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 for
multiple autovac workers.  There's discussion afoot about more complete
fixes, but a simple and back-patchable amelioration is to claim the table
and release the lock before rechecking stats.  If we find out there's no
longer a reason to process the table, re-taking the lock to un-claim the
table is cheap enough.

(This patch is quite old, but got lost amongst a discussion of more
aggressive fixes.  It's not clear when or if such a fix will be
accepted, 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 AutovacuumScheduleLock
not AutovacuumLock, since that is what is documented to protect the
wi_tableoid field.  This wasn't an actual bug in view of the fact that
readers of that field hold both locks, but it creates some concurrency
penalty against operations that need only AutovacuumLock.

Back-patch to all supported versions.

Jeff Janes

Discussion: https://postgr.es/m/26118.1520865816@sss.pgh.pa.us

src/backend/postmaster/autovacuum.c

@@ -212,9 +212,9 @@ typedef struct autovac_table
  * wi_launchtime Time at which this worker was launched
  * wi_cost_*	Vacuum cost-based delay parameters current in this worker
  *
- * All fields are protected by AutovacuumLock, except for wi_tableoid which is
+ * All fields are protected by AutovacuumLock, except for wi_tableoid and
- * protected by AutovacuumScheduleLock (which is read-only for everyone except
+ * wi_sharedrel which are protected by AutovacuumScheduleLock (note these
- * that worker itself).
+ * two fields are read-only for everyone except that worker itself).
  *-------------
  */
 typedef struct WorkerInfoData
@@ -2317,7 +2317,9 @@ do_autovacuum(void)
 	foreach(cell, table_oids)
 	{
 		Oid			relid = lfirst_oid(cell);
+		HeapTuple	classTup;
 		autovac_table *tab;
+		bool		isshared;
 		bool		skipit;
 		int			stdVacuumCostDelay;
 		int			stdVacuumCostLimit;
@@ -2342,9 +2344,23 @@ do_autovacuum(void)
 		}
 
 		/*
-		 * hold schedule lock from here until we're sure that this table still
+		 * Find out whether the table is shared or not.  (It's slightly
-		 * needs vacuuming.  We also need the AutovacuumLock to walk the
+		 * annoying to fetch the syscache entry just for this, but in typical
-		 * worker array, but we'll let go of that one quickly.
+		 * cases it adds little cost because table_recheck_autovac would
+		 * refetch the entry anyway.  We could buy that back by copying the
+		 * tuple here and passing it to table_recheck_autovac, but that
+		 * increases the odds of that function working with stale data.)
+		 */
+		classTup = SearchSysCache1(RELOID, ObjectIdGetDatum(relid));
+		if (!HeapTupleIsValid(classTup))
+			continue;			/* somebody deleted the rel, forget it */
+		isshared = ((Form_pg_class) GETSTRUCT(classTup))->relisshared;
+		ReleaseSysCache(classTup);
+
+		/*
+		 * Hold schedule lock from here until we've claimed the table.  We
+		 * also need the AutovacuumLock to walk the worker array, but that one
+		 * can just be a shared lock.
 		 */
 		LWLockAcquire(AutovacuumScheduleLock, LW_EXCLUSIVE);
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
@@ -2380,6 +2396,16 @@ do_autovacuum(void)
 			continue;
 		}
 
+		/*
+		 * Store the table's OID in shared memory before releasing the
+		 * schedule lock, so that other workers don't try to vacuum it
+		 * concurrently.  (We claim it here so as not to hold
+		 * AutovacuumScheduleLock while rechecking the stats.)
+		 */
+		MyWorkerInfo->wi_tableoid = relid;
+		MyWorkerInfo->wi_sharedrel = isshared;
+		LWLockRelease(AutovacuumScheduleLock);
+
 		/*
 		 * Check whether pgstat data still says we need to vacuum this table.
 		 * It could have changed if something else processed the table while
@@ -2396,18 +2422,13 @@ do_autovacuum(void)
 		if (tab == NULL)
 		{
 			/* someone else vacuumed the table, or it went away */
+			LWLockAcquire(AutovacuumScheduleLock, LW_EXCLUSIVE);
+			MyWorkerInfo->wi_tableoid = InvalidOid;
+			MyWorkerInfo->wi_sharedrel = false;
 			LWLockRelease(AutovacuumScheduleLock);
 			continue;
 		}
 
-		/*
-		 * Ok, good to go.  Store the table in shared memory before releasing
-		 * the lock so that other workers don't vacuum it concurrently.
-		 */
-		MyWorkerInfo->wi_tableoid = relid;
-		MyWorkerInfo->wi_sharedrel = tab->at_sharedrel;
-		LWLockRelease(AutovacuumScheduleLock);
-
 		/*
 		 * Remember the prevailing values of the vacuum cost GUCs.  We have to
 		 * restore these at the bottom of the loop, else we'll compute wrong
@@ -2522,10 +2543,10 @@ deleted:
 		 * settings, so we don't want to give up our share of I/O for a very
 		 * short interval and thereby thrash the global balance.
 		 */
-		LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
+		LWLockAcquire(AutovacuumScheduleLock, LW_EXCLUSIVE);
 		MyWorkerInfo->wi_tableoid = InvalidOid;
 		MyWorkerInfo->wi_sharedrel = false;
-		LWLockRelease(AutovacuumLock);
+		LWLockRelease(AutovacuumScheduleLock);
 
 		/* restore vacuum cost GUCs for the next iteration */
 		VacuumCostDelay = stdVacuumCostDelay;