linux-kernel - Re: Regarding dm-ioband tests

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Date:	Tue, 08 Sep 2009 15:24:08 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Ryo Tsuruta <ryov@...inux.co.jp>
CC:	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

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.

This means writes can always get in-between two reads,
sometimes multiple writes at a time, really disadvantaging
a group that is doing just disk reads.

This is a problem, because reads are generally more time
sensitive than writes.

>>> We regarded reducing throughput loss rather than reducing duration
>>> as the design of dm-ioband. Of course, it is possible to make a new
>>> policy which reduces duration.
>> ... while also reducing overall system throughput
>> by design?
> 
> I think it reduces system throughput compared to the current
> implementation, because it causes more overhead to do fine grained
> control. 

Except that the io scheduler based io controller seems
to be able to enforce fairness while not reducing
throughput.

Dm-ioband would have to address these issues to be a
serious contender, IMHO.

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