Tweak catalog cache management algorithms to reduce cost of

SearchCatCacheList and ReleaseCatCacheList.  Previously, we incremented
and decremented the refcounts of list member tuples along with the list
itself, but that's unnecessary, and very expensive when the list is big.
It's cheaper to change only the list refcount.  When we are considering
deleting a cache entry, we have to check not only its own refcount but
its parent list's ... but it's easy to arrange the code so that this
check is not made in any commonly-used paths, so the cost is really nil.
The bigger gain though is to refrain from DLMoveToFront'ing each individual
member tuple each time the list is referenced.  To keep some semblance
of fair space management, lists are just marked as used or not since the
last cache cleanout search, and we do a MoveToFront pass only when about
to run a cleanout.  In combination, these changes reduce the costs of
SearchCatCacheList and ReleaseCatCacheList from about 4.5% of pgbench
runtime to under 1%, according to my gprof results.

Author: tglsfdc

src/backend/utils/cache/catcache.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/cache/catcache.c,v 1.122 2005/08/08 19:17:22 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/cache/catcache.c,v 1.123 2005/08/13 22:18:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -97,6 +97,7 @@ static void CatalogCacheInitializeCache(CatCache *cache);
 static CatCTup *CatalogCacheCreateEntry(CatCache *cache, HeapTuple ntp,
 						uint32 hashValue, Index hashIndex,
 						bool negative);
+static void CatalogCacheCleanup(CatCTup *savect);
 static HeapTuple build_dummy_tuple(CatCache *cache, int nkeys, ScanKey skeys);
 
 
@@ -344,6 +345,7 @@ CatCachePrintStats(void)
  * Unlink and delete the given cache entry
  *
  * NB: if it is a member of a CatCList, the CatCList is deleted too.
+ * Both the cache entry and the list had better have zero refcount.
  */
 static void
 CatCacheRemoveCTup(CatCache *cache, CatCTup *ct)
@@ -481,8 +483,13 @@ CatalogCacheIdInvalidate(int cacheId,
 			if (ct->negative ||
 				ItemPointerEquals(pointer, &ct->tuple.t_self))
 			{
-				if (ct->refcount > 0)
+				if (ct->refcount > 0 ||
+					(ct->c_list && ct->c_list->refcount > 0))
+				{
 					ct->dead = true;
+					/* list, if any, was marked dead above */
+					Assert(ct->c_list == NULL || ct->c_list->dead);
+				}
 				else
 					CatCacheRemoveCTup(ccp, ct);
 				CACHE1_elog(DEBUG2, "CatalogCacheIdInvalidate: invalidated");
@@ -606,8 +613,13 @@ ResetCatalogCache(CatCache *cache)
 
 			nextelt = DLGetSucc(elt);
 
-			if (ct->refcount > 0)
+			if (ct->refcount > 0 ||
+				(ct->c_list && ct->c_list->refcount > 0))
+			{
 				ct->dead = true;
+				/* list, if any, was marked dead above */
+				Assert(ct->c_list == NULL || ct->c_list->dead);
+			}
 			else
 				CatCacheRemoveCTup(cache, ct);
 #ifdef CATCACHE_STATS
@@ -718,8 +730,14 @@ CatalogCacheFlushRelation(Oid relId)
 
 				if (tupRelid == relId)
 				{
-					if (ct->refcount > 0)
+					if (ct->refcount > 0 ||
+						(ct->c_list && ct->c_list->refcount > 0))
+					{
 						ct->dead = true;
+						/* parent list must be considered dead too */
+						if (ct->c_list)
+							ct->c_list->dead = true;
+					}
 					else
 						CatCacheRemoveCTup(cache, ct);
 #ifdef CATCACHE_STATS
@@ -1279,11 +1297,12 @@ ReleaseCatCache(HeapTuple tuple)
 	ct->refcount--;
 	ResourceOwnerForgetCatCacheRef(CurrentResourceOwner, &ct->tuple);
 
-	if (ct->refcount == 0
+	if (
 #ifndef CATCACHE_FORCE_RELEASE
-		&& ct->dead
+		ct->dead &&
 #endif
-		)
+		ct->refcount == 0 &&
+		(ct->c_list == NULL || ct->c_list->refcount == 0))
 		CatCacheRemoveCTup(ct->my_cache, ct);
 }
 
@@ -1377,24 +1396,18 @@ SearchCatCacheList(CatCache *cache,
 			continue;
 
 		/*
-		 * we found a matching list: move each of its members to the front
-		 * of the global LRU list.	Also move the list itself to the front
-		 * of the cache's list-of-lists, to speed subsequent searches. (We
-		 * do not move the members to the fronts of their hashbucket
+		 * We found a matching list: mark it as touched since the last
+		 * CatalogCacheCleanup() sweep.  Also move the list to the front
+		 * of the cache's list-of-lists, to speed subsequent searches.
+		 * (We do not move the members to the fronts of their hashbucket
 		 * lists, however, since there's no point in that unless they are
-		 * searched for individually.)	Also bump the members' refcounts.
-		 * (member refcounts are NOT registered separately with the
-		 * resource owner.)
+		 * searched for individually.)
 		 */
-		ResourceOwnerEnlargeCatCacheListRefs(CurrentResourceOwner);
-		for (i = 0; i < cl->n_members; i++)
-		{
-			cl->members[i]->refcount++;
-			DLMoveToFront(&cl->members[i]->lrulist_elem);
-		}
+		cl->touched = true;
 		DLMoveToFront(&cl->cache_elem);
 
 		/* Bump the list's refcount and return it */
+		ResourceOwnerEnlargeCatCacheListRefs(CurrentResourceOwner);
 		cl->refcount++;
 		ResourceOwnerRememberCatCacheListRef(CurrentResourceOwner, cl);
 
@@ -1413,7 +1426,7 @@ SearchCatCacheList(CatCache *cache,
 	 * relation.  For each matching tuple found in the relation, use an
 	 * existing cache entry if possible, else build a new one.
 	 *
-	 * We have to bump the member refcounts immediately to ensure they
+	 * We have to bump the member refcounts temporarily to ensure they
 	 * won't get dropped from the cache while loading other members.
 	 * We use a PG_TRY block to ensure we can undo those refcounts if
 	 * we get an error before we finish constructing the CatCList.
@@ -1488,6 +1501,7 @@ SearchCatCacheList(CatCache *cache,
 			}
 
 			/* Careful here: add entry to ctlist, then bump its refcount */
+			/* This way leaves state correct if lappend runs out of memory */
 			ctlist = lappend(ctlist, ct);
 			ct->refcount++;
 		}
@@ -1526,11 +1540,12 @@ SearchCatCacheList(CatCache *cache,
 			Assert(ct->c_list == NULL);
 			Assert(ct->refcount > 0);
 			ct->refcount--;
-			if (ct->refcount == 0
+			if (
 #ifndef CATCACHE_FORCE_RELEASE
-				&& ct->dead
+				ct->dead &&
 #endif
-				)
+				ct->refcount == 0 &&
+				(ct->c_list == NULL || ct->c_list->refcount == 0))
 				CatCacheRemoveCTup(cache, ct);
 		}
 
@@ -1544,6 +1559,7 @@ SearchCatCacheList(CatCache *cache,
 	cl->refcount = 0;			/* for the moment */
 	cl->dead = false;
 	cl->ordered = ordered;
+	cl->touched = false;		/* we already moved members to front */
 	cl->nkeys = nkeys;
 	cl->hash_value = lHashValue;
 	cl->n_members = nmembers;
@@ -1554,6 +1570,9 @@ SearchCatCacheList(CatCache *cache,
 		cl->members[i++] = ct = (CatCTup *) lfirst(ctlist_item);
 		Assert(ct->c_list == NULL);
 		ct->c_list = cl;
+		/* release the temporary refcount on the member */
+		Assert(ct->refcount > 0);
+		ct->refcount--;
 		/* mark list dead if any members already dead */
 		if (ct->dead)
 			cl->dead = true;
@@ -1575,38 +1594,22 @@ SearchCatCacheList(CatCache *cache,
 /*
  *	ReleaseCatCacheList
  *
- *	Decrement the reference counts of a catcache list.
+ *	Decrement the reference count of a catcache list.
  */
 void
 ReleaseCatCacheList(CatCList *list)
 {
-	int			i;
-
 	/* Safety checks to ensure we were handed a cache entry */
 	Assert(list->cl_magic == CL_MAGIC);
 	Assert(list->refcount > 0);
-
-	for (i = list->n_members; --i >= 0;)
-	{
-		CatCTup    *ct = list->members[i];
-
-		Assert(ct->refcount > 0);
-
-		ct->refcount--;
-
-		if (ct->dead)
-			list->dead = true;
-		/* can't remove tuple before list is removed */
-	}
-
 	list->refcount--;
 	ResourceOwnerForgetCatCacheListRef(CurrentResourceOwner, list);
 
-	if (list->refcount == 0
+	if (
 #ifndef CATCACHE_FORCE_RELEASE
-		&& list->dead
+		list->dead &&
 #endif
-		)
+		list->refcount == 0)
 		CatCacheRemoveCList(list->my_cache, list);
 }
 
@@ -1654,35 +1657,87 @@ CatalogCacheCreateEntry(CatCache *cache, HeapTuple ntp,
 
 	/*
 	 * If we've exceeded the desired size of the caches, try to throw away
-	 * the least recently used entry.  NB: be careful not to throw away
+	 * the least recently used entry(s).  NB: be careful not to throw away
 	 * the newly-built entry...
 	 */
 	if (CacheHdr->ch_ntup > CacheHdr->ch_maxtup)
-	{
-		Dlelem	   *elt,
-				   *prevelt;
+		CatalogCacheCleanup(ct);
 
-		for (elt = DLGetTail(&CacheHdr->ch_lrulist); elt; elt = prevelt)
-		{
-			CatCTup    *oldct = (CatCTup *) DLE_VAL(elt);
+	return ct;
+}
+
+/*
+ * CatalogCacheCleanup
+ *		Try to reduce the size of the catcaches when they get too big
+ *
+ * savect can be NULL, or a specific CatCTup not to remove even if it
+ * has zero refcount.
+ */
+static void
+CatalogCacheCleanup(CatCTup *savect)
+{
+	int			tup_target;
+	CatCache   *ccp;
+	Dlelem	   *elt,
+			   *prevelt;
 
-			prevelt = DLGetPred(elt);
+	/*
+	 * Each time we have to do this, try to cut the cache size down to
+	 * about 90% of the maximum.
+	 */
+	tup_target = (CacheHdr->ch_maxtup * 9) / 10;
+
+	/*
+	 * Our strategy for managing CatCLists is that, each time we have to
+	 * throw away some cache entries, we first move-to-front all the members
+	 * of CatCLists that have been touched since the last cleanup sweep.
+	 * Then we do strict LRU elimination by individual tuples, zapping a list
+	 * if any of its members gets zapped.  Before PostgreSQL 8.1, we moved
+	 * members to front each time their owning list was touched, which was
+	 * arguably more fair in balancing list members against standalone tuples
+	 * --- but the overhead for large lists was horrendous.  This scheme is
+	 * more heavily biased towards preserving lists, but that is not
+	 * necessarily bad either.
+	 */
+	for (ccp = CacheHdr->ch_caches; ccp; ccp = ccp->cc_next)
+	{
+		for (elt = DLGetHead(&ccp->cc_lists); elt; elt = DLGetSucc(elt))
+		{
+			CatCList   *cl = (CatCList *) DLE_VAL(elt);
 
-			if (oldct->refcount == 0 && oldct != ct)
+			Assert(cl->cl_magic == CL_MAGIC);
+			if (cl->touched && !cl->dead)
 			{
-				CACHE2_elog(DEBUG2, "CatCacheCreateEntry(%s): Overflow, LRU removal",
-							cache->cc_relname);
-#ifdef CATCACHE_STATS
-				oldct->my_cache->cc_discards++;
-#endif
-				CatCacheRemoveCTup(oldct->my_cache, oldct);
-				if (CacheHdr->ch_ntup <= CacheHdr->ch_maxtup)
-					break;
+				int		i;
+
+				for (i = 0; i < cl->n_members; i++)
+					DLMoveToFront(&cl->members[i]->lrulist_elem);
 			}
+			cl->touched = false;
 		}
 	}
 
-	return ct;
+	/* Now get rid of unreferenced tuples in reverse global LRU order */
+	for (elt = DLGetTail(&CacheHdr->ch_lrulist); elt; elt = prevelt)
+	{
+		CatCTup    *ct = (CatCTup *) DLE_VAL(elt);
+
+		prevelt = DLGetPred(elt);
+
+		if (ct->refcount == 0 &&
+			(ct->c_list == NULL || ct->c_list->refcount == 0) &&
+			ct != savect)
+		{
+#ifdef CATCACHE_STATS
+			ct->my_cache->cc_discards++;
+#endif
+			CatCacheRemoveCTup(ct->my_cache, ct);
+
+			/* Quit when we've removed enough tuples */
+			if (CacheHdr->ch_ntup <= tup_target)
+				break;
+		}
+	}
 }
 
 /*

src/include/utils/catcache.h

@@ -13,7 +13,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/catcache.h,v 1.54 2005/04/14 20:03:27 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/utils/catcache.h,v 1.55 2005/08/13 22:18:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -79,21 +79,25 @@ typedef struct catctup
 	 * Each tuple in a cache is a member of two Dllists: one lists all the
 	 * elements in all the caches in LRU order, and the other lists just
 	 * the elements in one hashbucket of one cache, also in LRU order.
-	 *
+	 */
+	Dlelem		lrulist_elem;	/* list member of global LRU list */
+	Dlelem		cache_elem;		/* list member of per-bucket list */
+
+	/*
 	 * The tuple may also be a member of at most one CatCList.	(If a single
 	 * catcache is list-searched with varying numbers of keys, we may have
 	 * to make multiple entries for the same tuple because of this
 	 * restriction.  Currently, that's not expected to be common, so we
 	 * accept the potential inefficiency.)
 	 */
-	Dlelem		lrulist_elem;	/* list member of global LRU list */
-	Dlelem		cache_elem;		/* list member of per-bucket list */
-	struct catclist *c_list;	/* containing catclist, or NULL if none */
+	struct catclist *c_list;	/* containing CatCList, or NULL if none */
 
 	/*
 	 * A tuple marked "dead" must not be returned by subsequent searches.
 	 * However, it won't be physically deleted from the cache until its
-	 * refcount goes to zero.
+	 * refcount goes to zero.  (If it's a member of a CatCList, the list's
+	 * refcount must go to zero, too; also, remember to mark the list dead
+	 * at the same time the tuple is marked.)
 	 *
 	 * A negative cache entry is an assertion that there is no tuple matching
 	 * a particular key.  This is just as useful as a normal entry so far
@@ -123,14 +127,13 @@ typedef struct catclist
 	 * none of these will be negative cache entries.)
 	 *
 	 * A CatCList is only a member of a per-cache list; we do not do separate
-	 * LRU management for CatCLists.  Instead, a CatCList is dropped from
-	 * the cache as soon as any one of its member tuples ages out due to
-	 * tuple-level LRU management.
+	 * LRU management for CatCLists.  See CatalogCacheCleanup() for the
+	 * details of the management algorithm.
 	 *
 	 * A list marked "dead" must not be returned by subsequent searches.
 	 * However, it won't be physically deleted from the cache until its
-	 * refcount goes to zero.  (Its member tuples must have refcounts at
-	 * least as large, so they won't go away either.)
+	 * refcount goes to zero.  (A list should be marked dead if any of its
+	 * member entries are dead.)
 	 *
 	 * If "ordered" is true then the member tuples appear in the order of the
 	 * cache's underlying index.  This will be true in normal operation,
@@ -141,6 +144,7 @@ typedef struct catclist
 	int			refcount;		/* number of active references */
 	bool		dead;			/* dead but not yet removed? */
 	bool		ordered;		/* members listed in index order? */
+	bool		touched;		/* used since last CatalogCacheCleanup? */
 	short		nkeys;			/* number of lookup keys specified */
 	uint32		hash_value;		/* hash value for lookup keys */
 	HeapTupleData tuple;		/* header for tuple holding keys */