Re: Scheduler accounting inflated for io bound processes.

[Ingo Molnar <mingo@kernel.org>]

* 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