Re: Slow vacuum performance

From: Andrew McMillan <andrew(at)catalyst(dot)net(dot)nz>
To: Patrick Hatcher <PHatcher(at)macys(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Slow vacuum performance
Date: 2004-06-21 10:11:49
Message-ID: 1087812709.1748.41.camel@lamb.mcmillan.net.nz
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On Fri, 2004-06-18 at 19:51 -0700, Patrick Hatcher wrote:
>
> Thanks!
>
> My effective_cache_size = 625000
>
> I thought that having the shared_buffers above 2k or 3k didn't gain
> any performance and may in fact degrade it?

Hi Patrick,

Quoting from:
http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html

shared_buffers
Sets the size of PostgreSQL's' memory buffer where queries are
held before being fed into the Kernel buffer of the host system.
It's very important to remember that this is only a holding
area, and not the total memory available for the server. As
such, resist the urge to set this number to a large portion of
your RAM, as this will actually degrade performance on many
operating systems. Members of the pgsql-performance mailing list
have found useful values in the range of 1000-6000, depending on
available RAM, database size, and number of concurrent queries.
For servers with very large amounts of available RAM (more than
1 GB) increasing this setting to 6-15% or available RAM has
worked well for some users. The real analysis of the precise
best setting is not fully understood and is more readily
determined through testing than calculation.

As a rule of thumb, observe shared memory usage of PostgreSQL
with tools like ipcs and determine the setting. Remember that
this is only half the story. You also need to set
effective_cache_size so that postgreSQL will use available
memory optimally.

Using this conservatively, on an 8G system, 6% would be roughly 60,000
pages - considerably higher than 2-3000...

One day when I wasn't timid (well, OK, I was desperate :-), I did see a
_dramatic_ performance improvement in a single very narrow activity by
setting shared_buffers to 300000 on a 4G RAM system (I was rolling back
a transaction involving an update to 2.8 million rows) , but afterwards
I set shared_buffers back to 10000, which I have now increased to 20000
on that system.

You may also want to look at:
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html

Or indeed, peruse the articles regularly as they appear:
http://www.varlena.com/varlena/GeneralBits/

Regards,
Andrew McMillan

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