Re: [PATCH 2/2] sched/fair: util_est: add running_sum tracking

Joel Fernandes
2018-06-05T19:43:44Z

On Tue, Jun 05, 2018 at 12:33:17PM -0700, Joel Fernandes wrote:
> On Tue, Jun 05, 2018 at 04:21:56PM +0100, Patrick Bellasi wrote:
[..]
> > To be more precise, at each ___update_load_avg we should really update
> > running_avg by:
> >
> > u32 divider = LOAD_AVG_MAX - 1024 + sa->period_contrib;
> > sa->running_avg = sa->running_sum / divider;
> >
> > but, this would imply tracking an additional signal in sched_avg and
> > doing an additional division at ___update_load_avg() time.
> >
> > Morten suggested that, if we accept the rounding errors due to
> > considering
> >
> > divider ~= LOAD_AVG_MAX
> >
> > thus discarding the (sa->period_contrib - 1024) correction, then we
> > can completely skip the tracking of running_avg (thus saving space in
> > sched_avg) and approximate it at dequeue time as per the code line,
> > just to compute the new util_est sample to accumulate.
> >
> > Does that make sense now?
>
> The patch always made sense to me.. I was just pointing out the extra
> division this patch adds. I agree since its done on dequeue-only, then its
> probably Ok to do..
>

One thing to note about this error is I remember not compensating for it
would make the utilization reduce... which is kind of weird. But yeah if we
can find a way compensate for the error and also keep the overhead low,
that's super.. I know this is probably low in priority considering the other
concerns Vincent brought up which are being discussed in the other thread,..
but just saying. :)

- Joel