Subject: [PATCH v7 1/3] CatCache expiration feature

From: Kyotaro Horiguchi <[email-masked]>
Date: Wed, 18 Nov 2020 16:54:31 +0900

---
 src/backend/access/transam/xact.c  |  3 ++
 src/backend/utils/cache/catcache.c | 87 +++++++++++++++++++++++++++++-
 src/backend/utils/misc/guc.c       | 12 +++++
 src/include/utils/catcache.h       | 19 +++++++
 4 files changed, 120 insertions(+), 1 deletion(-)

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index a2068e3fd4..86888d2409 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -1086,6 +1086,9 @@ static void
 AtStart_Cache(void)
 {
 	AcceptInvalidationMessages();
+
+	if (xactStartTimestamp != 0)
+		SetCatCacheClock(xactStartTimestamp);
 }
 
 /*
diff --git a/src/backend/utils/cache/catcache.c b/src/backend/utils/cache/catcache.c
index fa2b49c676..644d92dd9a 100644
--- a/src/backend/utils/cache/catcache.c
+++ b/src/backend/utils/cache/catcache.c
@@ -38,6 +38,7 @@
 #include "utils/rel.h"
 #include "utils/resowner_private.h"
 #include "utils/syscache.h"
+#include "utils/timestamp.h"

  /* #define CACHEDEBUG */	/* turns DEBUG elogs on */
@@ -60,9 +61,19 @@
 #define CACHE_elog(...)
 #endif
 
+/*
+ * GUC variable to define the minimum age of entries that will be considered
+ * to be evicted in seconds. -1 to disable the feature.
+ */
+int catalog_cache_prune_min_age = -1;
+uint64	prune_min_age_us;
+
 /* Cache management header --- pointer is NULL until created */
 static CatCacheHeader *CacheHdr = NULL;
 
+/* Clock for the last accessed time of a catcache entry. */
+uint64	catcacheclock = 0;
+
 static inline HeapTuple SearchCatCacheInternal(CatCache *cache,
 											   int nkeys,
 											   Datum v1, Datum v2,
@@ -74,6 +85,7 @@ static pg_noinline HeapTuple SearchCatCacheMiss(CatCache *cache,
 												Index hashIndex,
 												Datum v1, Datum v2,
 												Datum v3, Datum v4);
+static bool CatCacheCleanupOldEntries(CatCache *cp);
 
 static uint32 CatalogCacheComputeHashValue(CatCache *cache, int nkeys,
 										   Datum v1, Datum v2, Datum v3, Datum v4);
@@ -99,6 +111,15 @@ static void CatCacheFreeKeys(TupleDesc tupdesc, int nkeys, int *attnos,
 static void CatCacheCopyKeys(TupleDesc tupdesc, int nkeys, int *attnos,
 							 Datum *srckeys, Datum *dstkeys);
 
+/* GUC assign function */
+void
+assign_catalog_cache_prune_min_age(int newval, void *extra)
+{
+	if (newval < 0)
+		prune_min_age_us = UINT64_MAX;
+	else
+		prune_min_age_us = ((uint64) newval) * USECS_PER_SEC;
+}
 
 /*
  *					internal support functions
@@ -1264,6 +1285,9 @@ SearchCatCacheInternal(CatCache *cache,
 		 */
 		dlist_move_head(bucket, &ct->cache_elem);
 
+		/* Record the last access timestamp */
+		ct->lastaccess = catcacheclock;
+
 		/*
 		 * If it's a positive entry, bump its refcount and return it. If it's
 		 * negative, we can report failure to the caller.
@@ -1425,6 +1449,61 @@ SearchCatCacheMiss(CatCache *cache,
 	return &ct->tuple;
 }
 
+/*
+ * CatCacheCleanupOldEntries - Remove infrequently-used entries
+ *
+ * Catcache entries happen to be left unused for a long time for several
+ * reasons. Remove such entries to prevent catcache from bloating. It is based
+ * on the similar algorithm with buffer eviction. Entries that are accessed
+ * several times in a certain period live longer than those that have had less
+ * access in the same duration.
+ */
+static bool
+CatCacheCleanupOldEntries(CatCache *cp)
+{
+	int		nremoved = 0;
+	int		i;
+	long	oldest_ts = catcacheclock;
+	uint64	prune_threshold = catcacheclock - prune_min_age_us;
+
+	/* Scan over the whole hash to find entries to remove */
+	for (i = 0 ; i < cp->cc_nbuckets ; i++)
+	{
+		dlist_mutable_iter	iter;
+
+		dlist_foreach_modify(iter, &cp->cc_bucket[i])
+		{
+			CatCTup    *ct = dlist_container(CatCTup, cache_elem, iter.cur);
+
+			/* Don't remove referenced entries */
+			if (ct->refcount == 0 &&
+				(ct->c_list == NULL || ct->c_list->refcount == 0))
+			{
+				if (ct->lastaccess < prune_threshold)
+				{
+					CatCacheRemoveCTup(cp, ct);
+					nremoved++;
+
+					/* don't let the removed entry update oldest_ts */
+					continue;
+				}
+			}
+
+			/* update the oldest timestamp if the entry remains alive */
+			if (ct->lastaccess < oldest_ts)
+				oldest_ts = ct->lastaccess;
+		}
+	}
+
+	cp->cc_oldest_ts = oldest_ts;
+
+	if (nremoved > 0)
+		elog(DEBUG1, "pruning catalog cache id=%d for %s: removed %d / %d",
+			 cp->id, cp->cc_relname, nremoved, cp->cc_ntup + nremoved);
+
+	return nremoved > 0;
+}
+
 /*
  *	ReleaseCatCache
  *
@@ -1888,6 +1967,7 @@ CatalogCacheCreateEntry(CatCache *cache, HeapTuple ntp, Datum *arguments,
 	ct->dead = false;
 	ct->negative = negative;
 	ct->hash_value = hashValue;
+	ct->lastaccess = catcacheclock;
 
 	dlist_push_head(&cache->cc_bucket[hashIndex], &ct->cache_elem);
 
@@ -1899,7 +1979,12 @@ CatalogCacheCreateEntry(CatCache *cache, HeapTuple ntp, Datum *arguments,
 	 * arbitrarily, we enlarge when fill factor > 2.
 	 */
 	if (cache->cc_ntup > cache->cc_nbuckets * 2)
-		RehashCatCache(cache);
+	{
+		/* try removing old entries before expanding hash */
+		if (catcacheclock - cache->cc_oldest_ts < prune_min_age_us ||
+			!CatCacheCleanupOldEntries(cache))
+			RehashCatCache(cache);
+	}
 
 	return ct;
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 17579eeaca..255e9fa73d 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -88,6 +88,7 @@
 #include "utils/acl.h"
 #include "utils/builtins.h"
 #include "utils/bytea.h"
+#include "utils/catcache.h"
 #include "utils/float.h"
 #include "utils/guc_tables.h"
 #include "utils/memutils.h"
@@ -3445,6 +3446,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"catalog_cache_prune_min_age", PGC_USERSET, RESOURCES_MEM,
+			gettext_noop("System catalog cache entries that are living unused more than this seconds are considered for removal."),
+			gettext_noop("The value of -1 turns off pruning."),
+			GUC_UNIT_S
+		},
+		&catalog_cache_prune_min_age,
+		-1, -1, INT_MAX,
+		NULL, assign_catalog_cache_prune_min_age, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL, NULL
diff --git a/src/include/utils/catcache.h b/src/include/utils/catcache.h
index ddc2762eb3..291e857e38 100644
--- a/src/include/utils/catcache.h
+++ b/src/include/utils/catcache.h
@@ -22,6 +22,7 @@
 
 #include "access/htup.h"
 #include "access/skey.h"
+#include "datatype/timestamp.h"
 #include "lib/ilist.h"
 #include "utils/relcache.h"
 
@@ -61,6 +62,7 @@ typedef struct catcache
 	slist_node	cc_next;		/* list link */
 	ScanKeyData cc_skey[CATCACHE_MAXKEYS];	/* precomputed key info for heap
 											 * scans */
+	uint64		cc_oldest_ts;	/* timestamp (us) of the oldest tuple */
 
 	/*
 	 * Keep these at the end, so that compiling catcache.c with CATCACHE_STATS
@@ -119,6 +121,7 @@ typedef struct catctup
 	bool		dead;			/* dead but not yet removed? */
 	bool		negative;		/* negative cache entry? */
 	HeapTupleData tuple;		/* tuple management header */
+	uint64		lastaccess;		/* timestamp in us of the last usage */
 
 	/*
 	 * The tuple may also be a member of at most one CatCList.  (If a single
@@ -189,6 +192,22 @@ typedef struct catcacheheader
 /* this extern duplicates utils/memutils.h... */
 extern PGDLLIMPORT MemoryContext CacheMemoryContext;
 
+
+/* for guc.c, not PGDLLPMPORT'ed */
+extern int catalog_cache_prune_min_age;
+
+/* source clock for access timestamp of catcache entries */
+extern uint64 catcacheclock;
+
+/* SetCatCacheClock - set catcache timestamp source clock */
+static inline void
+SetCatCacheClock(TimestampTz ts)
+{
+	catcacheclock = (uint64) ts;
+}
+
+extern void assign_catalog_cache_prune_min_age(int newval, void *extra);
+
 extern void CreateCacheMemoryContext(void);
 
 extern CatCache *InitCatCache(int id, Oid reloid, Oid indexoid,
-- 
2.27.0

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