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

Am 03.11.2012 18:19, schrieb Petr Praus:
> On 3 November 2012 12:09, Gunnar "Nick" Bluth
> <gunnar(dot)bluth(at)pro-open(dot)de <mailto:gunnar(dot)bluth(at)pro-open(dot)de>> wrote:
>
> Am 03.11.2012 16:20, schrieb Petr Praus:
>>
>> Your CPUs are indeed pretty oldschool. FSB based, IIRC, not
>> NUMA. A process migration would be even more expensive there.
>>
>
> Ok, I've actually looked these up now... at the time these were
> current, I was in the lucky situation to only deal with Opterons.
> And actually, with these CPUs it is pretty possible that Scott
> Marlowe's hint (check vm.zone_reclaim_mode) was pointing in the
> right direction. Did you check that?
>
>
> I did check that, it's zero. I responded to his message, but my
> messages to the mailing list are getting delayed by ~24 hours because
> somebody has to always bless them.
>
>
>
>> Yes, same behaviour. I let the shared_buffers be the default
>> (which is 8MB). With work_mem 1MB the query runs fast, with
>> 96MB it runs slow (same times as before). It really seems
>> that the culprit is work_mem.
>>
>>
>
> Well, I'm pretty sure that having more work_mem is a good thing
> (tm) normally ;-)
>
>
> Well, that's what I always thought too! :-)
>
So, to sum this up (and make someone more competent bite on it maybe
;-), on your SMP, FSB, "fake-multicore" system all "hash"-related works
that potentially switch to different implementations internally (but
w/out telling us so) when given more work_mem are slower.

I'm pretty sure you're hitting some subtle, memory-access-related
cornercase here.

The L2 cache of your X7350 CPUs is 2MB, could you run the tests with,
say, 1, 2, 4 and 8MB of work_mem and post the results?

--
Gunnar "Nick" Bluth
RHCE/SCLA

Mobil +49 172 8853339
Email: gunnar(dot)bluth(at)pro-open(dot)de
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