Re: [PATCH v5 11/12] sched: replace capacity_factor by utilization

[Vincent Guittot]

On 16 September 2014 00:14, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> On 15 September 2014 13:42, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> On Sun, Sep 14, 2014 at 09:41:56PM +0200, Peter Zijlstra wrote:
>>> On Thu, Sep 11, 2014 at 07:26:48PM +0200, Vincent Guittot wrote:
>>> > On 11 September 2014 18:15, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>
>>> > > I'm confused about the utilization vs capacity_orig. I see how we should
>>> >
>>> > 1st point is that I should compare utilization vs capacity and not
>>> > capacity_orig.
>>> > I should have replaced capacity_orig by capacity in the functions
>>> > above when i move the utilization statistic from
>>> > rq->avg.runnable_avg_sum to cfs.usage_load_avg.
>>> > rq->avg.runnable_avg_sum was measuring all activity on the cpu whereas
>>> > cfs.usage_load_avg integrates only cfs tasks
>>> >
>>> > With this change, we don't need sgs->group_capacity_orig anymore but
>>> > only sgs->group_capacity. So sgs->group_capacity_orig can be removed
>>> > as it's no more used in the code as sg_capacity_factor has been
>>> > removed
>>>
>>> Yes, but.. so I suppose we need to add DVFS accounting and remove
>>> cpufreq from the capacity thing. Otherwise I don't see it make sense.
>>
>> OK, I've reconsidered _again_, I still don't get it.
>>
>> So fundamentally I think its wrong to scale with the capacity; it just
>> doesn't make any sense. Consider big.little stuff, their CPUs are
>> inherently asymmetric in capacity, but that doesn't matter one whit for
>> utilization numbers. If a core is fully consumed its fully consumed, no
>> matter how much work it can or can not do.
>>
>>
>> So the only thing that needs correcting is the fact that these
>> statistics are based on clock_task and some of that time can end up in
>> other scheduling classes, at which point we'll never get 100% even
>> though we're 'saturated'. But correcting for that using capacity doesn't
>> 'work'.
>
> I'm not sure to catch your last point because the capacity is the only
> figures that take into account the "time" consumed by other classes.
> Have you got in mind another way to take into account the other
> classes ?
>
> So we have cpu_capacity that is the capacity that can be currently
> used by cfs class
> We have cfs.usage_load_avg that is the sum of running time of cfs
> tasks on the CPU and reflect the % of usage of this CPU by CFS tasks
> We have to use the same metrics to compare available capacity for CFS
> and current cfs usage
>
> Now we have to use the same unit so we can either weight the
> cpu_capacity_orig with the cfs.usage_load_avg and compare it with
> cpu_capacity
> or with divide cpu_capacity by cpu_capacity_orig and scale it into the
> SCHED_LOAD_SCALE range. Is It what you are proposing ?

For the latter, we need to keep the  sgs->group_capacity_orig in order
to check if a group is overloaded whereas the 1st solution don't need
it anymore (once the correction i mentioned previously)

Vincent
>
> Vincent
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