Re: [PATCH] perf_events: add sampling period randomization support (v2)
From: Stephane Eranian
Date: Thu Mar 04 2010 - 21:34:20 EST
On Thu, Mar 4, 2010 at 1:13 PM, Ingo Molnar <mingo@xxxxxxx> wrote:
> * Stephane Eranian <eranian@xxxxxxxxxx> wrote:
>> On Thu, Mar 4, 2010 at 3:32 AM, Ingo Molnar <mingo@xxxxxxx> wrote:
>> > * eranian@xxxxxxxxxx <eranian@xxxxxxxxxx> 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.
rate of samples = rate of interrupts (if period < 32 bits on Intel).
> 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.
What would that buy you compared to what you already have?
> We know how long each sample takes so the result is precise, via
I understand that.
> -c is legacy in essence. The default is auto-freq and i doubt anyone uses -c
The -c option may be more convenient when not counting events correlated with
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