Greetins hackers,
I have mixed feelings if this welcome contribution as the potential gain is
relatively small in my tests, but still I would like to point out that
HASH_FFACTOR functionality from dynahash.c could be removed or optimized
(default fill factor is always 1, there's not a single place that uses custom
custom fill factor other than DEF_FFACTOR=1 inside PostgreSQL repository).
Because the functionality is present there seems to be division for every
buffer access [BufTableLookup()] / or every smgropen() call (everything call to
hash_search() is affected, provided it's not ShmemInitHash/HASH_PARTITION).
This division is especially visible via perf on single process StartupXLOG WAL
recovery process on standby in heavy duty 100% CPU conditions , as the top1 is
inside hash_search:
0x0000000000888751 <+449>: idiv r8
0x0000000000888754 <+452>: cmp rax,QWORD PTR [r15+0x338] <<-- in perf
annotate shows as 30-40%, even on default -O2, probably CPU pipelining for idiv
above
I've made a PoC test to skip that division assuming ffactor would be gone:
if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
- hctl->freeList[0].nentries / (long) (hctl->max_bucket +
1) >= hctl->ffactor &&
+ hctl->freeList[0].nentries >= (long) (hctl->max_bucket
+ 1) &&
For a stream of WAL 3.7GB I'm getting consistent improvement of ~4%, (yes I
know it's small, that's why I'm having mixed feelings):
gcc -O3: 104->100s
gcc -O2: 108->104s
pgbench -S -c 16 -j 4 -T 30 -M prepared: stays more or less the same (-s 100),
so no positive impact there
After removing HASH_FFACTOR PostgreSQL still compiles... Would removing it
break some external API/extensions ? I saw several optimization for the "idiv"
where it could be optimized e.g. see
https://github.com/ridiculousfish/libdivide Or maybe there is some other idea
to expose bottlenecks of BufTableLookup() ? I also saw codepath
PinBuffer()->GetPrivateRefCountEntry() -> dynahash that could be called pretty
often I have no idea what kind of pgbench stresstest could be used to
demonstrate the gain (or lack of it).
-Jakub Wartak.
