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

[Tvrtko Ursulin]

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?

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.

Regards,

Tvrtko