linux-kernel - Re: [PATCH] perf_events: add sampling period randomization support (v2)

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Date:	Thu, 4 Mar 2010 09:25:56 -0800
From:	Stephane Eranian <eranian@...gle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	paulus@...ba.org, fweisbec@...il.com, perfmon2-devel@...ts.sf.net,
	robert.richter@....com, eranian@...il.com, davem@...emloft.net,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH] perf_events: add sampling period randomization support 
	(v2)

On Thu, Mar 4, 2010 at 3:32 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * eranian@...gle.com <eranian@...gle.com> wrote:
>
>> This patch adds support for randomizing the sampling period.  Randomization
>> is very useful to mitigate the bias that exists with sampling. The random
>> number generator does not need to be sophisticated. This patch uses the
>> builtin random32() generator.
>
>> +     if (width > 63 || attr->freq)
>> +             return -EINVAL;
>
> Why not for freq counters? Those are semi-randomized already, but it might
> make sense to make them 'more' randomized in special circumstances. That would
> also allow us to enable the randomization in perf top and perf record, by
> default.
>
What's the goal of freq?
Achieve and maintain the target interrupt/rate.
In doing so, it has to adjust the period (not randomly).

Randomization may prevent achieving the rate, or it may slow
it down. What's the value add of that?

> Without that we'd have no immediate usecase and no way to ensure that this
> code works as intended.
>
Why?
With perf you also have fixed sampling period (-c option), you simply need to
express the fact you want it randomized.

> Thanks,
>
>        Ingo
>



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