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    SubjectRe: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity
    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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