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[46.139.12.213]) by smtp.gmail.com with ESMTPSA id v13sm41988211wrg.46.2019.04.25.10.33.35 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 25 Apr 2019 10:33:36 -0700 (PDT) Date: Thu, 25 Apr 2019 19:33:33 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Thara Gopinath , mingo@redhat.com, rui.zhang@intel.com, linux-kernel@vger.kernel.org, amit.kachhap@gmail.com, viresh.kumar@linaro.org, javi.merino@kernel.org, edubezval@gmail.com, daniel.lezcano@linaro.org, vincent.guittot@linaro.org, nicolas.dechesne@linaro.org, bjorn.andersson@linaro.org, dietmar.eggemann@arm.com, Quentin Perret , "Rafael J. Wysocki" Subject: Re: [PATCH V2 0/3] Introduce Thermal Pressure Message-ID: <20190425173333.GA4081@gmail.com> References: <1555443521-579-1-git-send-email-thara.gopinath@linaro.org> <20190417053626.GA47282@gmail.com> <5CB75FD9.3070207@linaro.org> <20190417182932.GB5140@gmail.com> <20190424163424.GG4038@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190424163424.GG4038@hirez.programming.kicks-ass.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 * Peter Zijlstra wrote: > On Wed, Apr 17, 2019 at 08:29:32PM +0200, Ingo Molnar wrote: > > Assuming PeterZ & Rafael & Quentin doesn't hate the whole thermal load > > tracking approach. > > I seem to remember competing proposals, and have forgotten everything > about them; the cover letter also didn't have references to them or > mention them in any way. > > As to the averaging and period, I personally prefer a PELT signal with > the windows lined up, if that really is too short a window, then a PELT > like signal with a natural multiple of the PELT period would make sense, > such that the windows still line up nicely. > > Mixing different averaging methods and non-aligned windows just makes me > uncomfortable. Yeah, so the problem with PELT is that while it nicely approximates variable-period decay calculations with plain additions, shifts and table lookups (i.e. accelerates pow()), AFAICS the most important decay parameter is fixed: the speed of decay, the dampening factor, which is fixed at 32: Documentation/scheduler/sched-pelt.c #define HALFLIFE 32 Right? Thara's numbers suggest that there's high sensitivity to the speed of decay. By using PELT we'd be using whatever averaging speed there is within PELT. Now we could make that parametric of course, but that would both complicate the PELT lookup code (one more dimension) and would negatively affect code generation in a number of places. Thanks, Ingo