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benchmarks/gem_syslatency: Subtract the clock_gettime() overhead

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Since clock_gettime() should be a fixed overhead that adds to the
latency result, subtract it from the result.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
This commit is contained in:
Chris Wilson 2016-03-10 10:14:25 +00:00
parent 2a41c4b183
commit 3e0d9ef02c
1 changed files with 18 additions and 4 deletions
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22
benchmarks/gem_syslatency.c
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@ -226,6 +226,18 @@ static double l_estimate(igt_stats_t *stats)
return igt_stats_get_mean(stats);
}
static double min_measurement_error(void)
{
struct timespec start, end;
int n;
clock_gettime(CLOCK_MONOTONIC, &start);
for (n = 0; n < 1024; n++)
clock_gettime(CLOCK_MONOTONIC, &end);
return elapsed(&start, &end) / n;
}
int main(int argc, char **argv)
{
struct gem_busyspin *busy;
@ -233,6 +245,7 @@ int main(int argc, char **argv)
pthread_attr_t attr;
int ncpus = sysconf(_SC_NPROCESSORS_ONLN);
igt_stats_t cycles, mean, max;
double min;
int time = 10;
int field = -1;
int enable_gem_sysbusy = 1;
@ -260,6 +273,7 @@ int main(int argc, char **argv)
/* Prevent CPU sleeps so that busy and idle loads are consistent. */
force_low_latency();
min = min_measurement_error();
busy = calloc(ncpus, sizeof(*busy));
if (enable_gem_sysbusy) {
@ -302,17 +316,17 @@ int main(int argc, char **argv)
default:
printf("gem_syslatency: cycles=%.0f, latency mean=%.3fus max=%.0fus\n",
igt_stats_get_mean(&cycles),
igt_stats_get_mean(&mean) / 1000,
l_estimate(&max) / 1000);
(igt_stats_get_mean(&mean) - min)/ 1000,
(l_estimate(&max) - min) / 1000);
break;
case 0:
printf("%.0f\n", igt_stats_get_mean(&cycles));
break;
case 1:
printf("%.3f\n", igt_stats_get_mean(&mean) / 1000);
printf("%.3f\n", (igt_stats_get_mean(&mean) - min) / 1000);
break;
case 2:
printf("%.0f\n", l_estimate(&max) / 1000);
printf("%.0f\n", (l_estimate(&max) - min) / 1000);
break;
}