Messages in this thread | | | Date | Wed, 26 Jun 2013 18:10:51 +0200 | From | Ingo Molnar <> | Subject | Re: Scheduler accounting inflated for io bound processes. |
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* David Ahern <dsahern@gmail.com> wrote:
> On 6/26/13 9:50 AM, Ingo Molnar wrote: > > > >* Peter Zijlstra <peterz@infradead.org> wrote: > > > >>On Wed, Jun 26, 2013 at 11:37:13AM +0200, Ingo Molnar wrote: > >>>Would be very nice to randomize the sampling rate, by randomizing the > >>>intervals within a 1% range or so - perf tooling will probably recognize > >>>the different weights. > >> > >>You're suggesting adding noise to the regular kernel tick? > > > >No, to the perf interval (which I assumed Mike was using to profile this?) > >- although slightly randomizing the kernel tick might make sense as well, > >especially if it's hrtimer driven and reprogrammed anyway. > > > >I might have gotten it all wrong though ... > > Sampled S/W events like cpu-clock have a fixed rate > (perf_swevent_init_hrtimer converts freq to sample_period). > > Sampled H/W events have an adaptive period that converges to the desired > sampling rate. The first few samples come in 10 usecs are so apart and > the time period expands to the desired rate. As I recall that adaptive > algorithm starts over every time the event is scheduled in.
Yes, but last I checked it (2 years ago? :-) the auto-freq code was converging pretty well to the time clock, with little jitter - in essence turning it into a fixed-period, fixed-frequency sampling method. That would explain Mike's results.
Thanks,
Ingo
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