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[88.147.66.34]) by smtp.gmail.com with ESMTPSA id h140sm3832714wme.22.2019.11.01.09.56.48 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 01 Nov 2019 09:56:50 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 12.4 \(3445.104.8\)) Subject: Re: [PATCH 08/10] blkcg: implement blk-iocost From: Paolo Valente In-Reply-To: <20191101161506.GA28212@blackbody.suse.cz> Date: Fri, 1 Nov 2019 17:56:48 +0100 Cc: Tejun Heo , Johannes Weiner , clm@fb.com, dennisz@fb.com, Josef Bacik , kernel-team@fb.com, newella@fb.com, lizefan@huawei.com, axboe@kernel.dk, Rik van Riel , josef@toxicpanda.com, cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: References: <20190828220600.2527417-1-tj@kernel.org> <20190828220600.2527417-9-tj@kernel.org> <20190910125513.GA6399@blackbody.suse.cz> <20190910160855.GS2263813@devbig004.ftw2.facebook.com> <20191003145106.GC6678@blackbody.suse.cz> <20191003164552.GA3247445@devbig004.ftw2.facebook.com> <20191009153629.GA5400@blackbody.suse.cz> <20191014153643.GD18794@devbig004.ftw2.facebook.com> <20191101161506.GA28212@blackbody.suse.cz> To: =?utf-8?Q?Michal_Koutn=C3=BD?= X-Mailer: Apple Mail (2.3445.104.8) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Il giorno 1 nov 2019, alle ore 17:15, Michal Koutn=C3=BD = ha scritto: >=20 > Hello >=20 Hi Michal, > (I realize it's likely late for the remark but I'd like to bring it up > anyway.) >=20 > On Mon, Oct 14, 2019 at 08:36:43AM -0700, Tejun Heo = wrote: >> We likely can talk on the subject >> for a really long time probalby because there's no clearly = technically >> better choice here, so... > I agree with you that functionally the two options are equal and also > from configuration POV they seem both sensible. >=20 > I checked where BFQ stores its per-device parameters and its under the > sysfs directory of given device's iosched directory. So from the user > perspective it'd be more consistent if all similar tunables resided > under that location. >=20 > (OTOH, I admit I'm not that familiar with block layer internals to > identify the overlap between IO scheduler and IO controller.) >=20 If useful for you to know, BFQ parameters are not meant to changed (apart from the low_latency tunable, if one wants full control on weights). Parameters are a testing aid, to use in case of an anomaly. After solving the anomaly, default values should be used again. Thanks, Paolo >> Yeah, it's kinda unfortunate that it requires this many parameters = but >> at least my opinion is that that's reflecting the inherent >> complexities of the underlying devices and how workloads interact = with >> them. > After I learnt about the existence of BFQ tunables, I'm no longer > concerned by the complexity of the parameter space. >=20 > Thanks for the explanations of QoS purpose. >=20 >> For QoS parameters, Andy is currently working on a method to = determine >> the set of parametesr which are at the edge of total work cliff - >> ie. the point where tighetning QoS params further starts reducing the >> total amount of work the device can do significantly. > The QoS description in the Documentation/ describes the interpretation > of the individual parameters, however, this purpose and how it works = was > not clear to be from that. I think the QoS policy would deserve = similar > description in the Documentation/. >=20 >> Nothing can issue IOs indefinitely without some of them completing = and >> the total amount of work a workload can do is conjoined with the >> completion latencies. [...] > I may reply to this point later. However, if that provably works, I'm > likely missing something in my understanding, so that'd be irrelevant. >=20 > Cheers, > Michal