Re: Caching by Postgres

On Tue, 23 Aug 2005 10:10:45 -0700 (PDT)
gokulnathbabu manoharan <gokulnathbabu(at)yahoo(dot)com> wrote:

> Hi all,
>
> I like to know the caching policies of Postgresql.
> What parameter in the postgresql.conf affects the
> cache size used by the Postgresql? As far as I have
> searched my knowledge of the parameters are
>
> 1. shared_buffers - Sets the limit on the amount of
> shared memory used. If I take this is as the cache
> size then my performance should increase with the
> increase in the size of shared_buffers. But it seems
> it is not the case and my performance actually
> decreases with the increase in the shared_buffers. I
> have a RAM size of 32 GB. The table which I use more
> frequently has around 68 million rows. Can I cache
> this entire table in RAM?

increasing shared_buffers to a point helps, but after
a certain threshold it can actually degree performance.

> 2. work_mem - It is the amount of memory used by an
> operation. My guess is once the operation is complete
> this is freed and hence has nothing to do with the
> caching.

This is the amount of memory used for things like sorts and
order bys on a per backend process basis.

> 3. effective_cache_size - The parameter used by the
> query planner and has nothing to do with the actual
> caching.

The instructs the query planner on how large the operating
system's disk cache is. There isn't a built in cache, PostgreSQL
relies on the operating system to cache the on disk information
based on how often it is used. In most cases this is probably
more accurate anyway.

I wrote an article on PostgreSQL performance tuning that has
links to several other related sites, you can find it here:

http://www.revsys.com/writings/postgresql-performance.html

---------------------------------
Frank Wiles <frank(at)wiles(dot)org>
http://www.wiles.org
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