linux-kernel - Re: Higher than expected disk write(2) latency

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Message-Id: <20080710061822.38975133.akpm@linux-foundation.org>
Date:	Thu, 10 Jul 2008 06:18:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Martin Sustrik <sustrik@...tmq.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Martin Lucina <mato@...elna.sk>, linux-kernel@...r.kernel.org
Subject: Re: Higher than expected disk write(2) latency

On Thu, 10 Jul 2008 15:17:47 +0200 Martin Sustrik <sustrik@...tmq.com> wrote:

> Hi Alan,
> 
> >> What we see is that AIO performs rather bad while we are still 
> >> enqueueing more writes (it misses right position on the disk and has to 
> >> do superfluous disk revolvings), however, once we stop enqueueing new 
> >> write request, those already in the queue are processed swiftly.
> > 
> > Which disk scheduler are you using - some of the disk schedulers
> > intentionally delay writes to try and get better block merging.
> 
> It's CFQ. Does it delay writes? And if so, what should we use instead?
> 

noop is the simplest scheduler.  deadline is the simplest real scheduler,
and deadline doesn't have any delaying logic.

If CFQ or anticipatory _are_ putting delays into this workload, that'd be
a bug.
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