linux-kernel - Re: [PATCH] cfq: enable idle for seeky processes on rotational NCQ devices

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Date:	Sun, 4 Oct 2009 20:29:33 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [PATCH] cfq: enable idle for seeky processes on rotational NCQ 
	devices

On Sun, Oct 4, 2009 at 7:36 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
> I think this one is a bit problematic. What I'd like seeky processes to
> do is enable 'idle unless other sync read comes in' for such cases,
> otherwise it will cost us a lot of performance on the seeky vs seeky
> cases because we don't get to take advantage of queuing.

Are we sure that queuing is beneficial in this workload, on non-raid
rotational devices?
If the seeks are still quite local (e.g. when accessing a single
file), given that seek time is proportional to seek length, idling
should provide higher throughput.

Anyway, I'm working on an other patch that will group together all
seeky queues and dispatch them without idling, and idle only on the
last one, so if you prefer, this can be postponed until the other
patch is ready.

Thanks,
Corrado

> It would be
> perfectly fine to continue waiting if just async IO comes in, but if we
> have other seeky readers then they should get a turn.
>
> I realize that this does skew potential priority issues.
>
> --
> Jens Axboe
>
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