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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: (view raw, whole thread or download thread mbox)
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.


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.

 Tel: +44 (0)29 2081 5056

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