Message-ID: <4BE7FD1F.1050903@tech.sdb.it>
Date: Mon, 10 May 2010 14:33:35 +0200
From: Silvio Brandani <silvio.brandani@tech.sdb.it>
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To: Giles Lean <giles.lean@pobox.com>
Cc: pgsql-admin@postgresql.org
Subject: Re: postgres invoked oom-killer
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	<20100508030541.43086.qmail@sapphire.netherstone.net>
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Giles Lean ha scritto:
> Silvio Brandani <silvio.brandani@tech.sdb.it> wrote:
>
>   
>> yes the server is dedicated to PostgreSQL.
>>
>> Could be a bug of PostgreSQL the fact that the system went Out of 
>> Memory?? Wath can be the cause of it?
>>
>> Regards,
>> Silvio
>>     
>
> The out of memory killer is a totally bogus misfeature of
> Linux.  At least these days they let you turn it off, and
> everyone should turn it off _and_ should bug their OS vendor
> to request that they ship their distribution with memory
> overcommit and the out of memory killer disabled.
>
> The "out of memory" killer only exists because Linux by
> default will allow more memory to be allocated than exists in
> the system. This is called "memory overcommit" and is kown in
> operating system circles (outside of Linux) to be a bad thing.
>
> The rationale for memory overcommit being a bad thing is that
> if your OS doesn't allow allocation of more memory than there
> is in the system, then applications are forced to deal with
> memory requests that fail, instead of seeing those requests
> "succeed" and being killed some random time later.
>
> Allowing more memory to be "allocated" than exists in the
> machine is only useful in two circumstances that I know of,
> and neither apply to a dedicated PostgreSQL system, running
> Linux or not:
>
> 1. When badly written applications allocate far more memory
>    than they use.  PostgreSQL isn't like that.
>
>    The technically correct solution is to fix the
>    applications; allowing memory overcommit is a dangerous
>    workaround that _will_ cause problems.
>
> 2. When very large processes need to fork (e.g. a scientific
>    numerical analysis program using most of physical memory)
>    might want to fork to send an email notification or
>    something similar).
>
>    The technically correct solution is to use vfork().
>
> I won't make myself popular with the Linux zealots, but memory
> overcommit should just be removed from Linux.  It does way more
> harm than good on any operating system I've ever seen it used
> on, and the problems it has caused just on Linux are legion.
>
> The only OS that had _any_ excuse for memory overcommit I've
> seen was one that lacked a vfork() system call; they added
> that call and the justification for memory overcommit went
> away.
>
> Regards,
>
> Giles
>
>   
Thanks  a lot,

I' m going to disable the memory killer, moreover I have run the 
Operative System watcher scripts to monitor the undergoing activity in 
case it happens again.


Best Regards,

Silvio Brandani

-- 
Silvio Brandani
Infrastructure Administrator
SDB Information Technology
Phone: +39.055.3811222
Fax:   +39.055.5201119

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