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

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Date:	Sat, 21 Aug 2010 11:30:36 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Matt Fleming <matt@...sole-pimps.org>,
	Zhang Rui <rui.zhang@...el.com>,
	"Lin, Ming M" <ming.m.lin@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"robert.richter@....com" <robert.richter@....com>,
	"acme@...hat.com" <acme@...hat.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"dzickus@...hat.com" <dzickus@...hat.com>,
	"gorcunov@...il.com" <gorcunov@...il.com>,
	"Brown, Len" <lenb@...nel.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [RFC PATCH 0/3] perf: show package power consumption in perf


* Frederic Weisbecker <fweisbec@...il.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
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