linux-kernel - Re: [PATCH 18/31] perf, core: Add a concept of a weightened sample

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Date:	Fri, 28 Sep 2012 19:09:31 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 18/31] perf, core: Add a concept of a weightened sample

On Fri, Sep 28, 2012 at 4:57 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> I came to the conclusion that yes we need something like a weight or cost
>> as a generic way of reporting that in some modes the period is not really
>> the right measure to evaluate the "cost" of an event.
>
> I'm not fully sure if you're for or against it. I think
> the patch is mostly orthogonal to what you're proposing
>
I am for it. It does not have to be specific to TSX or PEBS-LL.
I have one form of it in my PEBS-LL patch. And yes, it appears
as a sort key (for memory sampling mode only right now).

> My main target is the TSX abort cost, the memory latencies
> I just added as a bonus.
>
>>
>> I was testing my PEBS Load Latency patch this week, I came to that
>> conclusion. The way perf report sorts samples based on aggregated
>> periods per IP does not work for PEBS Load Latency (and possibly other
>> modes). The sorting needs to be based on some cost that may be distinct
>> from the period. By default, it would be the period, but for PEBS LL that
>> would be the latency of the load at a specific IP. That would more reflect
>> was is going on.
>
>
> I originally folded the weight into nr_events, but in the end it turned
> out it's fairly useful to expose both explicitely as sort keys.
>
> In some cases you want the average weight, in others the total weight
> (SUM(weight) * nr_events). I haven't tried to mess with the period
> so far.
>
> -Andi
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