[Intel-gfx] [PATCH igt] igt/perf_pmu: Bump batch_duration for legacy sampling inaccuracy

[Chris Wilson]

Quoting Tvrtko Ursulin (2017-11-23 07:14:13)
> 
> On 23/11/2017 00:08, Chris Wilson wrote:
> > Since the legacy ringbuffer uses a sampling technique, it is limited to
> > an accuracy based on a 200Hz timer, or 5ms. We assert that measurements
> > are within 5%, so with a 100ms duration that gives us no room for the
> > systemmatic error in our sampling. Bump the duration to 500ms to give us
> > plenty of safety margin, if it then fails, it should not be due to the
> > sampling.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > ---
> >   tests/perf_pmu.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tests/perf_pmu.c b/tests/perf_pmu.c
> > index 61da224e..50ca7895 100644
> > --- a/tests/perf_pmu.c
> > +++ b/tests/perf_pmu.c
> > @@ -44,7 +44,7 @@
> >   IGT_TEST_DESCRIPTION("Test the i915 pmu perf interface");
> >   
> >   const double tolerance = 0.05f;
> > -const unsigned long batch_duration_ns = 100e6;
> > +const unsigned long batch_duration_ns = 500e6;
> >   
> >   static int open_pmu(uint64_t config)
> >   {
> > 
> 
> Hm, it is definitely too short in sampling mode as you describe in the 
> commit.
> 
> I am only a bit unhappy that 5x increase makes the total test run much 
> longer. Embedding knowledge in the test on what counters are sampling 
> and what not would be too bad?
> 
> Or perhaps a compromise on those by extending the batch duration a 
> little bit less, and increasing the tolerance a bit?

My rough estimate with the current tolerance we need a minimum of 300ms
batch to hide the sampling inaccuracy (liberal use of Nyquist plus error
accumulation). 500ms then to give enough slack to be sure it's not a
systematic error from sampling.

Increasing tolerance is a bit harder to sell, I think. You do want some
notion of accuracy and 5% is a "happy" value.

> That would mean adding variables like sampling_batch_duration_ns and 
> sampling_tolerance and busyness based tests would also pick based on gen.
> 
> If you would be happy with that I'll implement it.

You want something more complicated go for it. Personally, even with .5s
batch duration total runtime wasn't an issue for me. (It's the pauses on
frequency, interrupts  and rc6 that start to get me worried!)

Total runtime with .5s is just under 40s.
-Chris