524-
LWLockAcquire(AutovacuumLock,LW_EXCLUSIVE);
2545+
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
25252546
MyWorkerInfo->wi_tableoid=InvalidOid;
25262547
MyWorkerInfo->wi_sharedrel= false;
2527-
LWLockRelease(AutovacuumLock);
2548+
LWLockRelease(AutovacuumScheduleLock);
25282549

25292550
/* restore vacuum cost GUCs for the next iteration */
25302551
VacuumCostDelay=stdVacuumCostDelay;

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