linux-kernel - Re: [PATCH v3 3/3] perf, tools, script: Allow computing metrics in perf script

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Date:   Tue, 21 Nov 2017 22:28:25 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Andi Kleen <andi@...stfloor.org>, acme@...nel.org,
        jolsa@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] perf, tools, script: Allow computing metrics in
 perf script

On Tue, Nov 21, 2017 at 09:07:14AM -0800, Andi Kleen wrote:
> On Tue, Nov 21, 2017 at 10:28:06AM +0100, Jiri Olsa wrote:
> > On Mon, Nov 20, 2017 at 08:03:06AM -0800, Andi Kleen wrote:
> > > > > Yes it is.
> > > > > 
> > > > > It's for the complete sampling period because it is computed
> > > > > over the delta from the last sample to the previous sample.
> > > > > 
> > > > > There isn't really a metric at a point, it is always over a interval.
> > > > 
> > > > agreed, it's the count we meassured from the last sample.. but the
> > > > 'averaged' word above implies to me we compute some average over the
> > > > 'sampling' period, which we dont do
> > > 
> > > Do you have a better word in mind?
> > > 
> > > AFAIK average is the right word for this because it's the summary
> > > for that time period.
> > 
> > the way I understand it is that we take the values from the current
> > sample and count the metric value.. so the phrase:
> > 
> > .. the metric computed is averaged over the whole sampling period,
> > not just for the sample point ...
> > 
> > does not make sense to me.. because we take the value of that
> > single 'sample point'.. I dont see any average sum in there
> 
> The current samples contains the sum of event counts for a sampling period.
> 
> EventA-1           EventA-2                EventA-3      EventA-4
> EventB-1     EventB-2                             EventC-3       
> 
>                          gap with no events                overflow
> |-----------------------------------------------------------------|
> period-start                                             period-end
> ^                                                                 ^
> |                                                                 |
> previous sample                                      current sample
> 
> 
> So EventA = 4 and EventB = 3 at the sample point
> 
> I generate a metric, let's say EventA / EventB. It applies
> to the whole period.
> 
> But the metric is over a longer time which does not have the same
> behavior. For example the gap above doesn't have any events, while
> they are clustered at the beginning and end of the sample period.
> 
> But we're summing everything together. The metric doesn't know
> that the gap is different than the busy period. 
> 
> That's what I'm trying to express with averaging.

I see, so averaging to express the sum of the uneven distribution
of the counts.. ook

thanks for bearing with me ;-)
jirka