From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752408AbcELNLt (ORCPT ); Thu, 12 May 2016 09:11:49 -0400 Received: from mail-wm0-f47.google.com ([74.125.82.47]:38708 "EHLO mail-wm0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751395AbcELNLr (ORCPT ); Thu, 12 May 2016 09:11:47 -0400 From: Paolo Subject: Re: [PATCH RFC 10/22] block, bfq: add full hierarchical scheduling and cgroups support To: Tejun Heo References: <1454364778-25179-1-git-send-email-paolo.valente@linaro.org> <1454364778-25179-11-git-send-email-paolo.valente@linaro.org> <20160211222824.GD3741@mtj.duckdns.org> <57174CA7.5000706@linaro.org> <20160422181321.GV7822@mtj.duckdns.org> <20160422184110.GX7822@mtj.duckdns.org> <20160422193221.GY7822@mtj.duckdns.org> <57F65679-CD8E-43BA-8C46-C165B4C20677@linaro.org> <20160425192436.GE7822@mtj.duckdns.org> <571E7E63.6010103@linaro.org> <2D83ACA0-2513-4019-9A0D-3166FDDE7485@linaro.org> Cc: Jens Axboe , 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, Jeff Moyer Message-ID: <5734810E.3060001@linaro.org> Date: Thu, 12 May 2016 15:11:42 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.2 MIME-Version: 1.0 In-Reply-To: <2D83ACA0-2513-4019-9A0D-3166FDDE7485@linaro.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Il 06/05/2016 22:20, Paolo Valente ha scritto: > > ... > > I can now confirm that, because of a little bug, a fraction ranging > from one third to half of the writeback bios for the writer is wrongly > associated with the root group. I'm sending a bugfix. > > I'm retesting BFQ after this blk fix. If I understand correctly, now > you agree that BFQ is well suited for cgroups too, at least in > principle. So I will apply all your suggestions and corrections, and > submit a fresh patchset. > Hi, this is just to report another apparently important blkio malfunction (unless what I describe below is, for some reason, normal). This time the malfunction is related to CFQ. Even after applying my fix for bio cloning, CFQ may fail to guarantee the expected resource-time sharing. It happens, for example, in the following simple scenario with one sequential writer, in a group, and one sequential reader, in another group. Both groups have the same weight (and are memory.high limited to 16MB). Yet the writer is served for only about 4% of the time, instead of 50%. Its bw is consequently very low. Being this an unwanted accident, this percentage probably varies with the characteristics of the system at hand. The causes of the problem seem to be buried in CFQ logic, and do not seem trivial. So I guess that solving this problem is not worth the effort, as BFQ seems to have hope to replace CFQ altogether. I'm then focusing on BFQ: it suffers from a similar problem, but because of a rather simple reason. Thanks, Paolo > Thanks, > Paolo > >> Ok (if there is some quick way to get this information without >> instrumenting the code, then any suggestion or pointer is welcome). >> >> Thanks, >> Paolo >> >>> Thanks. >