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

[Peter Zijlstra]

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.

> 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.

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.