On Thu, Mar 4, 2010 at 1:13 PM, Ingo Molnar <mi...@elte.hu> wrote: > > * Stephane Eranian <eran...@google.com> wrote: > >> On Thu, Mar 4, 2010 at 3:32 AM, Ingo Molnar <mi...@elte.hu> wrote: >> > >> > * eran...@google.com <eran...@google.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. > 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 > PERF_SAMPLE_PERIOD. > I understand that. > -c is legacy in essence. The default is auto-freq and i doubt anyone uses -c > anymore. > The -c option may be more convenient when not counting events correlated with time. ------------------------------------------------------------------------------ Download Intel® Parallel Studio Eval Try the new software tools for yourself. Speed compiling, find bugs proactively, and fine-tune applications for parallel performance. See why Intel Parallel Studio got high marks during beta. http://p.sf.net/sfu/intel-sw-dev _______________________________________________ perfmon2-devel mailing list perfmon2-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/perfmon2-devel
