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Subject: Re: [PATCH 0/4] linux-2.6-block: deactivating pagecache for
	benchmarks
From: Badari Pulavarty <pbadari@gmail.com>
To: Jeff Garzik <jgarzik@pobox.com>
Cc: Andrew Morton <akpm@osdl.org>, Dirk Henning Gerdes <mail@dirk-gerdes.de>,
       axboe@suse.de, lkml <linux-kernel@vger.kernel.org>
In-Reply-To: <20051202013408.GA4225@havoc.gtf.org>
References: <1133443051.6110.32.camel@noti>
	 <20051201172520.7095e524.akpm@osdl.org>
	 <20051202013408.GA4225@havoc.gtf.org>
Content-Type: text/plain
Date: Fri, 02 Dec 2005 11:19:21 -0800
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On Thu, 2005-12-01 at 20:34 -0500, Jeff Garzik wrote:
> On Thu, Dec 01, 2005 at 05:25:20PM -0800, Andrew Morton wrote:
> > Dirk Henning Gerdes <mail@dirk-gerdes.de> wrote:
> > >
> > >  For doing benchmarks on the I/O-Schedulers, I thought it would be very
> > >  useful to disable the pagecache.
> > 
> > That's an FAQ.   Something like this?
> > 
> > 
> > From: Andrew Morton <akpm@osdl.org>
> > 
> > Add /proc/sys/vm/drop-pagecache.  When written to, this will cause the kernel
> > to discard as much pagecache and reclaimable slab objects as it can.
> > 
> > It won't drop dirty data, so the user should run `sync' first.
> > 
> > Caveats:
> > 
> > a) Holds inode_lock for exorbitant amounts of time.
> > 
> > b) Needs to be taught about NUMA nodes: propagate these all the way through
> >    so the discarding can be controlled on a per-node basis.
> > 
> > c) The pagecache shrinking and slab shrinking should probably have separate
> >    controls.
> > 
> > 
> > Signed-off-by: Andrew Morton <akpm@osdl.org>
> 
> ACK, I've wanted something like this for a while.
> 
> I really think it should be a config option, though, to discourage
> people from building with it :)

Why ? Since its controlled through /proc, if some one "echo" stuff into
it, they might get crappy performance (like other /proc tunables). 
Isn't it expected ?

Thanks,
Badari

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