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

[Luca Abeni]

On 12/15/2015 01:43 PM, 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 ?
> > > > > 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.
> > > >
> > > > I think a more "theoretically sound" approach would be to track the
> > > > _active_ utilisation (informally speaking, the sum of the utilisations
> > > > of the tasks that are actually active on a core - the exact definition
> > > > of "active" is the trick here).
> > >
> > >
> > > The point is that we probably need 2 definitions of "active" tasks.
> >
> > Ok; thanks for clarifying. I do not know much about the remaining capacity
> > used by CFS; however, from what you write I guess CFS really need an
> > "average"
> > utilisation (while frequency scaling needs the active utilisation).
>
> yes. this patch is only about the "average" utilization
Ok; so, I think that the approach of this patch is too pessimistic (it uses
the "worst-case" utilisation as an estimation of the average one).

> > > This one
> > > should be updated quite often with the wake up and the sleep of tasks
> > > as well as the throttling.
> >
> > Strictly speaking, the active utilisation must be updated when a task
> > wakes up and when a task sleeps/terminates (but when a task
> > sleeps/terminates
> > you cannot decrease the active utilisation immediately: you have to wait
> > some time because the task might already have used part of its "future
> > utilisation").
> > The active utilisation must not be updated when a task is throttled: a
> > task is throttled when its current runtime is 0, so it already used all
> > of its utilisation for the current period (think about two tasks with
> > runtime=50ms and period 100ms: they consume 100% of the time on a CPU,
> > and when the first task consumed all of its runtime, you cannot decrease
> > the active utilisation).
>
>   I haven't read the paper you pointed in the previous email but it's
> on my todo list. Does the GRUB-PA take into account the frequency
> transition when selecting the best frequency ?
I do not know...
As far as I understand, the GRUB-PA approach is simple: if the active
utilisation of SCHED_DEADLINE tasks is Ua, then the CPU frequency can be
reduced to the maximum possible frequency multiplied by Ua (of course,
this must be adjusted a little bit, because the original GRUB-PA paper
only considered real-time/SCHED_DEADLINE tasks... To leave some CPU time
for other tasks, you have to increase Ua a little bit).

Some time ago one of the authors of the GRUB-PA paper told me that they
evaluated the performance of the algorithm by simulating the behaviour of
a real CPU, but I do not know the details, and I do not know if they took
the frequency transition into account.



				Luca
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