linux-kernel - Re: [PATCH 15/20] blkio: Take care of preemptions across groups

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Date:	Wed, 04 Nov 2009 14:30:13 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-kernel@...r.kernel.org, jens.axboe@...cle.com,
	nauman@...gle.com, dpshah@...gle.com, lizf@...fujitsu.com,
	ryov@...inux.co.jp, fernando@....ntt.co.jp, s-uchida@...jp.nec.com,
	taka@...inux.co.jp, guijianfeng@...fujitsu.com,
	balbir@...ux.vnet.ibm.com, righi.andrea@...il.com,
	m-ikeda@...jp.nec.com, akpm@...ux-foundation.org, riel@...hat.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 15/20] blkio: Take care of preemptions across groups

Vivek Goyal <vgoyal@...hat.com> writes:

> It will not as we traverse up the hierarchy and look for the ioprio class
> of the group entity.
>
> So if you got following configuration where G1 and G2 are two groups. G1
> is prio class RT and G2 is prio class BE, then any queue in G1 will
> preempt any queue in G2 as at highest level, G1 and G2 are different class
> altogether.
>
> 			root
> 			/  \
> 		       G1  G2
>
> Normal cfqq preemption checks will not catch this. So if G2 has some BE
> cfqq running, and some BE queue gets backlogged in G1, this new queue wil
> not preempt the queue in G2 and it should have.
>
> That's why preemption checks at group level.
>
> Secondly if G1 and G2 are of ioprioclass BE and all the jobs in G1 are of
> RT nature, they will not preempt the queues in G2, hence providing
> isolation.

Thanks for the explanation, Vivek.  I somehow missed that we were
checking the class of the group entity and not the cfqq's entity.

Cheers,
Jeff
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