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Re: Limited performance on multi core server

From: Matthew Lunnon <mlunnon(at)rwa-net(dot)co(dot)uk>
To: Claus Guttesen <kometen(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Limited performance on multi core server
Date: 2007-12-12 11:12:42
Message-ID: 475FC22A.2070505@rwa-net.co.uk (view raw or flat)
Thread:
Lists: pgsql-performance
Thanks for the information Claus,  Why would reducing the effective 
cache size help the processor usage?  It seems that there is plenty of 
resources on the box although I can see that 10MB of sort space could 
mount up if we had 500 connections but at the moment we do not have 
anything like that number.

Thanks
Matthew.

Claus Guttesen wrote:
>> I have a 4 * dual core 64bit AMD OPTERON server with 16G of RAM, running
>> postgres 7.4.3.  This has been recompiled on the server for 64 stored
>> procedure parameters, (I assume this makes postgres 64 bit but are not
>> sure).  When the server gets under load from database connections
>> executing reads, lets say 20 - 40 concurrent reads, the CPU's seem to
>> limit at about 30-35% usage with no iowait reported. If I run a simple
>> select at this time it takes 5 seconds, the same query runs in 300
>> millis when the server is not under load so it seems that the database
>> is not performing well even though there is plenty of spare CPU.  There
>> does not appear to be large amounts of disk IO and my database is about
>> 5.5G so this should fit comfortably in RAM.
>>
>> changes to postgresql.sql:
>>
>> max_connections = 500
>> shared_buffers = 96000
>> sort_mem = 10240
>> effective_cache_size = 1000000
>>
>> Does anyone have any ideas what my bottle neck might be and what I can
>> do about it?
>>     
>
> You might want to lower shared_buffers to a lower value. Mine is set
> at 32768. Is your db performing complex sort? Remember that this value
> is per connection. Maby 1024. effective_cache_size should also be
> lowered to something like 32768. As far as I understand shared_buffers
> and effective_cache_size have to be altered "in reverse", ie. when
> lowering one the other can be raised.
>
> HTH.
>
>   

-- 
Matthew Lunnon
Technical Consultant
RWA Ltd.

 mlunnon(at)rwa-net(dot)co(dot)uk
 Tel: +44 (0)29 2081 5056
 www.rwa-net.co.uk
--

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