From patchwork Mon Feb 1 22:12:52 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Paolo Valente X-Patchwork-Id: 60983 Delivered-To: patch@linaro.org Received: by 10.112.130.2 with SMTP id oa2csp369953lbb; Mon, 1 Feb 2016 14:51:59 -0800 (PST) X-Received: by 10.66.193.233 with SMTP id hr9mr42207669pac.18.1454367116872; Mon, 01 Feb 2016 14:51:56 -0800 (PST) Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id hb8si26794514pac.55.2016.02.01.14.51.56; Mon, 01 Feb 2016 14:51:56 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932880AbcBAWvz (ORCPT + 30 others); Mon, 1 Feb 2016 17:51:55 -0500 Received: from spostino.sms.unimo.it ([155.185.44.3]:56071 "EHLO spostino.sms.unimo.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932736AbcBAWvL (ORCPT ); Mon, 1 Feb 2016 17:51:11 -0500 Received: from [185.14.10.195] (port=35339 helo=localhost.localdomain) by spostino.sms.unimo.it with esmtpsa (TLS1.2:RSA_AES_128_CBC_SHA256:128) (Exim 4.80) (envelope-from ) id 1aQMk3-0004Vs-V4; Mon, 01 Feb 2016 23:14:40 +0100 From: Paolo Valente To: Jens Axboe , Tejun Heo Cc: Fabio Checconi , Arianna Avanzini , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, ulf.hansson@linaro.org, linus.walleij@linaro.org, broonie@kernel.org, Paolo Valente Subject: [PATCH RFC 16/22] block, bfq: preserve a low latency also with NCQ-capable drives Date: Mon, 1 Feb 2016 23:12:52 +0100 Message-Id: <1454364778-25179-17-git-send-email-paolo.valente@linaro.org> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1454364778-25179-1-git-send-email-paolo.valente@linaro.org> References: <1454364778-25179-1-git-send-email-paolo.valente@linaro.org> UNIMORE-X-SA-Score: -2.9 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.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 Signed-off-by: Arianna Avanzini --- block/cfq-iosched.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) -- 1.9.1 diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 281943e..40feb47 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -4439,7 +4439,8 @@ 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 &&