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Re: Limited performance on multi core server

From: "Claus Guttesen" <kometen(at)gmail(dot)com>
To: mlunnon(at)rwa-net(dot)co(dot)uk
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Limited performance on multi core server
Date: 2007-12-12 10:45:24
Message-ID: b41c75520712120245k40994522g562954e02899a8d2@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
> 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.

-- 
regards
Claus

When lenity and cruelty play for a kingdom,
the gentlest gamester is the soonest winner.

Shakespeare

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