[igt-dev] [PATCH i-g-t 1/2] igt/perf_pmu: Aim for a fixed number of iterations for calibrating accuracy

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-08-09 12:54:41)
> 
> On 08/08/2018 15:59, Chris Wilson wrote:
> > Our observation is that the systematic error is proportional to the
> > number of iterations we perform; the suspicion is that it directly
> > correlates with the number of sleeps. Reduce the number of iterations,
> > to try and keep the error in check.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > ---
> >   tests/perf_pmu.c | 34 +++++++++++++++++++++-------------
> >   1 file changed, 21 insertions(+), 13 deletions(-)
> > 
> > diff --git a/tests/perf_pmu.c b/tests/perf_pmu.c
> > index 9a20abb6b..5a26d5272 100644
> > --- a/tests/perf_pmu.c
> > +++ b/tests/perf_pmu.c
> > @@ -1521,14 +1521,13 @@ static void __rearm_spin_batch(igt_spin_t *spin)
> >   
> >   static void
> >   accuracy(int gem_fd, const struct intel_execution_engine2 *e,
> > -      unsigned long target_busy_pct)
> > +      unsigned long target_busy_pct,
> > +      unsigned long target_iters)
> >   {
> > -     unsigned long busy_us = 10000 - 100 * (1 + abs(50 - target_busy_pct));
> > -     unsigned long idle_us = 100 * (busy_us - target_busy_pct *
> > -                             busy_us / 100) / target_busy_pct;
> >       const unsigned long min_test_us = 1e6;
> > -     const unsigned long pwm_calibration_us = min_test_us;
> > -     const unsigned long test_us = min_test_us;
> > +     unsigned long pwm_calibration_us;
> > +     unsigned long test_us;
> > +     unsigned long cycle_us, busy_us, idle_us;
> >       double busy_r, expected;
> >       uint64_t val[2];
> >       uint64_t ts[2];
> > @@ -1538,18 +1537,27 @@ accuracy(int gem_fd, const struct intel_execution_engine2 *e,
> >       /* Sampling platforms cannot reach the high accuracy criteria. */
> >       igt_require(gem_has_execlists(gem_fd));
> >   
> > -     while (idle_us < 2500) {
> > +     /* Aim for approximately 100 iterations for calibration */
> > +     cycle_us = min_test_us / target_iters;
> > +     busy_us = cycle_us * target_busy_pct / 100;
> > +     idle_us = cycle_us - busy_us;
> 
> 2% load, 1s / 10 iters
>         cycles_us = 100ms
>         busy_us = 2ms
>         idle_us = 98ms
> ...
> 
> > +
> > +     while (idle_us < 2500 || busy_us < 2500) {
> >               busy_us *= 2;
> >               idle_us *= 2;
> 
> ...
> 
> busy_us = 4ms
> idle_us = 196ms

Currently it is 250ms per 98:2 cycle and about 20ms per 50:50 cycle. So
we are only doing 4 and 50 iterations respectively.

10 cycles is strictly an improvement :-p
-Chris