postgres.git / summary / log / commit / refs

commit    8085a4f7510191b8fe7f8f7b5846fdbc79abcf8d
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Fri May 12 22:30:02 2017 +0000

    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 c790309caae..612f0f3a0d1 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. */


[parent: 5e2af609e14e]