linux-kernel - Re: Block IO Controller V4

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Date:	Thu, 3 Dec 2009 18:51:53 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Gui Jianfeng <guijianfeng@...fujitsu.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, jmoyer@...hat.com, righi.andrea@...il.com,
	m-ikeda@...jp.nec.com, czoccolo@...il.com, Alan.Brunelle@...com
Subject: Re: Block IO Controller V4

On Thu, Dec 03, 2009 at 01:10:03PM -0500, Vivek Goyal wrote:

[..]
> Hi Gui,
> 
> Can you please try following patch and see if it helps you. If not, then
> we need to figure out why we choose to not idle and delete the group from
> service tree.
> 

Hi Gui,

Please try this version of the patch instead of previous one. During more
testing I saw some additional deletions where we should have waited and
the reason being that we were hitting boundary condition. At the request
completion time slice has not expired but after 4-5 ns, select_queue hits
and jiffy has incremented by then and slice expires.

ttime_mean, is not covering this condition because this workload is so
sequential that ttime_mean=0.

So I am checking for new condition where if we are into last ms of slice,
mark the queue wait_busy.

Thanks
Vivek

Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
---
 block/cfq-iosched.c |   30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

Index: linux11/block/cfq-iosched.c
===================================================================
--- linux11.orig/block/cfq-iosched.c	2009-12-03 15:10:53.000000000 -0500
+++ linux11/block/cfq-iosched.c	2009-12-03 18:37:35.000000000 -0500
@@ -3248,6 +3248,29 @@ static void cfq_update_hw_tag(struct cfq
 		cfqd->hw_tag = 0;
 }
 
+static inline bool
+cfq_should_wait_busy(struct cfq_data *cfqd, struct cfq_queue *cfqq)
+{
+	struct cfq_io_context *cic = cfqd->active_cic;
+
+	/* If there are other queues in the group, don't wait */
+	if (cfqq->cfqg->nr_cfqq > 1)
+		return false;
+
+	if (cfq_slice_used(cfqq))
+		return true;
+
+	if (cfqq->slice_end - jiffies == 1)
+		return true;
+
+	/* if slice left is less than think time, wait busy */
+	if (cic && sample_valid(cic->ttime_samples)
+	    && (cfqq->slice_end - jiffies < cic->ttime_mean))
+		return true;
+
+	return false;
+}
+
 static void cfq_completed_request(struct request_queue *q, struct request *rq)
 {
 	struct cfq_queue *cfqq = RQ_CFQQ(rq);
@@ -3286,11 +3309,10 @@ static void cfq_completed_request(struct
 		}
 
 		/*
-		 * If this queue consumed its slice and this is last queue
-		 * in the group, wait for next request before we expire
-		 * the queue
+		 * Should we wait for next request to come in before we expire
+		 * the queue.
 		 */
-		if (cfq_slice_used(cfqq) && cfqq->cfqg->nr_cfqq == 1) {
+		if (cfq_should_wait_busy(cfqd, cfqq)) {
 			cfqq->slice_end = jiffies + cfqd->cfq_slice_idle;
 			cfq_mark_cfqq_wait_busy(cfqq);
 		}
--
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