Date: Wed, 23 Jan 2008 19:31:51 -0500 (EST)
From: Greg Smith <gsmith@gregsmith.com>
To: Tory M Blue <tmblue@gmail.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Postgres 8.2 memory weirdness
In-Reply-To: <8a547c840801231023q55796128nb2e88b8c8cfe86e@mail.gmail.com>
Message-ID: <Pine.GSO.4.64.0801231923420.12679@westnet.com>
References: <8a547c840801231023q55796128nb2e88b8c8cfe86e@mail.gmail.com>
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On Wed, 23 Jan 2008, Tory M Blue wrote:

> I have hundreds of thousands of updates, inserts a day. But what I'm 
> seeing is my server appears to "deallocate" memory (for the lack of a 
> better term) and performance goes to heck, slow response, a sub second 
> query takes anywhere from 6-40 seconds to complete when this happens.

Generally if you have a system doing many updates and inserts that slows 
for that long, it's because it hit a checkpoint.  I'm not sure what your 
memory-related issues are but it's possible that might be from a backlog 
of sessions using memory that are stuck behind the checkpoint, 
particularly since you mention simple query connections stacking up during 
these periods.

In any case you should prove/disprove this is checkpoint-related behavior 
before you chase down something more esoteric.  There's a quick intro to 
this area in the "Monitoring checkpoints" section of 
http://www.westnet.com/~gsmith/content/postgresql/chkp-bgw-83.htm and the 
later sections go into what you can do about it.

> I suspect I've outgrown our initial postgres config, or there are more
> sysctl or other kernel tweaks that need to happen.

You should post a list of what you're changed from the defaults.  You're 
analyzing from the perspective where you assume it's a memory problem and 
a look at your config will give a better idea whether that's possible or 
not.  Other good things to mention:  exact 8.2 version, OS, total memory, 
outline of disk configuration.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD