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    SubjectRe: [PATCH] perf_events: add sampling period randomization support (v2)

    * Stephane Eranian <> wrote:

    > On Thu, Mar 4, 2010 at 3:32 AM, Ingo Molnar <> wrote:
    > >
    > > * <> 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

    > > 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

    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.


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