On Fri, Jun 28, 2019 at 11:08:14AM +0100, Patrick Bellasi wrote: > On 26-Jun 13:40, Vincent Guittot wrote: > > Hi Patrick, > > > > On Thu, 20 Jun 2019 at 17:06, Patrick Bellasi <[email protected]> > > wrote: > > > > > > The estimated utilization for a task is currently defined based on: > > > - enqueued: the utilization value at the end of the last activation > > > - ewma: an exponential moving average which samples are the enqueued > > > values > > > > > > According to this definition, when a task suddenly change it's bandwidth > > > requirements from small to big, the EWMA will need to collect multiple > > > samples before converging up to track the new big utilization. > > > > > > Moreover, after the PELT scale invariance update [1], in the above > > > scenario we > > > can see that the utilization of the task has a significant drop from the > > > first > > > big activation to the following one. That's implied by the new > > > "time-scaling" > > > > Could you give us more details about this? I'm not sure to understand > > what changes between the 1st big activation and the following one ? > > We are after a solution for the problem Douglas Raillard discussed at > OSPM, specifically the "Task util drop after 1st idle" highlighted in > slide 6 of his presentation: > > > http://retis.sssup.it/ospm-summit/Downloads/02_05-Douglas_Raillard-How_can_we_make_schedutil_even_more_effective.pdf >
So I see the problem, and I don't hate the patch, but I'm still struggling to understand how exactly it related to the time-scaling stuff. Afaict the fundamental problem here is layering two averages. The second (EWMA in our case) will always lag/delay the input of the first (PELT). The time-scaling thing might make matters worse, because that helps PELT ramp up faster, but that is not the primary issue. Or am I missing something?
