linux-kernel - [PATCH]cfq-iosched: don't take requests with long distence as close

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Date:	Mon, 28 Dec 2009 10:03:48 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>
Subject: [PATCH]cfq-iosched: don't take requests with long distence as close

On Fri, Dec 25, 2009 at 06:16:27PM +0800, Corrado Zoccolo wrote:
> Hi Shaohua,
> On Thu, Dec 24, 2009 at 1:55 AM, Shaohua Li <shaohua.li@...el.com> wrote:
> > df5fe3e8e13883f58dc97489076bbcc150789a21
> > b3b6d0408c953524f979468562e7e210d8634150
> > The coop merge is too aggressive. For example, if two tasks are reading two
> > files where the two files have some adjecent blocks, cfq will immediately
> > merge them. cfq_rq_close() also has trouble, sometimes the seek_mean is very
> > big. I did a test to make cfq_rq_close() always checks the distence according
> > to CIC_SEEK_THR, but still saw a lot of wrong merge. (BTW, why we take a long
> > distence far away request as close. Taking them close doesn't improve any thoughtput
> > to me. Maybe we should always use CIC_SEEK_THR as close criteria).
> Yes, when deciding if two queues are going to be merged, we should use
> the constant CIC_SEEK_THR.

seek_mean could be very big sometimes, using it as close criteria is meanless
as this doen't improve any performance. So if it's big, let's fallback to
default value.

Signed-off-by: Shaohua Li<shaohua.li@...el.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index e2f8046..8025605 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1682,6 +1682,10 @@ static inline int cfq_rq_close(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 	if (!sample_valid(cfqq->seek_samples))
 		sdist = CFQQ_SEEK_THR;
 
+	/* if seek_mean is big, using it as close criteria is meanless */
+	if (sdist > CFQQ_SEEK_THR)
+		sdist = CFQQ_SEEK_THR;
+
 	return cfq_dist_from_last(cfqd, rq) <= sdist;
 }
 
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