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Re: select on 22 GB table causes "An I/O error occured while sending to the backend." exception

From: Steve Atkins <steve(at)blighty(dot)com>
To: PostgreSQL Performance <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: 2008-08-28 15:05:37
Message-ID: EADBB336-A14E-492C-A8CA-105C8F77A43A@blighty.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Aug 28, 2008, at 6:26 AM, Matthew Wakeling wrote:

> On Wed, 27 Aug 2008, david(at)lang(dot)hm wrote:
>> if memory overcommit is disabled, the kernel checks to see if you  
>> have an extra 1G of ram available, if you do it allows the process  
>> to continue, if you don't it tries to free memory (by throwing away  
>> cache, swapping to disk, etc), and if it can't free the memory will  
>> return a memroy allocation error (which I believe will cause  
>> firefox to exit).
>
> Remember that the memory overcommit check is checking against the  
> amount of RAM + swap you have - not just the amount of RAM. When a  
> fork occurs, hardly any extra actual RAM is used (due to copy on  
> write), but the potential is there for the process to use it. If  
> overcommit is switched off, then you just need to make sure there is  
> *plenty* of swap to convince the kernel that it can actually fulfil  
> all of the memory requests if all the processes behave badly and all  
> shared pages become unshared. Then the consequences of processes  
> actually using that memory are that the machine will swap, rather  
> than the OOM killer having to act.
>
> Of course, it's generally bad to run a machine with more going on  
> than will fit in RAM.
>
> Neither swapping nor OOM killing are particularly good - it's just a  
> consequence of the amount of memory needed being unpredictable.
>
> Probably the best solution is to just tell the kernel somehow to  
> never kill the postmaster.

Or configure adequate swap space?

Cheers,
   Steve

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Next:From: Jerry ChamplinDate: 2008-08-28 15:47:19
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