Message-ID: <42963C3F.4000905@arbash-meinel.com>
Date: Thu, 26 May 2005 16:14:39 -0500
From: John Arbash Meinel <john@arbash-meinel.com>
User-Agent: Mozilla Thunderbird 1.0.2 (Macintosh/20050317)
MIME-Version: 1.0
To: Josh Close <narshe@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: slow queries, possibly disk io
References: <4a0cafe205052613242e0d4d10@mail.gmail.com>
In-Reply-To: <4a0cafe205052613242e0d4d10@mail.gmail.com>
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Josh Close wrote:

>I have some queries that have significan't slowed down in the last
>couple days. It's gone from 10 seconds to over 2 mins.
>
>The cpu has never gone over 35% in the servers lifetime, but the load
>average is over 8.0 right now. I'm assuming this is probably due to
>disk io.
>
>I need some help setting up postgres so that it doesn't need to go to
>disk. I think the shared_buffers and effective_cache_size values are
>the one's I need to look at.
>
>Would setting shmmax and smmall to 90% or so of available mem and
>putting a lot for postgres be helpful?
>
>
Setting shared buffers above something like 10-30% of memory is counter
productive.

>Effective cach size says this:
>Sets the planner's assumption about the effective size of the disk
>cache (that is, the portion of the kernel's disk cache that will be
>used for PostgreSQL data files).
>
>Does that mean the total available ram? Or what's left over from shared_buffers?
>
>I've tried different things and not much has been working. Is there a
>good way to ensure that most of the tables accessed in postgres will
>be cached in mem and not have to go to disk?
>
>If I'm joining a lot of tables, should the sort_mem be set high also?
>Do shared_buffers, effective_cache_size, and sort_mem all use
>different mem? Or are they seperate?
>
>
>
Increasing sort_mem can help with various activities, but increasing it
too much can cause you to swap, which kills performance. The caution is
that you will likely use at least 1 sort_mem per connection, and can
likely use more than one if the query is complicated.

effective_cache_size changes how Postgres plans queries, but given the
same query plan, it doesn't change performance at all.

>I've looked for information and haven't found any useful pages about this.
>
>Any help would be greatly appreciated.
>
>Thanks.
>
>-Josh
>
>

John
=:->


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