linux-kernel - Re: Regarding dm-ioband tests

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Date:	Wed, 09 Sep 2009 18:24:04 +0900 (JST)
From:	Ryo Tsuruta <ryov@...inux.co.jp>
To:	fchecconi@...il.com
Cc:	riel@...hat.com, vgoyal@...hat.com, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, jens.axboe@...cle.com, agk@...hat.com,
	akpm@...ux-foundation.org, nauman@...gle.com,
	guijianfeng@...fujitsu.com, jmoyer@...hat.com,
	balbir@...ux.vnet.ibm.com
Subject: Re: Regarding dm-ioband tests

Hi,

Fabio Checconi <fchecconi@...il.com> wrote:
> Hi,
> 
> > From: Rik van Riel <riel@...hat.com>
> > Date: Tue, Sep 08, 2009 03:24:08PM -0400
> >
> > Ryo Tsuruta wrote:
> > >Rik van Riel <riel@...hat.com> wrote:
> > 
> > >>Are you saying that dm-ioband is purposely unfair,
> > >>until a certain load level is reached?
> > >
> > >Not unfair, dm-ioband(weight policy) is intentionally designed to
> > >use bandwidth efficiently, weight policy tries to give spare bandwidth
> > >of inactive groups to active groups.
> > 
> > This sounds good, except that the lack of anticipation
> > means that a group with just one task doing reads will
> > be considered "inactive" in-between reads.
> > 
> 
>   anticipation helps in achieving fairness, but CFQ currently disables
> idling for nonrot+NCQ media, to avoid the resulting throughput loss on
> some SSDs.  Are we really sure that we want to introduce anticipation
> everywhere, not only to improve throughput on rotational media, but to
> achieve fairness too?

I'm also not sure if it's worth introducing anticipation everywhere.
The storage devices are becoming faster and smarter every year. In
practice, I did a benchmark with a SAN storage and the noop scheduler
got the best result.

However, I'll consider how IO from one task should take care of.

Thanks,
Ryo Tsuruta
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