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SubjectRe: [RFC PATCH v1 8/8] sched/deadline: make bandwidth enforcement scale-invariant
On Tue, 25 Jul 2017 15:51:05 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Jul 25, 2017 at 09:03:08AM +0200, Luca Abeni wrote:
>
> > > I'm still confused..
> > >
> > > So GRUB does:
> > >
> > > dq = Uact -dt
> > >
> > > right?
> >
> > Right. This is what the original (single processor) GRUB did. And
> > this was used by the "GRUB-PA" algorithm:
> > https://www.researchgate.net/profile/Giuseppe_Lipari/publication/220800940_Using_resource_reservation_techniques_for_power-aware_scheduling/links/09e41513639b2703fc000000.pdf
> >
> > (basically, GRUB-PA uses GRUB for reclaiming, and scales the CPU
> > frequency based on Uact)
> >
> >
> > > Now, you do DVFS using that same Uact. If we lower the clock, we
> > > need more time, so would we then not end up with something like:
> > >
> > > dq = 1/Uact -dt
> >
> > Well, in the GRUB-PA algorithm GRUB reclaiming is the mechanism
> > used to give more runtime to the task... Since Uact is < 1, doing
> > dq = - Uact * dt
> > means that we decrease the current runtime by a smaller amount of
> > time. And so we end up giving more runtime to the task: instead of
> > giving dl_runtime every dl_period, we give "dl_runtime / Uact" every
> > dl_period... And since the CPU is slower (by a ratio Uact), this is
> > equivalent to giving dl_runtime at the maximum CPU speed / frequency
> > (at least, in theory :).
> >
> >
> > > After all; our budget assignment is such that we're able to
> > > complete our work at max freq. Therefore, when we lower the
> > > frequency, we'll have to increase budget pro rata, otherwise
> > > we'll not complete our work and badness happens.
> >
> > Right. But instead of increasing dl_runtime, GRUB-PA decreases the
> > amount of time accounted to the current runtime.
> >
> >
> > > Say we have a 1 Ghz part and Uact=0.5 we'd select 500 Mhz and need
> > > double the time to complete.
> > >
> > > Now, if we fold these two together, you'd get:
> > >
> > > dq = Uact/Uact -dt = -dt
> >
> > Not sure why " / Uact"... According to the GRUB-PA algorithm, you
> > just do
> > dq = - Uact * dt = -0.5dt
> > and you end up giving the CPU to the task for 2 * dl_runtime every
> > dl_period (as expected)
>
> Yeah, I seem to have gone off the rails there... Bah I'm terminally
> confused now. Let me try and get my brain the right way up.

This stuff always confuses me too... :)

The parts that gives me more headaches is how to combine GRUB-PA with
non-reclaiming tasks, and how to cope with "real world issues" (such as
an actual DVFS frequency different from the theoretical one, GRUB
reclaiming less than 100% of the CPU time, etc...)

Anyway, Claudio is running some experiments with this patchset,
measuring power saving and missed deadlines for various sets of
periodic real-time tasks... We hope to present the results at RTLWS.


Luca

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