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

[Nicolas Pitre <nicolas.pitre@linaro.org>]

On Mon, 15 Sep 2014, Peter Zijlstra 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@infradead.org> 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.

Let's suppose a task running on a 1GHz CPU producing a load of 100.

The same task on a 100MHz CPU would produce a load of 1000 because that 
CPU is 10x slower.  So to properly evaluate the load of a task when 
moving it around, we want to normalize its load based on the CPU 
performance.  In this case the correction factor would be 0.1.

Given those normalized loads, we need to scale CPU capacity as well.  If 
the 1GHz CPU can handle 50 of those tasks it has a capacity of 5000.

In theory the 100MHz CPU could handle only 5 of those tasks, meaning it 
has a normalized capacity of 500, but only if the load metric is already 
normalized as well.

Or am I completely missing the point here?


Nicolas





> 
> 
> 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'.
> 
>