Add a small cache of locks owned by a resource owner in ResourceOwner.

This speeds up reassigning locks to the parent owner, when the transaction
holds a lot of locks, but only a few of them belong to the current resource
owner. This is particularly helps pg_dump when dumping a large number of
objects.

The cache can hold up to 15 locks in each resource owner. After that, the
cache is marked as overflowed, and we fall back to the old method of
scanning the whole local lock table. The tradeoff here is that the cache has
to be scanned whenever a lock is released, so if the cache is too large,
lock release becomes more expensive. 15 seems enough to cover pg_dump, and
doesn't have much impact on lock release.

Jeff Janes, reviewed by Amit Kapila and Heikki Linnakangas.

src/backend/utils/resowner/resowner.c

@@ -27,6 +27,23 @@
 #include "utils/rel.h"
 #include "utils/snapmgr.h"
 
+/*
+ * To speed up bulk releasing or reassigning locks from a resource owner to
+ * its parent, each resource owner has a small cache of locks it owns. The
+ * lock manager has the same information in its local lock hash table, and
+ * we fall back on that if cache overflows, but traversing the hash table
+ * is slower when there are a lot of locks belonging to other resource owners.
+ *
+ * MAX_RESOWNER_LOCKS is the size of the per-resource owner cache. It's
+ * chosen based on some testing with pg_dump with a large schema. When the
+ * tests were done (on 9.2), resource owners in a pg_dump run contained up
+ * to 9 locks, regardless of the schema size, except for the top resource
+ * owner which contained much more (overflowing the cache). 15 seems like a
+ * nice round number that's somewhat higher than what pg_dump needs. Note that
+ * making this number larger is not free - the bigger the cache, the slower
+ * it is to release locks (in retail), when a resource owner holds many locks.
+ */
+#define MAX_RESOWNER_LOCKS 15
 
 /*
  * ResourceOwner objects look like this
@@ -43,6 +60,10 @@ typedef struct ResourceOwnerData
 	Buffer	   *buffers;		/* dynamically allocated array */
 	int			maxbuffers;		/* currently allocated array size */
 
+	/* We can remember up to MAX_RESOWNER_LOCKS references to local locks. */
+	int			nlocks;		/* number of owned locks */
+	LOCALLOCK  *locks[MAX_RESOWNER_LOCKS];	/* list of owned locks */
+
 	/* We have built-in support for remembering catcache references */
 	int			ncatrefs;		/* number of owned catcache pins */
 	HeapTuple  *catrefs;		/* dynamically allocated array */
@@ -272,11 +293,30 @@ ResourceOwnerReleaseInternal(ResourceOwner owner,
 			 * subtransaction, we do NOT release its locks yet, but transfer
 			 * them to the parent.
 			 */
+			LOCALLOCK **locks;
+			int			nlocks;
+
 			Assert(owner->parent != NULL);
-			if (isCommit)
-				LockReassignCurrentOwner();
+
+			/*
+			 * Pass the list of locks owned by this resource owner to the lock
+			 * manager, unless it has overflowed.
+			 */
+			if (owner->nlocks > MAX_RESOWNER_LOCKS)
+			{
+				locks = NULL;
+				nlocks = 0;
+			}
 			else
-				LockReleaseCurrentOwner();
+			{
+				locks = owner->locks;
+				nlocks = owner->nlocks;
+			}
+
+			if (isCommit)
+				LockReassignCurrentOwner(locks, nlocks);
+			else
+				LockReleaseCurrentOwner(locks, nlocks);
 		}
 	}
 	else if (phase == RESOURCE_RELEASE_AFTER_LOCKS)
@@ -357,6 +397,7 @@ ResourceOwnerDelete(ResourceOwner owner)
 
 	/* And it better not own any resources, either */
 	Assert(owner->nbuffers == 0);
+	Assert(owner->nlocks == 0 || owner->nlocks == MAX_RESOWNER_LOCKS + 1);
 	Assert(owner->ncatrefs == 0);
 	Assert(owner->ncatlistrefs == 0);
 	Assert(owner->nrelrefs == 0);
@@ -588,6 +629,56 @@ ResourceOwnerForgetBuffer(ResourceOwner owner, Buffer buffer)
 	}
 }
 
+/*
+ * Remember that a Local Lock is owned by a ResourceOwner
+ *
+ * This is different from the other Remember functions in that the list of
+ * locks is only a lossy cache. It can hold up to MAX_RESOWNER_LOCKS entries,
+ * and when it overflows, we stop tracking locks. The point of only remembering
+ * only up to MAX_RESOWNER_LOCKS entries is that if a lot of locks are held,
+ * ResourceOwnerForgetLock doesn't need to scan through a large array to find
+ * the entry.
+ */
+void
+ResourceOwnerRememberLock(ResourceOwner owner, LOCALLOCK * locallock)
+{
+	if (owner->nlocks > MAX_RESOWNER_LOCKS)
+		return;		/* we have already overflowed */
+
+	if (owner->nlocks < MAX_RESOWNER_LOCKS)
+		owner->locks[owner->nlocks] = locallock;
+	else
+	{
+		/* overflowed */
+	}
+	owner->nlocks++;
+}
+
+/*
+ * Forget that a Local Lock is owned by a ResourceOwner
+ */
+void
+ResourceOwnerForgetLock(ResourceOwner owner, LOCALLOCK *locallock)
+{
+	int			i;
+
+	if (owner->nlocks > MAX_RESOWNER_LOCKS)
+		return;		/* we have overflowed */
+
+	Assert(owner->nlocks > 0);
+	for (i = owner->nlocks - 1; i >= 0; i--)
+	{
+		if (locallock == owner->locks[i])
+		{
+			owner->locks[i] = owner->locks[owner->nlocks - 1];
+			owner->nlocks--;
+			return;
+		}
+	}
+	elog(ERROR, "lock reference %p is not owned by resource owner %s",
+		 locallock, owner->name);
+}
+
 /*
  * Make sure there is room for at least one more entry in a ResourceOwner's
  * catcache reference array.