To: "Ryan Hansen" <ryan.hansen@brightbuilders.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Memory Allocation 
In-reply-to: <011101c95013$b6fca0a0$24f5e1e0$@hansen@brightbuilders.com> 
References: <011101c95013$b6fca0a0$24f5e1e0$@hansen@brightbuilders.com>
Comments: In-reply-to "Ryan Hansen" <ryan.hansen@brightbuilders.com>
	message dated "Wed, 26 Nov 2008 15:09:55 -0700"
Date: Wed, 26 Nov 2008 17:58:33 -0500
Message-ID: <26199.1227740313@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Ryan Hansen" <ryan.hansen@brightbuilders.com> writes:
> I have a fairly robust server running Ubuntu Hardy Heron, 24 GB of memory,
> and I've tried to commit half the memory to PG's shared buffer, but it seems
> to fail.  I'm setting the kernel shared memory accordingly using sysctl,
> which seems to work fine, but when I set the shared buffer in PG and restart
> the service, it fails if it's above about 8 GB.

Fails how?  And what PG version is that?

FWIW, while there are various schools of thought on how large to make
shared_buffers, pretty much everybody agrees that half of physical RAM
is not the sweet spot.  What you're likely to get is maximal
inefficiency with every active disk page cached twice --- once in kernel
space and once in shared_buffers.

			regards, tom lane