Date: Fri, 15 Feb 2008 14:34:33 -0500
From: "Jignesh K. Shah" <J.K.Shah@Sun.COM>
Subject: Re: shared_buffers in 8.3 w/ lots of RAM on dedicated PG
 machine
In-reply-to: <Pine.GSO.4.64.0802150916380.28161@westnet.com>
To: Greg Smith <gsmith@gregsmith.com>
Cc: Peter Schuller <peter.schuller@infidyne.com>,
	pgsql-performance@postgresql.org
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Greg Smith wrote:
> On Fri, 15 Feb 2008, Peter Schuller wrote:
>
>> Or is it a matter of PostgreSQL doing non-direct I/O, such that
>> anything cached in shared_buffers will also be cached by the OS?
>
> PostgreSQL only uses direct I/O for writing to the WAL; everything 
> else goes through the regular OS buffer cache unless you force it to 
> do otherwise at the OS level (like some Solaris setups do with 
> forcedirectio).  This is one reason it still make not make sense to 
> give an extremely high percentage of RAM to PostgreSQL even with 
> improvements in managing it.  Another is that shared_buffers memory 
> has to be reconciled with disk at every checkpoint, where OS buffers 
> do not.  A third is that your OS may just be more efficient at 
> buffering--it knows more about the underlying hardware, and the 
> clock-sweep method used internally by PostgreSQL to simulate a LRU 
> cache is not extremely sophisticated.
>
> However, don't feel limited by the general 25% rule; it's certainly 
> worth exploring whether 50% or more works better for your workload.  
> You'll have to benchmark that yourself though, and I'd suggest using 
> pg_buffercache: 
> http://www.postgresql.org/docs/8.3/static/pgbuffercache.html to get an 
> idea just what the pages are being used for.
>

As per the test that I have done mostly with forcedirectio on Solaris, I 
have seen gains with increasing the buffercache to about somewhere 
between 10GB and thats when thing seem to take a turn...

So in my case I am generally comfortable for Postgres to use about 
8-10GB beyond which I am cautious.

Also with tests with UFS buffered for table/index and forcedirectio it 
seems to perform better with forcedirectio .. However if you do want to 
exploit the extra RAM with UFS then you have to do some tunings for UFS 
in Solaris.. Now with machines with 32GB becoming common this is 
something worth pursuing depending on the storage if it can handle the 
directio load or not.


Regards,
Jignesh