Messages in this thread | | | Date | Wed, 28 Nov 2018 15:21:33 +0000 | From | Patrick Bellasi <> | Subject | Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT |
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On 28-Nov 15:55, Vincent Guittot wrote: > On Wed, 28 Nov 2018 at 15:40, Patrick Bellasi <patrick.bellasi@arm.com> wrote: > > > > 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 ? > > In the example of a task that runs 39ms in period of 80ms that we used > during previous version, > the utilization on the big core will reach 709 so will util_est too > When the task migrates on little core (512), util_est is higher than > current cpu capacity
Right, and what's the problem ?
1) We know that PELT is calibrated to 32ms period task and in your example, since the runtime is higher then the half-life, it's correct to estimate a utilization higher then 50%.
PELT utilization is defined _based on the half-life_: thus your task having a 50% duty cycle does not mean we are not correct if report a utilization != 50%. It would be as broken as reporting 10% utilization for a task running 100ms every 1s.
2) If it was a 70% task on a previous activation, once it's moved into a lower capacity CPU it's still correct to assume that it's likely going to require the same bandwidth and thus will be under-provisioned.
I still don't see where we are wrong in this case :/
To me it looks different then the problem I described.
> > 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 ?
What's your opinion on the above instead ?
-- #include <best/regards.h>
Patrick Bellasi
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