From f7d0a31ce94e78d8e69a5e2cc8ae44344d85f9db Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 12 May 2017 18:30:02 -0400 Subject: [PATCH] Reduce initial size of RelfilenodeMapHash. 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 --- src/backend/utils/cache/relfilenodemap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/backend/utils/cache/relfilenodemap.c b/src/backend/utils/cache/relfilenodemap.c index 894dacb5e7..a2b74ad0b0 100644 --- 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. */