Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 15 Dec 2015 05:43:44 +0100 | Subject | Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity |
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On 14 December 2015 at 17:51, Peter Zijlstra <peterz@infradead.org> wrote: > On Mon, Dec 14, 2015 at 04:56:17PM +0100, Vincent Guittot wrote: >> I agree that if the WCET is far from reality, we will underestimate >> available capacity for CFS. Have you got some use case in mind which >> overestimates the WCET ? > > Pretty much any 'correct' WCET is pessimistic. There's heaps of smart > people working on improving WCET bounds, but they're still out there. > This is mostly because of the .00001% tail cases that 'never' happen but > would make your tokamak burn a hole just when you're outside. > >> If we can't rely on this parameters to evaluate the amount of capacity >> used by deadline scheduler on a core, this will imply that we can't >> also use it for requesting capacity to cpufreq and we should fallback >> on a monitoring mechanism which reacts to a change instead of >> anticipating it. > > No, since the WCET can and _will_ happen, its the best you can do with > cpufreq. If you were to set it lower you could not be able to execute > correctly in your 'never' tail cases.
In the context of frequency scaling, This mean that we will never reach low frequency
> > There 'might' be smart pants ways around this, where you run part of the > execution at lower speed and switch to a higher speed to 'catch' up if > you exceed some boundary, such that, on average, you run at the same > speed the WCET mandates, but I'm not sure that's worth it. Juri/Luca > might know.
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