linux-kernel - Re: [PATCH] Don't wait if queue already has requests.

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Date:	Wed, 9 Feb 2011 14:15:56 +0100
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Justin TerAvest <teravest@...gle.com>
CC:	"vgoyal@...hat.com" <vgoyal@...hat.com>,
	"ctalbott@...gle.com" <ctalbott@...gle.com>,
	"mrubin@...gle.com" <mrubin@...gle.com>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	"guijianfeng@...fujitsu.com" <guijianfeng@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Don't wait if queue already has requests.

On 2011-02-08 20:18, Justin TerAvest wrote:
> Commit 7667aa0630407bc07dc38dcc79d29cc0a65553c1 added logic to wait for
> the last queue of the group to become busy (have at least one request),
> so that the group does not lose out for not being continuously
> backlogged. The commit did not check for the condition that the last
> queue already has some requests. As a result, if the queue already has
> requests, wait_busy is set. Later on, cfq_select_queue() checks the
> flag, and decides that since the queue has a request now and wait_busy
> is set, the queue is expired.  This results in early expiration of the
> queue.
> 
> This patch fixes the problem by adding a check to see if queue already
> has requests. If it does, wait_busy is not set. As a result, time slices
> do not expire early.
> 
> The queues with more than one request are usually buffered writers.
> Testing shows improvement in isolation between buffered writers.

Thanks for catching this in due time, I'll queue it up for 2.6.38 and
mark it for stable backport.

-- 
Jens Axboe

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