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From: Paolo Valente <paolo.valente@linaro.org>
To: Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	ulf.hansson@linaro.org, linus.walleij@linaro.org,
	broonie@kernel.org, hare@suse.de, arnd@arndb.de,
	bart.vanassche@sandisk.com, grant.likely@secretlab.ca,
	jack@suse.cz, James.Bottomley@HansenPartnership.com,
	Paolo Valente <paolo.valente@linaro.org>,
	Arianna Avanzini <avanzini.arianna@gmail.com>
Subject: [PATCH 08/14] block, bfq: preserve a low latency also with NCQ-capable drives
Date: Wed, 26 Oct 2016 11:28:01 +0200	[thread overview]
Message-ID: <1477474082-2846-9-git-send-email-paolo.valente@linaro.org> (raw)
In-Reply-To: <1477474082-2846-1-git-send-email-paolo.valente@linaro.org>

I/O schedulers typically allow NCQ-capable drives to prefetch I/O
requests, as NCQ boosts the throughput exactly by prefetching and
internally reordering requests.

Unfortunately, as discussed in detail and shown experimentally in [1],
this may cause fairness and latency guarantees to be violated. The
main problem is that the internal scheduler of an NCQ-capable drive
may postpone the service of some unlucky (prefetched) requests as long
as it deems serving other requests more appropriate to boost the
throughput.

This patch addresses this issue by not disabling device idling for
weight-raised queues, even if the device supports NCQ. This allows BFQ
to start serving a new queue, and therefore allows the drive to
prefetch new requests, only after the idling timeout expires. At that
time, all the outstanding requests of the expired queue have been most
certainly served.

[1] P. Valente and M. Andreolini, "Improving Application
    Responsiveness with the BFQ Disk I/O Scheduler", Proceedings of
    the 5th Annual International Systems and Storage Conference
    (SYSTOR '12), June 2012.
    Slightly extended version:
    http://algogroup.unimore.it/people/paolo/disk_sched/bfq-v1-suite-
							results.pdf

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
---
 block/bfq-iosched.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index d9b0900..3b11772 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5746,7 +5746,8 @@ static void bfq_update_idle_window(struct bfq_data *bfqd,
 
 	if (atomic_read(&bic->icq.ioc->active_ref) == 0 ||
 	    bfqd->bfq_slice_idle == 0 ||
-		(bfqd->hw_tag && BFQQ_SEEKY(bfqq)))
+		(bfqd->hw_tag && BFQQ_SEEKY(bfqq) &&
+			bfqq->wr_coeff == 1))
 		enable_idle = 0;
 	else if (bfq_sample_valid(bic->ttime.ttime_samples)) {
 		if (bic->ttime.ttime_mean > bfqd->bfq_slice_idle &&
-- 
2.10.0

  parent reply	other threads:[~2016-10-26  9:29 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-10-26  9:27 [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler Paolo Valente
2016-10-26  9:27 ` [PATCH 01/14] block, bfq: " Paolo Valente
2016-10-26  9:27 ` [PATCH 02/14] block, bfq: add full hierarchical scheduling and cgroups support Paolo Valente
2016-10-26  9:27 ` [PATCH 03/14] block, bfq: improve throughput boosting Paolo Valente
2016-10-26  9:27 ` [PATCH 04/14] block, bfq: modify the peak-rate estimator Paolo Valente
2016-10-26  9:27 ` [PATCH 05/14] block, bfq: add more fairness with writes and slow processes Paolo Valente
2016-10-26  9:27 ` [PATCH 06/14] block, bfq: improve responsiveness Paolo Valente
2016-10-26  9:28 ` [PATCH 07/14] block, bfq: reduce I/O latency for soft real-time applications Paolo Valente
2016-10-26  9:28 ` Paolo Valente [this message]
2016-10-26  9:28 ` [PATCH 09/14] block, bfq: reduce latency during request-pool saturation Paolo Valente
2016-10-26 10:19 ` [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler Christoph Hellwig
2016-10-26 11:34   ` Jan Kara
2016-10-26 15:05     ` Bart Van Assche
2016-10-26 15:13       ` Arnd Bergmann
2016-10-26 15:29         ` Christoph Hellwig
2016-10-26 15:32           ` Jens Axboe
2016-10-26 16:04             ` Paolo Valente
2016-10-26 16:12               ` Jens Axboe
2016-10-27  9:26                 ` Jan Kara
2016-10-27 14:34                   ` Grozdan
2016-10-27 15:55                     ` Heinz Diehl
2016-10-27 16:28                     ` Jens Axboe
2016-10-27 16:26                   ` Jens Axboe
2016-10-28  7:59                     ` Jan Kara
2016-10-28 14:10                       ` Jens Axboe
2016-10-27 17:32                 ` Ulf Hansson
2016-10-27 17:43                   ` Jens Axboe
2016-10-27 18:13                     ` Ulf Hansson
2016-10-27 18:21                       ` Jens Axboe
2016-10-27 19:34                         ` Ulf Hansson
2016-10-27 21:08                           ` Jens Axboe
2016-10-27 22:27                             ` Linus Walleij
2016-10-28  9:32                               ` Linus Walleij
2016-10-28 14:22                                 ` Jens Axboe
2016-10-28 20:38                                   ` Linus Walleij
2016-10-28 15:29                                 ` Christoph Hellwig
2016-10-28 21:09                                   ` Linus Walleij
2016-10-28 15:30                                 ` Jens Axboe
2016-10-28 15:58                                   ` Bartlomiej Zolnierkiewicz
2016-10-28 16:05                                   ` Arnd Bergmann
2016-10-28 17:17                                     ` Mark Brown
2016-10-28 14:07                               ` Jens Axboe
2016-10-28  6:36                             ` Ulf Hansson
2016-10-28 14:17                               ` Jens Axboe
2016-10-28 17:12                                 ` Mark Brown
2016-10-27 19:41                         ` Mark Brown
2016-10-27 19:45                           ` Christoph Hellwig
2016-10-27 22:01                             ` Mark Brown
2016-10-28 12:07                       ` Arnd Bergmann
2016-10-28 12:17                         ` Richard Weinberger
2016-10-29  5:38                 ` Paolo Valente
2016-10-29 13:12                   ` Bart Van Assche
2016-10-29 14:12                   ` Jens Axboe
2016-10-30  3:06                     ` Paolo Valente
2016-10-26 12:37   ` Paolo Valente

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