From bc22de9bcdb2249150fb5b3c48fdc4f6bedd3ad7 Mon Sep 17 00:00:00 2001 From: Jiri Olsa Date: Mon, 23 Apr 2018 11:08:20 +0200 Subject: [PATCH] perf stat: Display time in precision based on std deviation Ingo suggested to display elapsed time for multirun workload (perf stat -e) with precision based on the precision of the standard deviation. In his own words: > This output is a slightly bit misleading: > Performance counter stats for 'make -j128' (10 runs): > 27.988995256 seconds time elapsed ( +- 0.39% ) > The 9 significant digits in the result, while only 1 is valid, suggests accuracy > where none exists. > It would be better if 'perf stat' would display elapsed time with a precision > adjusted to stddev, it should display at most 2 more significant digits than > the stddev inaccuracy. > I.e. in the above case 0.39% is 0.109, so we only have accuracy for 1 digit, and > so we should only display 3: > 27.988 seconds time elapsed ( +- 0.39% ) Plus a suggestion about the output, which is small enough and connected with the above change that I merged both changes together. > Small output style nit - I think it would be nice if with --repeat the stddev was > also displayed in absolute values, besides percentage: > > 27.988 +- 0.109 seconds time elapsed ( +- 0.39% ) The output is now: Performance counter stats for './perf bench sched pipe' (5 runs): SNIP 13.3667 +- 0.0256 seconds time elapsed ( +- 0.19% ) Suggested-by: Ingo Molnar Signed-off-by: Jiri Olsa Tested-by: Arnaldo Carvalho de Melo Cc: Alexander Shishkin Cc: Andi Kleen Cc: David Ahern Cc: Namhyung Kim Cc: Peter Zijlstra Link: http://lkml.kernel.org/r/20180423090823.32309-7-jolsa@kernel.org Signed-off-by: Arnaldo Carvalho de Melo --- tools/perf/builtin-stat.c | 30 ++++++++++++++++++++++++------ 1 file changed, 24 insertions(+), 6 deletions(-) diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c index f17dc601b0f3..3a94647ef6bc 100644 --- a/tools/perf/builtin-stat.c +++ b/tools/perf/builtin-stat.c @@ -1764,19 +1764,37 @@ static void print_header(int argc, const char **argv) } } +static int get_precision(double num) +{ + if (num > 1) + return 0; + + return lround(ceil(-log10(num))); +} + static void print_footer(void) { + double avg = avg_stats(&walltime_nsecs_stats) / NSEC_PER_SEC; FILE *output = stat_config.output; int n; if (!null_run) fprintf(output, "\n"); - fprintf(output, " %17.9f seconds time elapsed", - avg_stats(&walltime_nsecs_stats) / NSEC_PER_SEC); - if (run_count > 1) { - fprintf(output, " "); - print_noise_pct(stddev_stats(&walltime_nsecs_stats), - avg_stats(&walltime_nsecs_stats)); + + if (run_count == 1) { + fprintf(output, " %17.9f seconds time elapsed", avg); + } else { + double sd = stddev_stats(&walltime_nsecs_stats) / NSEC_PER_SEC; + /* + * Display at most 2 more significant + * digits than the stddev inaccuracy. + */ + int precision = get_precision(sd) + 2; + + fprintf(output, " %17.*f +- %.*f seconds time elapsed", + precision, avg, precision, sd); + + print_noise_pct(sd, avg); } fprintf(output, "\n\n");