From: Vincent Fu <vincent.fu@xxxxxxx> Since we now have ntime_since() we can use nsec in the division for get_cycles_per_msec(). This makes the cycles_per_msec value that fio ultimately uses more stable since fio will no longer be using values truncated to usec. While we're here also modify some debug prints to make it explicit that fio ultimately uses a trimmed mean in its time calculations. On platforms where only gettimeofday() is available this will be no worse than the original code since we will revert back to using times truncated to usec. With the patch, the last three digits of the trimmed mean have a tight range from 784-786. Without the patch, the corresponding value (labeled avg) has a much wider range from 821-958. Notice also that the standard error S is an order of magnitude smaller with the patch. *** WITH PATCH *** $ ./fio --debug=time --cpuclock-test | grep mean time 315 min=3466785, max=3466796, mean=3466789.120000, S=0.057234, N=50 time 315 trimmed mean=3466789, N=29 $ ./fio --debug=time --cpuclock-test | grep mean time 329 min=3466784, max=3466795, mean=3466789.480000, S=0.053641, N=50 time 329 trimmed mean=3466788, N=36 $ ./fio --debug=time --cpuclock-test | grep mean time 343 min=3466786, max=3466794, mean=3466789.220000, S=0.036654, N=50 time 343 trimmed mean=3466788, N=42 $ ./fio --debug=time --cpuclock-test | grep mean time 357 min=3466785, max=3466794, mean=3466789.080000, S=0.053580, N=50 time 357 trimmed mean=3466788, N=33 $ ./fio --debug=time --cpuclock-test | grep mean time 371 min=3466785, max=3466794, mean=3466789.600000, S=0.043519, N=50 time 371 trimmed mean=3466789, N=36 $ ./fio --debug=time --cpuclock-test | grep mean time 385 min=3466785, max=3466794, mean=3466789.280000, S=0.052534, N=50 time 385 trimmed mean=3466789, N=32 $ ./fio --debug=time --cpuclock-test | grep mean time 407 min=3466786, max=3466796, mean=3466789.520000, S=0.042616, N=50 time 407 trimmed mean=3466789, N=41 $ ./fio --debug=time --cpuclock-test | grep mean time 513 min=3466785, max=3466796, mean=3466789.220000, S=0.051316, N=50 time 513 trimmed mean=3466789, N=35 *** WIHTOUT PATCH *** $ ./fio-3.11 --debug=time --cpuclock-test | grep 'mean\|avg' time 439 avg: 3466821 time 439 min=3466790, max=3467006, mean=3466838.060000, S=0.959748 $ ./fio-3.11 --debug=time --cpuclock-test | grep 'mean\|avg' time 455 avg: 3466958 time 455 min=3466796, max=3466996, mean=3466945.880000, S=0.789178 $ ./fio-3.11 --debug=time --cpuclock-test | grep 'mean\|avg' time 469 avg: 3466887 time 469 min=3466790, max=3466984, mean=3466887.500000, S=0.825069 $ ./fio-3.11 --debug=time --cpuclock-test | grep 'mean\|avg' time 485 avg: 3466951 time 485 min=3466818, max=3466996, mean=3466939.420000, S=0.873334 $ ./fio-3.11 --debug=time --cpuclock-test | grep 'mean\|avg' time 499 avg: 3466957 time 499 min=3466793, max=3467009, mean=3466950.140000, S=0.764626 $ ./fio-3.11 --debug=time --cpuclock-test | grep 'mean\|avg' time 608 avg: 3466954 time 608 min=3466818, max=3466981, mean=3466937.040000, S=0.862232 --- gettime.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/gettime.c b/gettime.c index 7702193d..272a3e62 100644 --- a/gettime.c +++ b/gettime.c @@ -239,13 +239,13 @@ static unsigned long get_cycles_per_msec(void) __fio_gettime(&e); c_e = get_cpu_clock(); - elapsed = utime_since(&s, &e); - if (elapsed >= 1280) + elapsed = ntime_since(&s, &e); + if (elapsed >= 1280000) break; } while (1); fio_clock_source = old_cs; - return (c_e - c_s) * 1000 / elapsed; + return (c_e - c_s) * 1000000 / elapsed; } #define NR_TIME_ITERS 50 @@ -298,10 +298,10 @@ static int calibrate_cpu_clock(void) avg /= samples; cycles_per_msec = avg; - dprint(FD_TIME, "avg: %llu\n", (unsigned long long) avg); - dprint(FD_TIME, "min=%llu, max=%llu, mean=%f, S=%f\n", + dprint(FD_TIME, "min=%llu, max=%llu, mean=%f, S=%f, N=%d\n", (unsigned long long) minc, - (unsigned long long) maxc, mean, S); + (unsigned long long) maxc, mean, S, NR_TIME_ITERS); + dprint(FD_TIME, "trimmed mean=%llu, N=%d\n", (unsigned long long) avg, samples); max_ticks = MAX_CLOCK_SEC * cycles_per_msec * 1000ULL; max_mult = ULLONG_MAX / max_ticks; -- 2.17.1
