Subject: Re: effective cache size on FreeBSD (WAS: Performance on SUSE w/
From: Sven Willenberger <sven@dmv.com>
To: Claus Guttesen <kometen@gmail.com>
Cc: Jon Brisbin <jon.brisbin@npcinternational.com>,
	pgsql-performance@postgresql.org
In-Reply-To: <b41c75520510110754g534d9fa8u@mail.gmail.com>
References: <b41c75520510110754g534d9fa8u@mail.gmail.com>
Content-Type: text/plain
Date: Tue, 11 Oct 2005 11:04:24 -0400
Message-Id: <1129043064.1975.22.camel@lanshark.dmv.com>
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On Tue, 2005-10-11 at 16:54 +0200, Claus Guttesen wrote:
> > > I have a postgresql 7.4.8-server with 4 GB ram.
> > > #effective_cache_size = 1000    # typically 8KB each
> > >
> > > This is computed by sysctl -n vfs.hibufspace / 8192 (on FreeBSD). So I
> > > changed it to:
> > >
> > > effective_cache_size = 27462    # typically 8KB each
> >
> > Apparently this formula is no longer relevant on the FreeBSD systems as
> > it can cache up to almost all the available RAM. With 4GB of RAM, one
> > could specify most of the RAM as being available for caching, assuming
> > that nothing but PostgreSQL runs on the server -- certainly 1/2 the RAM
> > would be a reasonable value to tell the planner.
> >
> > (This was verified by using dd:
> > dd if=/dev/zero of=/usr/local/pgsql/iotest bs=128k count=16384 to create
> > a 2G file then
> > dd if=/usr/local/pgsql/iotest of=/dev/null
> >
> > If you run systat -vmstat 2 you will see 0% diskaccess during the read
> > of the 2G file indicating that it has, in fact, been cached)
> 
> Thank you for your reply. Does this apply to FreeBSD 5.4 or 6.0 on
> amd64 (or both)?
> 

Not sure about 6.0 (but I don't know why it would change) but definitely
on 5.4 amd64 (and I would imagine i386 as well).

Sven