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Commit4b0e717

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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 parent44a36a8 commit4b0e717

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‎src/backend/postmaster/autovacuum.c

Lines changed: 37 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -209,9 +209,9 @@ typedef struct autovac_table
209209
* wi_launchtime Time at which this worker was launched
210210
* wi_cost_*Vacuum cost-based delay parameters current in this worker
211211
*
212-
* All fields are protected by AutovacuumLock, except for wi_tableoidwhich is
213-
* protected by AutovacuumScheduleLock (which is read-only for everyone except
214-
* that worker itself).
212+
* All fields are protected by AutovacuumLock, except for wi_tableoidand
213+
*wi_sharedrel which areprotected by AutovacuumScheduleLock (note these
214+
*two fields are read-only for everyone exceptthat worker itself).
215215
*-------------
216216
*/
217217
typedefstructWorkerInfoData
@@ -2197,7 +2197,9 @@ do_autovacuum(void)
21972197
foreach(cell,table_oids)
21982198
{
21992199
Oidrelid=lfirst_oid(cell);
2200+
HeapTupleclassTup;
22002201
autovac_table*tab;
2202+
boolisshared;
22012203
boolskipit;
22022204
intstdVacuumCostDelay;
22032205
intstdVacuumCostLimit;
@@ -2222,9 +2224,23 @@ do_autovacuum(void)
22222224
}
22232225

22242226
/*
2225-
* hold schedule lock from here until we're sure that this table still
2226-
* needs vacuuming. We also need the AutovacuumLock to walk the
2227-
* worker array, but we'll let go of that one quickly.
2227+
* Find out whether the table is shared or not. (It's slightly
2228+
* annoying to fetch the syscache entry just for this, but in typical
2229+
* cases it adds little cost because table_recheck_autovac would
2230+
* refetch the entry anyway. We could buy that back by copying the
2231+
* tuple here and passing it to table_recheck_autovac, but that
2232+
* increases the odds of that function working with stale data.)
2233+
*/
2234+
classTup=SearchSysCache1(RELOID,ObjectIdGetDatum(relid));
2235+
if (!HeapTupleIsValid(classTup))
2236+
continue;/* somebody deleted the rel, forget it */
2237+
isshared= ((Form_pg_class)GETSTRUCT(classTup))->relisshared;
2238+
ReleaseSysCache(classTup);
2239+
2240+
/*
2241+
* Hold schedule lock from here until we've claimed the table. We
2242+
* also need the AutovacuumLock to walk the worker array, but that one
2243+
* can just be a shared lock.
22282244
*/
22292245
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
22302246
LWLockAcquire(AutovacuumLock,LW_SHARED);
@@ -2260,6 +2276,16 @@ do_autovacuum(void)
22602276
continue;
22612277
}
22622278

2279+
/*
2280+
* Store the table's OID in shared memory before releasing the
2281+
* schedule lock, so that other workers don't try to vacuum it
2282+
* concurrently. (We claim it here so as not to hold
2283+
* AutovacuumScheduleLock while rechecking the stats.)
2284+
*/
2285+
MyWorkerInfo->wi_tableoid=relid;
2286+
MyWorkerInfo->wi_sharedrel=isshared;
2287+
LWLockRelease(AutovacuumScheduleLock);
2288+
22632289
/*
22642290
* Check whether pgstat data still says we need to vacuum this table.
22652291
* It could have changed if something else processed the table while
@@ -2276,18 +2302,13 @@ do_autovacuum(void)
22762302
if (tab==NULL)
22772303
{
22782304
/* someone else vacuumed the table, or it went away */
2305+
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
2306+
MyWorkerInfo->wi_tableoid=InvalidOid;
2307+
MyWorkerInfo->wi_sharedrel= false;
22792308
LWLockRelease(AutovacuumScheduleLock);
22802309
continue;
22812310
}
22822311

2283-
/*
2284-
* Ok, good to go. Store the table in shared memory before releasing
2285-
* the lock so that other workers don't vacuum it concurrently.
2286-
*/
2287-
MyWorkerInfo->wi_tableoid=relid;
2288-
MyWorkerInfo->wi_sharedrel=tab->at_sharedrel;
2289-
LWLockRelease(AutovacuumScheduleLock);
2290-
22912312
/*
22922313
* Remember the prevailing values of the vacuum cost GUCs. We have to
22932314
* restore these at the bottom of the loop, else we'll compute wrong
@@ -2397,10 +2418,10 @@ do_autovacuum(void)
23972418
* settings, so we don't want to give up our share of I/O for a very
23982419
* short interval and thereby thrash the global balance.
23992420
*/
2400-
LWLockAcquire(AutovacuumLock,LW_EXCLUSIVE);
2421+
LWLockAcquire(AutovacuumScheduleLock,LW_EXCLUSIVE);
24012422
MyWorkerInfo->wi_tableoid=InvalidOid;
24022423
MyWorkerInfo->wi_sharedrel= false;
2403-
LWLockRelease(AutovacuumLock);
2424+
LWLockRelease(AutovacuumScheduleLock);
24042425

24052426
/* restore vacuum cost GUCs for the next iteration */
24062427
VacuumCostDelay=stdVacuumCostDelay;

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