Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

[Vincent Guittot]

On Mon, 20 Feb 2023 at 11:13, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Feb 09, 2023 at 05:16:46PM +0100, Vincent Guittot wrote:
>
> > > The results is very similar to PELT halflife reduction. The advantage is
> > > that 'util_est_faster' is only activated selectively when the runtime of
> > > the current task in its current activation is long enough to create this
> > > CPU util boost.
> >
> > IIUC how util_est_faster works, it removes the waiting time when
> > sharing cpu time with other tasks. So as long as there is no (runnable
> > but not running time), the result is the same as current util_est.
>
> Uh.. it's double the speed, no? Even if there is no contention, the
> fake/in-situ pelt sum runs at double time and thus will ramp up faster
> than normal.

Ah yes. I haven't noticed it was (delta * 2) and not delta

>
> > util_est_faster makes a difference only when the task alternates
> > between runnable and running slices.
>
> UTIL_EST was supposed to help mitigate some of that, but yes. Also note
> that _FASTER sorta sucks here because it starts from 0 every time, if it
> were to start from the state saved by util_est_dequeue(), it would ramp
> up faster still.

Yes.

>
> Patch has a comment along those lines I think.
>
> > Have you considered using runnable_avg metrics in the increase of cpu
> > freq ? This takes into the runnable slice and not only the running
> > time and increase faster than util_avg when tasks compete for the same
> > CPU
>
> Interesting! Indeed, that's boosting the DVFS for contention. And as
> deggeman's reply shows, it seems to work well.
>
> I wonder if that one place where it regresses is exactly the case
> without contention.

Yes that might be the case indeed. I would expect uclamp_min to help
for ensuring a min frequency such scenario