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

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Date:	Tue, 30 Jun 2009 21:28:27 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Gui Jianfeng <guijianfeng@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, dm-devel@...hat.com,
	jens.axboe@...cle.com, nauman@...gle.com, dpshah@...gle.com,
	lizf@...fujitsu.com, mikew@...gle.com, fchecconi@...il.com,
	paolo.valente@...more.it, ryov@...inux.co.jp,
	fernando@....ntt.co.jp, s-uchida@...jp.nec.com, taka@...inux.co.jp,
	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

On Tue, Jun 30, 2009 at 02:40:36PM +0800, Gui Jianfeng wrote:
> Vivek Goyal wrote:
> ...
> > +
> > +/*
> > + * Do the accounting. Determine how much service (in terms of time slices)
> > + * current queue used and adjust the start, finish time of queue and vtime
> > + * of the tree accordingly.
> > + *
> > + * Determining the service used in terms of time is tricky in certain
> > + * situations. Especially when underlying device supports command queuing
> > + * and requests from multiple queues can be there at same time, then it
> > + * is not clear which queue consumed how much of disk time.
> > + *
> > + * To mitigate this problem, cfq starts the time slice of the queue only
> > + * after first request from the queue has completed. This does not work
> > + * very well if we expire the queue before we wait for first and more
> > + * request to finish from the queue. For seeky queues, we will expire the
> > + * queue after dispatching few requests without waiting and start dispatching
> > + * from next queue.
> > + *
> > + * Not sure how to determine the time consumed by queue in such scenarios.
> > + * Currently as a crude approximation, we are charging 25% of time slice
> > + * for such cases. A better mechanism is needed for accurate accounting.
> > + */
> 
>   Hi Vivek,
> 
>   The comment is out of date, would you update it accordingly?
> 

Thanks Gui. Yes, I will update it in next posting.

Vivek
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