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[84.3.50.134]) by smtp.gmail.com with ESMTPSA id 19sm26137589wrc.47.2019.11.18.05.15.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 18 Nov 2019 05:15:48 -0800 (PST) Date: Mon, 18 Nov 2019 14:15:46 +0100 From: Ingo Molnar To: Mel Gorman Cc: Vincent Guittot , linux-kernel@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, pauld@redhat.com, valentin.schneider@arm.com, srikar@linux.vnet.ibm.com, quentin.perret@arm.com, dietmar.eggemann@arm.com, Morten.Rasmussen@arm.com, hdanton@sina.com, parth@linux.ibm.com, riel@surriel.com Subject: Re: [PATCH v4 00/10] sched/fair: rework the CFS load balance Message-ID: <20191118131546.GA66833@gmail.com> References: <1571405198-27570-1-git-send-email-vincent.guittot@linaro.org> <20191021075038.GA27361@gmail.com> <20191030162440.GO3016@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191030162440.GO3016@techsingularity.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mel Gorman wrote: > On Mon, Oct 21, 2019 at 09:50:38AM +0200, Ingo Molnar wrote: > > > > > > > Thanks, that's an excellent series! > > > > Agreed despite the level of whining and complaining I made during the > review. I saw no whining and complaining whatsoever, and thanks for the feedback! :-) > > > I've queued it up in sched/core with a handful of readability edits to > > comments and changelogs. > > > > There are some upstreaming caveats though, I expect this series to be a > > performance regression magnet: > > > > - load_balance() and wake-up changes invariably are such: some workloads > > only work/scale well by accident, and if we touch the logic it might > > flip over into a less advantageous scheduling pattern. > > > > - In particular the changes from balancing and waking on runnable load > > to full load that includes blocking *will* shift IO-intensive > > workloads that you tests don't fully capture I believe. You also made > > idle balancing more aggressive in essence - which might reduce cache > > locality for some workloads. > > > > A full run on Mel Gorman's magic scalability test-suite would be super > > useful ... > > > > I queued this back on the 21st and it took this long for me to get back > to it. > > What I tested did not include the fix for the last patch so I cannot say > the data is that useful. I also failed to include something that exercised > the IO paths in a way that idles rapidly as that can catch interesting > details (usually cpufreq related but sometimes load-balancing related). > There was no real thinking behind this decision, I just used an old > collection of tests to get a general feel for the series. I have just applied Vincent's fix to find_idlest_group(), so that will probably modify some of the results. (Hopefully for the better.) Will push it out later today-ish. > Most of the results were performance-neutral and some notable gains > (kernel compiles were 1-6% faster depending on the -j count). Hackbench > saw a disproportionate gain in terms of performance but I tend to be > wary of hackbench as improving it is rarely a universal win. There > tends to be some jitter around the point where a NUMA nodes worth of > CPUs gets overloaded. tbench (mmtests configuation network-tbench) on a > NUMA machine showed gains for low thread counts and high thread counts > but a loss near the boundary where a single node would get overloaded. > > Some NAS-related workloads saw a drop in performance on NUMA machines > but the size class might be too small to be certain, I'd have to rerun > with the D class to be sure. The biggest strange drop in performance > was the elapsed time to run the git test suite (mmtests configuration > workload-shellscripts modified to use a fresh XFS partition) took > 17.61% longer to execute on a UMA Skylake machine. This *might* be due > to the missing fix because it is mostly a single-task workload. Thanks a lot for your testing! > I'm not going to go through the results in detail because I think > another full round of testing would be required to take the fix into > account. I'd also prefer to wait to see if the review results in any > material change to the series. I'll try to make sure it all gets addressed. Thanks, Ingo