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

Am 02.11.2012 17:12, schrieb Petr Praus:

Your CPUs are indeed pretty oldschool. FSB based, IIRC, not NUMA. A
process migration would be even more expensive there.

> Might be worth to
> - manually pin (with taskset) the session you test this in to a
> particular CPU (once on each socket) to see if the times change
>
>
> I tested this and it does not seem to have any effect (assuming I used
> taskset correctly but I think so: taskset 02 psql to pin down to CPU
> #1 and taskset 01 psql to pin to CPU #0).
Well, that pinned your _client_ to the CPUs, not the server side session ;-)
You'd have to spot for the PID of the new "IDLE" server process and pin
that using "taskset -p". Also, 01 and 02 are probably cores in the same
package/socket. Try "lscpu" first and spot for "NUMA node*" lines at the
bottom.
But anyway... let's try something else first:
>
> - try reducing work_mem in the session you're testing in (so you
> have large SHM, but small work mem)
>
>
> Did this and it indicates to me that shared_buffers setting actually
> does not have an effect on this behaviour as I previously thought it
> has. It really boils down to work_mem: when I set shared_buffers to
> something large (say 4GB) and just play with work_mem the problem
> persists.
This only confirms what we've seen before. As soon as your work_mem
permits an in-memory sort of the intermediate result set (which at that
point in time is where? In the SHM, or in the private memory of the
backend? I can't tell, tbth), the sort takes longer than when it's using
a temp file.

What if you reduce the shared_buffers to your original value and only
increase/decrease the session's work_mem? Same behaviour?

Cheers,

--
Gunnar "Nick" Bluth
RHCE/SCLA

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