Re: RFC on io-stalls patch

From: Andrea Arcangeli <andrea@suse.de>
To: Jens Axboe <axboe@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>,
	Marcelo Tosatti <marcelo@conectiva.com.br>,
	Chris Mason <mason@suse.com>, lkml <linux-kernel@vger.kernel.org>,
	"Stephen C. Tweedie" <sct@redhat.com>,
	Jeff Garzik <jgarzik@pobox.com>, Andrew Morton <akpm@digeo.com>,
	Alexander Viro <viro@math.psu.edu>
Subject: Re: RFC on io-stalls patch
Date: Wed, 16 Jul 2003 15:11:28 +0200	[thread overview]
Message-ID: <20030716131128.GG4978@dualathlon.random> (raw)
In-Reply-To: <20030716130442.GZ833@suse.de>

On Wed, Jul 16, 2003 at 03:04:42PM +0200, Jens Axboe wrote:
> On Wed, Jul 16 2003, Andrea Arcangeli wrote:
> > On Wed, Jul 16, 2003 at 02:46:56PM +0200, Jens Axboe wrote:
> > > Well it's a combined problem. Threshold too high on dirty memory,
> > > someone doing a read well get stuck flushing out as well.
> > 
> > a pure read not. the write throttling should be per-process, then there
> > will be little risk.
> 
> A read from user space, dirtying data along the way.

it doesn't necessairly block on dirty memory. We even _free_ ram clean
if needed, exactly because of that. You can raise the amount of _free_
ram up to 99% of the whole ram in your box to be almost guaranteed to
never wait on dirty memory freeing. Of course the default tries to
optimize for writeback cache and there's a reasonable margin to avoid
writing dirty stuff. the sysctl is there for special usages where you
want to never block in a read from userspace regardless whatever the
state of the system.

Andrea