Re: Re: Increasing work_mem and shared_buffers on Postgres 9.2 significantly slows down queries

Regards, Petr.
Tuning PostgreSQL is not just change the postgresql.conf, it includes
more things like:
- the filesystem that you are using
- the kernel version that you using (particularly in Linux systems)
- the tuning to kernel variables
- the type of discs that you are using (SSDs are very fast, like you saw
in your iMac system)

On 10/30/2012 02:44 PM, Petr Praus wrote:
> I just found one particularly interesting fact: when I perform the
> same test on my mid-2010 iMac (OSX 10.7.5) also with Postgres 9.2.1
> and 16GB RAM, I don't experience the slow down.
> Specifically:
> set work_mem='1MB';
> select ...; // running time is ~1800 ms
> set work_mem='96MB';
> select ...' // running time is ~1500 ms
>
> When I do exactly the same query (the one from my previous post) with
> exactly the same data on the server:
> I get 2100 ms with work_mem=1MB and 3200 ms with 96 MB.
>
> The Mac has SSD so it's understandably faster, but it exhibits a
> behavior I would expect. What am I doing wrong here?
>
> Thanks.
>
> On 30 October 2012 14:08, Petr Praus <petr(at)praus(dot)net
> <mailto:petr(at)praus(dot)net>> wrote:
>
> 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
> <http://c.id>=cp.contestId
> left outer join teammember tm on tm.contestparticipantid=cp.id
> <http://cp.id>
> left outer join staffmember sm on cp.id
> <http://cp.id>=sm.contestparticipantid
> left outer join person p on p.id <http://p.id>=cp.personid
> left outer join personinfo pi on pi.id
> <http://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-down
> but a few people suggested
>
>

--

Marcos Luis Ortíz Valmaseda
about.me/marcosortiz <http://about.me/marcosortiz>
@marcosluis2186 <http://twitter.com/marcosluis2186>

10mo. ANIVERSARIO DE LA CREACION DE LA UNIVERSIDAD DE LAS CIENCIAS INFORMATICAS...
CONECTADOS AL FUTURO, CONECTADOS A LA REVOLUCION

http://www.uci.cu
http://www.facebook.com/universidad.uci
http://www.flickr.com/photos/universidad_uci