From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8AF21C433DB for ; Sun, 7 Feb 2021 12:50:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5604264E31 for ; Sun, 7 Feb 2021 12:50:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229536AbhBGMuE (ORCPT ); Sun, 7 Feb 2021 07:50:04 -0500 Received: from szxga04-in.huawei.com ([45.249.212.190]:12146 "EHLO szxga04-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229510AbhBGMuE (ORCPT ); Sun, 7 Feb 2021 07:50:04 -0500 Received: from DGGEMS412-HUB.china.huawei.com (unknown [172.30.72.59]) by szxga04-in.huawei.com (SkyGuard) with ESMTP id 4DYTTW2GyCz164w6; Sun, 7 Feb 2021 20:47:59 +0800 (CST) Received: from [10.174.179.198] (10.174.179.198) by DGGEMS412-HUB.china.huawei.com (10.3.19.212) with Microsoft SMTP Server id 14.3.498.0; Sun, 7 Feb 2021 20:49:11 +0800 Subject: Re: question about relative control for sync io using bfq To: Paolo Valente CC: Jens Axboe , Ming Lei , "Christoph Hellwig" , linux-block , chenzhou , "houtao (A)" References: <7E41BC22-33EA-4D0F-9EBD-3AB0824E3F2E@linaro.org> <7c28a80f-dea9-d701-0399-a22522c4509b@huawei.com> <554AE702-9A13-4FB5-9B29-9AF11F09CE5B@linaro.org> From: "yukuai (C)" Message-ID: <97ce5ede-0f7e-ce63-7a92-01c3356f4e44@huawei.com> Date: Sun, 7 Feb 2021 20:49:10 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <554AE702-9A13-4FB5-9B29-9AF11F09CE5B@linaro.org> Content-Type: text/plain; charset="gbk"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.179.198] X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 2021/02/05 15:49, Paolo Valente wrote: > > >> Il giorno 29 gen 2021, alle ore 09:28, yukuai (C) ha scritto: >> >> Hi, >> >> Thanks for your response, and my apologize for the delay, my tmie >> is very limited recently. >> > > I do know that problem ... > >> On 2021/01/22 18:09, Paolo Valente wrote: >>> Hi, >>> this is a core problem, not of BFQ but of any possible solution that >>> has to provide bandwidth isolation with sync I/O. One of the examples >> >> I'm not sure about this, so I test it with iocost in mq and cfq in sq, >> result shows that they do can provide bandwidth isolation with sync I/O >> without significant performance degradation. > > Yep, that means just that, with your specific workload, bandwidth > isolation gets guaranteed without idling. So that's exactly one of > the workloads for which I'm suggesting my handling of a special case. > > >>> is the one I made for you in my other email. At any rate, the problem >>> that you report seems to occur with just one group. We may think of >>> simply changing my condition >>> bfqd->num_groups_with_pending_reqs > 0 >>> to >>> bfqd->num_groups_with_pending_reqs > 1 >> >> We aredy tried this, the problem will dispeare if only one group is >> active. And I think this modification is reasonable because >> bandwidth isolation is not necessary in this case. >> > > Thanks for your feedback. I'll consider submitting this change. > >> However, considering the common case, when more than one >> group is active, and one of the group is issuing sync IO, I think >> we need to find a way to prevent the preformance degradation. > > I agree. What do you think of my suggestion for solving the problem? > Might you help with that? Hi Do you mead the suggestion that you mentioned in another email: "a varied_rq_size flag, similar to the varied_weights flag" ? I'm afraid that's just a circumvention plan, not a solution to the special case. By the way, I'm glad if there is anything I can help, however it'll wait for a few days cause the Spring Festival is coming. Thanks, Yu Kuai > > Thanks, > Paolo > >>> If this simple solution does solve the problem you report, then I >>> could run my batch of tests to check whether it causes some >>> regression. >>> What do you think? >>> Thanks. >>> Paolo >> >> Thanks >> Yu Kuai >>> . > > . >