Avoid O(N^2) behavior when the standby process releases many locks.

tglsfdc · tglsfdc · commit 6301c3adabd9 · 2021-10-31T15:31:29.000-04:00
When replaying a transaction that held many exclusive locks on the primary, a standby server's startup process would expend O(N^2) effort on manipulating the list of locks. This code was fine when written, but commit 1cff1b9 made repetitive list_delete_first() calls inefficient, as explained in its commit message. Fix by just iterating the list normally, and releasing storage only when done. (This'd be inadequate if we needed to recover from an error occurring partway through; but we don't.) Back-patch to v13 where 1cff1b9 came in. Nathan Bossart Discussion: https://postgr.es/m/CD2F0E7F-9822-45EC-A411-AE56F14DEA9F@amazon.com
diff --git a/src/backend/storage/ipc/standby.c b/src/backend/storage/ipc/standby.c
@@ -986,9 +986,11 @@ StandbyAcquireAccessExclusiveLock(TransactionId xid, Oid dbOid, Oid relOid)
 static void
 StandbyReleaseLockList(List *locks)
 {
-	while (locks)
+	ListCell   *lc;
+
+	foreach(lc, locks)
 	{
-		xl_standby_lock *lock = (xl_standby_lock *) linitial(locks);
+		xl_standby_lock *lock = (xl_standby_lock *) lfirst(lc);
 		LOCKTAG		locktag;
 
 		elog(trace_recovery(DEBUG4),
@@ -1002,9 +1004,9 @@ StandbyReleaseLockList(List *locks)
 				 lock->xid, lock->dbOid, lock->relOid);
 			Assert(false);
 		}
-		pfree(lock);
-		locks = list_delete_first(locks);
 	}
+
+	list_free_deep(locks);
 }
 
 static void

Original file line number	Diff line number	Diff line change
`@@ -986,9 +986,11 @@ StandbyAcquireAccessExclusiveLock(TransactionId xid, Oid dbOid, Oid relOid)`
`986`	`986`	`static void`
`987`	`987`	`StandbyReleaseLockList(List *locks)`
`988`	`988`	`{`
`989`		`- while (locks)`
	`989`	`+ ListCell *lc;`
	`990`	`+`
	`991`	`+ foreach(lc, locks)`
`990`	`992`	`{`
`991`		`- xl_standby_lock lock = (xl_standby_lock ) linitial(locks);`
	`993`	`+ xl_standby_lock lock = (xl_standby_lock ) lfirst(lc);`
`992`	`994`	`LOCKTAG locktag;`
`993`	`995`
`994`	`996`	`elog(trace_recovery(DEBUG4),`
`@@ -1002,9 +1004,9 @@ StandbyReleaseLockList(List *locks)`
`1002`	`1004`	`lock->xid, lock->dbOid, lock->relOid);`
`1003`	`1005`	`Assert(false);`
`1004`	`1006`	`}`
`1005`		`- pfree(lock);`
`1006`		`- locks = list_delete_first(locks);`
`1007`	`1007`	`}`
	`1008`	`+`
	`1009`	`+ list_free_deep(locks);`
`1008`	`1010`	`}`
`1009`	`1011`
`1010`	`1012`	`static void`