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

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Date:	Thu, 4 Mar 2010 22:13:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Stephane Eranian <eranian@...gle.com>
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)


* Stephane Eranian <eranian@...gle.com> wrote:

> 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).

No, the goal of auto-freq is to keep a steady average rate of sampling.

There is no requirement to keep it 'steady' - each sample comes with a 
specific weight.

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

Why do you assume that the two are incompatible? We can randomize auto-freq 
and still have a perfectly stable average rate.

We know how long each sample takes so the result is precise, via 
PERF_SAMPLE_PERIOD.

> > 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.

-c is legacy in essence. The default is auto-freq and i doubt anyone uses -c 
anymore.

Why would they use it? Auto-freq is so much more convenient - it adapts to the 
workload and achieves a steady state of sampling, regardless of how frequent 
the hardware events are and regardless of how dynamic the workload itself is.

	Ingo
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