Re: Allow sorts to use more available memory

On 09/12/2011 12:47 PM, Andy Colson wrote:

> work_mem is not the total a query can use. I believe each step can
> use that much, and each backend can use it for multiple bits. So if
> you had two backends, each doing 2 sorts, you'd use 2*2 = 4 * 2GB =
> 8GB.

Exactly. Find a big query somewhere in your system. Use EXPLAIN to
examine it. Chances are, that one query has one or more sorts. Each one
of those gets its own work_mem. Each sort. The query have four sorts? It
may use 4*work_mem. On a whim a while back, I doubled our 8MB setting to
16MB on a test system. During a load test, the machine ran out of
memory, swapped out, and finally crashed after the OOM killer went nuts.

Set this value *at your own risk* and only after *significant* testing.
Having it too high can have rather unexpected consequences. Setting it
to 1 or 2GB, unless you have VERY few threads, or a TON of memory, is a
very, very bad idea.

--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 800 | Chicago IL, 60604
312-676-8870
sthomas(at)peak6(dot)com

______________________________________________

See http://www.peak6.com/email-disclaimer/ for terms and conditions related to this email