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([2600:380:9e2c:9b66:68e0:4c59:dc75:6d4b]) by smtp.gmail.com with ESMTPSA id r6sm597826ywd.47.2019.06.13.01.37.30 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 13 Jun 2019 01:37:36 -0700 (PDT) Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup controller To: "Srivatsa S. Bhat" , Jan Kara Cc: Paolo Valente , linux-fsdevel@vger.kernel.org, linux-block , linux-ext4@vger.kernel.org, cgroups@vger.kernel.org, kernel list , Jeff Moyer , Theodore Ts'o , amakhalov@vmware.com, anishs@vmware.com, srivatsab@vmware.com, Ulf Hansson , Linus Walleij , Greg Kroah-Hartman , Stable References: <6FE0A98F-1E3D-4EF6-8B38-2C85741924A4@linaro.org> <2A58C239-EF3F-422B-8D87-E7A3B500C57C@linaro.org> <5b71028c-72f0-73dd-0cd5-f28ff298a0a3@csail.mit.edu> <0d6e3c02-1952-2177-02d7-10ebeb133940@csail.mit.edu> <7B74A790-BD98-412B-ADAB-3B513FB1944E@linaro.org> <6a6f4aa4-fc95-f132-55b2-224ff52bd2d8@csail.mit.edu> <7c5e9d11-4a3d-7df4-c1e6-7c95919522ab@csail.mit.edu> <20190612130446.GD14578@quack2.suse.cz> From: Jens Axboe Message-ID: Date: Thu, 13 Jun 2019 02:37:29 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/12/19 1:36 PM, Srivatsa S. Bhat wrote: > > [ Adding Greg to CC ] > > On 6/12/19 6:04 AM, Jan Kara wrote: >> On Tue 11-06-19 15:34:48, Srivatsa S. Bhat wrote: >>> On 6/2/19 12:04 AM, Srivatsa S. Bhat wrote: >>>> On 5/30/19 3:45 AM, Paolo Valente wrote: >>>>> >>> [...] >>>>> At any rate, since you pointed out that you are interested in >>>>> out-of-the-box performance, let me complete the context: in case >>>>> low_latency is left set, one gets, in return for this 12% loss, >>>>> a) at least 1000% higher responsiveness, e.g., 1000% lower start-up >>>>> times of applications under load [1]; >>>>> b) 500-1000% higher throughput in multi-client server workloads, as I >>>>> already pointed out [2]. >>>>> >>>> >>>> I'm very happy that you could solve the problem without having to >>>> compromise on any of the performance characteristics/features of BFQ! >>>> >>>> >>>>> I'm going to prepare complete patches. In addition, if ok for you, >>>>> I'll report these results on the bug you created. Then I guess we can >>>>> close it. >>>>> >>>> >>>> Sounds great! >>>> >>> >>> Hi Paolo, >>> >>> Hope you are doing great! >>> >>> I was wondering if you got a chance to post these patches to LKML for >>> review and inclusion... (No hurry, of course!) >>> >>> Also, since your fixes address the performance issues in BFQ, do you >>> have any thoughts on whether they can be adapted to CFQ as well, to >>> benefit the older stable kernels that still support CFQ? >> >> Since CFQ doesn't exist in current upstream kernel anymore, I seriously >> doubt you'll be able to get any performance improvements for it in the >> stable kernels... >> > > I suspected as much, but that seems unfortunate though. The latest LTS > kernel is based on 4.19, which still supports CFQ. It would have been > great to have a process to address significant issues on older > kernels too. > > Greg, do you have any thoughts on this? The context is that both CFQ > and BFQ I/O schedulers have issues that cause I/O throughput to suffer > upto 10x - 30x on certain workloads and system configurations, as > reported in [1]. > > In this thread, Paolo posted patches to fix BFQ performance on > mainline. However CFQ suffers from the same performance collapse, but > CFQ was removed from the kernel in v5.0. So obviously the usual stable > backporting path won't work here for several reasons: > > 1. There won't be a mainline commit to backport from, as CFQ no > longer exists in mainline. > > 2. This is not a security/stability fix, and is likely to involve > invasive changes. > > I was wondering if there was a way to address the performance issues > in CFQ in the older stable kernels (including the latest LTS 4.19), > despite the above constraints, since the performance drop is much too > significant. I guess not, but thought I'd ask :-) > > [1]. https://lore.kernel.org/lkml/8d72fcf7-bbb4-2965-1a06-e9fc177a8938@csail.mit.edu/ This issue has always been there. There will be no specific patches made for stable for something that doesn't even exist in the newer kernels. -- Jens Axboe