linux-kernel - Re: [PATCH 02/20] io-controller: Common flat fair queuing code in elevaotor layer

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Date:	Fri, 29 May 2009 18:57:16 +0200
From:	Fabio Checconi <fchecconi@...il.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Nauman Rafique <nauman@...gle.com>, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, dm-devel@...hat.com,
	jens.axboe@...cle.com, dpshah@...gle.com, lizf@...fujitsu.com,
	mikew@...gle.com, paolo.valente@...more.it, ryov@...inux.co.jp,
	fernando@....ntt.co.jp, s-uchida@...jp.nec.com, taka@...inux.co.jp,
	guijianfeng@...fujitsu.com, jmoyer@...hat.com,
	dhaval@...ux.vnet.ibm.com, balbir@...ux.vnet.ibm.com,
	righi.andrea@...il.com, m-ikeda@...jp.nec.com, jbaron@...hat.com,
	agk@...hat.com, snitzer@...hat.com, akpm@...ux-foundation.org,
	peterz@...radead.org
Subject: Re: [PATCH 02/20] io-controller: Common flat fair queuing code in elevaotor layer

> From: Vivek Goyal <vgoyal@...hat.com>
> Date: Fri, May 29, 2009 12:06:10PM -0400
>
> On Thu, May 28, 2009 at 12:41:27PM -0700, Nauman Rafique wrote:
...
> > I have some concerns about the new preemption logic.
> 
> Actually we need a more proper definition of in-class preemption. Across
> class preemption means that RT class always gets to run first.
> 
> What does in-class preemption mean? If I look at the current CFQ code,
> it does look like that preempting process will gain share. It is always
> added to the front of the tree with "rb_key=0" and that means, this new
> queue will get fresh time slice (even if it got time slice very recently).
> 
> Currently I have just tried to make the behavior same as CFQ to reduce
> the possiblility of regressions. That's a different thing that we can
> discuss what should be the exact behavior in case of in-class preemption
> and first it needs to be fixed in CFQ, if current behavior is an issue.
> 
> On the other hand, I am not sure if previous bfq preemption logic was
> working. We were checking if the new request belonged to the queue which
> will be served next, then preempt the existing queue. While looking
> for the next queue, I think we did not consider the current active
> entity (as it was not on the tree). So after expiry of the current
> queue, it might get selected next if it has not got its share. So there
> was no point in preempting the queue. If queue already got its share, then
> anyway the next queue will be selected next and there is no point in
> preempting the current queue.
> 

BFQ had no preemption logic, as far as I know; it simply was not
preemptive, and the guarantees it provided took that into account.

I don't know what is the best way to introduce a CFQ-like preemption logic
into the wf2q+ code; for sure anything that does not schedule according
to the algorithm's timestamps is a good candidate to break the guarantees
the scheduler can provide, making it an extremely complex way to get
the same worst-case delays of a (much simpler) round-robin scheduler.
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