Date: Wed, 01 Jun 2005 19:22:42 +1200
From: Mark Kirkwood <markir@paradise.net.nz>
Subject: Re: postgresql-8.0.1 performance tuning
In-reply-to: <429D5942.6020502@streppone.it>
To: Cosimo Streppone <cosimo@streppone.it>
Cc: Postgresql Performance <pgsql-performance@postgresql.org>
Message-id: <429D6242.6000804@paradise.net.nz>
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References: <1117201312.7060.6.camel@fandelm.ecommit.de>
	<429D47FD.7090501@streppone.it> <429D56B2.9070302@paradise.net.nz>
	<429D5942.6020502@streppone.it>

Cosimo Streppone wrote:
> Mark Kirkwood ha scritto:
> 
>> Cosimo Streppone wrote:
>>
>>> ######### Config ############
>>>
>>>> /etc/sysctl.conf:
>>>> kernel.shmall = 786432000
>>>> kernel.shmmax = 786432000
>>>
>>>
>>> I think you have a problem here.
>>> kernel.shmmax should *not* be set to an amount of RAM, but
> 
> 
> Sorry, I thought "shmall" but written "shmmax".
> Thanks Mark!
> 

Hehe - happens to me all the time!

On the shmall front - altho there is *probably* no real performance 
impact setting it to the same as shmmax (i.e. allowing 4096 allocations 
of size shmmax!), it is overkill. In addition it does allow for a DOS by 
a program that allocates thousands of segments (or somehow starts 
thousands of Pg servers on different ports...)!

For a dedicated Pg server I would size shmall using a calculation along 
the lines of:

shmall = (no. of postgresql servers) * (shmmax/4096)


If there are other daemons on the box that need to use shared memory, 
then add their likely requirements to shmall too!

cheers

Mark