From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Return-Path: From: Paolo Valente To: Jens Axboe , Tejun Heo 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 , Arianna Avanzini Subject: [PATCH 08/14] block, bfq: preserve a low latency also with NCQ-capable drives Date: Wed, 26 Oct 2016 11:28:01 +0200 Message-Id: <1477474082-2846-9-git-send-email-paolo.valente@linaro.org> In-Reply-To: <1477474082-2846-1-git-send-email-paolo.valente@linaro.org> References: <1477474082-2846-1-git-send-email-paolo.valente@linaro.org> List-ID: 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 Signed-off-by: Arianna Avanzini --- 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