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

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Message-ID: <20171121170714.GI28112@tassilo.jf.intel.com>
Date:   Tue, 21 Nov 2017 09:07:14 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.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 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.

If you still don't like the word I will remove the sentence.
I don't have a better term to describe the above.

-Andi