Reduce initial size of RelfilenodeMapHash.

tglsfdc · tglsfdc · commit f7d0a31ce94e · 2017-05-12T18:30:02.000-04:00
A test case provided by Mathieu Fenniak shows that hash_seq_search'ing this hashtable can consume a very significant amount of overhead during logical decoding, which triggers frequent cache invalidation. Testing suggests that the actual population of the hashtable is often no more than a few dozen entries, so we can cut the overhead just by dropping the initial number of buckets down from 1024 --- I chose to cut it to 64. (In situations where we do have a significant number of entries, we shouldn't get any real penalty from doing this, as the dynahash.c code will resize the hashtable automatically.) This gives a further factor-of-two savings in Mathieu's test case. That may be overly optimistic for real-world benefit, as real cases may have larger average table populations, but it's hard to see it turning into a net negative for any workload. Back-patch to 9.4 where relfilenodemap.c was introduced. Discussion: https://postgr.es/m/CAHoiPjzea6N0zuCi=+f9v_j94nfsy6y8SU7-=bp4=7qw6_i=Rg@mail.gmail.com
diff --git a/src/backend/utils/cache/relfilenodemap.c b/src/backend/utils/cache/relfilenodemap.c
@@ -123,7 +123,7 @@ InitializeRelfilenodeMap(void)
 	 * error.
 	 */
 	RelfilenodeMapHash =
-		hash_create("RelfilenodeMap cache", 1024, &ctl,
+		hash_create("RelfilenodeMap cache", 64, &ctl,
 					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
 
 	/* Watch for invalidation events. */
