Re: [RFC PATCH 0/3] perf: show package power consumption in perf

[Ingo Molnar <mingo@elte.hu>]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> > Right, short counters (like SH when not chained) need something to 
> > accumulate deltas into the larger u64. You can indeed use timers for 
> > that, hr or otherwise, but you don't need the swcounter hrtimer 
> > infrastructure for that.
> 
> So what is the point in simulating a PMI using an hrtimer? It won't be 
> based on periods on the interesting counter but on time periods. This 
> is not how we want the samples. If we want timer based samples, we can 
> just launch a seperate software timer based event.

If we then measure the delta of the count during that constant-time 
period, we'll get a 'weight' to consider.

So for example if we sample with a period of every 1000 cache-misses, 
regular same-counter-PMU-IRQ sampling goes like this:

   1000
   1000
   1000
   1000
   1000
   ....

While if we use a hrtimer, we get variations:

   1050
    711
   1539
   2210
    400

But using that variable period as a weight will, statistically, 
compensate for the variation.

It's similar to how the auto-freq code works - that too has variable 
periods (due to the self-adjustment) - which we compensate with weight.

Thanks,

	Ingo