Hello,
I have a PostgreSQL 9.2 instance running on RHEL 6.3, 8-core machine with
16GB of RAM. The server is dedicated to this database, the disks are local
RAID10. Given that the default postgresql.conf is quite conservative
regarding memory settings, I thought it might be a good idea to allow
Postgres to use more memory. To my surprise, following advice in the
performance tuning guide on Postgres wiki[2] significantly slowed down
practically every query I run but it's more noticeable on the more complex
queries.
I also tried running pgtune[1] which gave the following recommendation with
more parameters tuned, but that didn't change anything. It suggests
shared_buffers of 1/4 of RAM size which seems to in line with advice
elsewhere (and on PG wiki in particular).
default_statistics_target = 50
maintenance_work_mem = 960MB
constraint_exclusion = on
checkpoint_completion_target = 0.9
effective_cache_size = 11GB
work_mem = 96MB
wal_buffers = 8MB
checkpoint_segments = 16
shared_buffers = 3840MB
max_connections = 80
I tried reindexing the whole database after changing the settings (using
REINDEX DATABASE), but that didn't help either. I played around with
shared_buffers and work_mem. Gradually changing them from the very
conservative default values (128k / 1MB) also gradually decreased
performance.
I ran EXPLAIN (ANALYZE,BUFFERS) on a few queries and the culprit seems to
be that Hash Join is significantly slower. It's not clear to me why.
To give some specific example, I have the following query. It runs in
~2100ms on the default configuration and ~3300ms on the configuration with
increased buffer sizes:
select count(*) from contest c
left outer join contestparticipant cp on c.id=cp.contestId
left outer join teammember tm on tm.contestparticipantid=cp.id
left outer join staffmember sm on cp.id=sm.contestparticipantid
left outer join person p on p.id=cp.personid
left outer join personinfo pi on pi.id=cp.personinfoid
where pi.lastname like '%b%' or pi.firstname like '%a%';
EXPLAIN (ANALYZE,BUFFERS) for the query above:
- Default buffers: http://explain.depesz.com/s/xaHJ
- Bigger buffers: http://explain.depesz.com/s/Plk
The tables don't have anything special in them
The question is why am I observing decreased performance when I increase
buffer sizes? The machine is definitely not running out of memory.
Allocation if shared memory in OS is (`shmmax` and `shmall`) is set to very
large values, that should not be a problem. I'm not getting any errors in
the Postgres log either. I'm running autovacuum in the default
configuration but I don't expect that has anything to do with it. All
queries were run on the same machine few seconds apart, just with changed
configuration (and restarted PG).
I also found a blog post [3] which experiments with various work_mem values
that run into similar behavior I'm experiencing but it doesn't really
explain it.
[1]: http://pgfoundry.org/projects/pgtune/
[2]: http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server
[3]:
http://www.depesz.com/2011/07/03/understanding-postgresql-conf-work_mem/
Thanks,
Petr Praus
PS:
I also posted the question here:
http://dba.stackexchange.com/questions/27893/increasing-work-mem-and-shared-buffers-on-postgres-9-2-significantly-slows-downbut
a few people suggested
