From: | "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com> |
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To: | david(at)lang(dot)hm |
Cc: | "Andrew Sullivan" <ajs(at)commandprompt(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: select on 22 GB table causes "An I/O error occured while sending to the backend." exception |
Date: | 2008-08-27 22:28:56 |
Message-ID: | dcc563d10808271528x3fb78aa3r87f42765591b05b0@mail.gmail.com |
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Lists: | pgsql-performance |
On Wed, Aug 27, 2008 at 4:22 PM, <david(at)lang(dot)hm> wrote:
> I disagree with you. I think goof Postgres operation is so highly dependant
> on caching as much data as possible that disabling overcommit (and throwing
> away a lot of memory that could be used for cache) is a solution that's as
> bad or worse than the problem it's trying to solve.
>
> I find that addign a modest amount of swap to the system and leaving
> overcommit enabled works better for me, if the system starts swapping I have
> a chance of noticing and taking action, but it will ride out small
> overloads. but the biggest thing is that it's not that much more acceptable
> for me to have other programs on the box failing due to memory allocation
> errors, and those will be much more common with overcommit disabled then the
> OOM killer would be with it enabled
I don't generally find this to be the case because I usually allocate
about 20-25% of memory to shared_buffers, use another 10-20% for
work_mem across all backends, and let the OS cache with the other
50-60% or so of memory. In this situation allocations rarely, if
ever, fail.
Note I also turn swappiness to 0 or 1 or something small too.
Otherwise linux winds up swapping out seldom used shared_buffers to
make more kernel cache, which is counter productive in the extreme.
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