On Thu, Jan 24, 2019 at 02:04:32PM +0000, Patrick Bellasi wrote: > > So if I'm not mistaken we then have 3 cases: > > > > 1) runnable == util <= capacity > > > > no contention, idle > > > > 2) runnable == util > capacity > > > > no contention, no idle > > > > 3) runnable > util > > > > contention, no idle > > > > For 1) we can use: 'util' > > For 2) we can use: 'capacity' > > For 3) we can use: 'util * capacity >> 10' > > > > (note that 2 is a special case of 3 when u=1) > > > > This should work right? > > I think there is a case, similar to 2, in which the new 'util' could > potentially be used. That's the case for example of a 20% (estimated) > utilization task running alone on a 15% capacity CPU, for a single > activation. In that case such a task will complete and be dequeued > with: > > runnable == util > capacity > > The problem is that we need to be sure there was not contention... and > that seems to be difficult to detect.
When there is contention runnable and util should diverge. Given this is all discrete stuff, there's a few funnies, but who cares about those :-) > > Now, instead of doing complicated things like that, you instead figure > > that when there's no idle there's also no dequeue happening and we can > > simply short-cut by skipping the entire thing, forgetting everything > > about 2,3. > > > > Did I get that right? > > More or less... just saying that 1 is the only easy to detect scenario > in which we are granted the utilization represents an actual bandwidth > request and thus the only safe values to sample for estimated > utilization. For the other cases, since anyway: > > util_est := max(max(ewma, last_util), util_avg) > > util_est will just keep representing a safe and actually measured > lower-bound for the expected utilization of a task, without > side-affecting the EWMA which has a "slow" update dynamic. Right, so maybe we should expound the comment here a bit; but otherwise I'm inclined to merge that v9 Vincent posted.
