Messages in this thread | | | Date | Wed, 28 Nov 2018 14:40:39 +0000 | From | Patrick Bellasi <> | Subject | Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT |
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On 28-Nov 14:33, Vincent Guittot wrote: > On Wed, 28 Nov 2018 at 12:53, Patrick Bellasi <patrick.bellasi@arm.com> wrote: > > > > On 28-Nov 11:02, Peter Zijlstra wrote: > > > On Wed, Nov 28, 2018 at 10:54:13AM +0100, Vincent Guittot wrote: > > > > > > > Is there anything else that I should do for these patches ? > > > > > > IIRC, Morten mention they break util_est; Patrick was going to explain. > > > > I guess the problem is that, once we cross the current capacity, > > strictly speaking util_avg does not represent anymore a utilization. > > > > With the new signal this could happen and we end up storing estimated > > utilization samples which will overestimate the task requirements. > > > > We will have a spike in estimated utilization at next wakeup, since we > > use MAX(util_avg@dequeue_time, ewma). Potentially we also inflate the EWMA in > > case we collect multiple samples above the current capacity. > > TBH I don't see how it's different from current implementation with a > task that was scheduled on big core and now wakes up on little core. > The util_est is overestimated as well.
While running below the capacity of a CPU, either big or LITTLE, we can still measure the actual used bandwidth as long as we have idle time. If the task is then moved into a lower capacity core, I think it's still safe to assume that, likely, it would need more capacity.
Why do you say it's the same ?
With your new signal instead, once we cross the current capacity, utilization is just not anymore utilization. Thus, IMHO it make sense avoid to accumulate a sample for what we call "estimated utilization".
I would also say that, with the current implementation which caps utilization to the current capacity, we get better estimation in general. At least we can say with absolute precision:
"the task needs _at least_ that amount of capacity".
Potentially we can also flag the task as being under-provisioned, in case there was not idle time, and _let a policy_ decide what to do with it and the granted information we have.
While, with your new signal, once we are over the current capacity, the "utilization" is just a sort of "random" number at best useful to drive some conclusions about how long the task has been delayed.
IOW, I fear that we are embedding a policy within a signal which is currently representing something very well defined: how much cpu bandwidth a task used. While, latency/under-provisioning policies perhaps should be better placed somewhere else.
Perhaps I've missed it in some of the previous discussions: have we have considered/discussed this signal-vs-policy aspect ?
-- #include <best/regards.h>
Patrick Bellasi
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