testing/dispatch_drift.c

   1 #include <dispatch/dispatch.h>
   2 #include <mach/mach_time.h>
   3 #include <sys/time.h>
   4 #include <unistd.h>
   5 #include <stdio.h>
   6 #include <stdlib.h>
   7 #include "dispatch_test.h"
   8
   9 int
  10 main(int argc __attribute__((unused)), char* argv[] __attribute__((unused)))
  11 {
  12         __block uint32_t count = 0;
  13         __block uint64_t first_time_m = 0ULL;
  14         __block double first_time_d;
  15         __block double last_jitter = 0;
  16         // 10 times a second
  17         uint64_t interval = 1000000000 / 10;
  18         double interval_d = interval / 1000000000.0;
  19         // for 25 seconds
  20         unsigned int target = 25 / interval_d;
  21
  22         test_start("Timer drift test");
  23
  24         dispatch_source_t t = dispatch_source_timer_create(DISPATCH_TIMER_INTERVAL, interval, 0, NULL, dispatch_get_main_queue(),
  25                 ^(dispatch_event_t event __attribute__((unused))) {
  26                     struct timeval now_tv;
  27                     static double first = 0;
  28                     gettimeofday(&now_tv, NULL);
  29                     double now = now_tv.tv_sec + now_tv.tv_usec / 1000000.0;
  30
  31                     if (count == 0) {
  32                             // Because this is taken at 1st timer fire,
  33                             // later jitter values may be negitave.
  34                             // This doesn't effect the drift calculation.
  35                             first = now;
  36                     }
  37                     double goal = first + interval_d * count;
  38                     double jitter = goal - now;
  39                     double drift = jitter - last_jitter;
  40
  41                     printf("%4d: jitter %f, drift %f\n", count, jitter, drift);
  42 #if 0
  43                     test_double_less_than("drift", now_d - expected_fire_time_d, .001);
  44 #endif
  45
  46                     if (target <= ++count) {
  47                             if (drift < 0) {
  48                                     drift = -drift;
  49                             }
  50                             test_double_less_than("drift", drift, 0.0001);
  51                             test_stop();
  52                     }
  53                     last_jitter = jitter;
  54                 });
  55
  56         test_ptr_notnull("timer source", t);
  57
  58         dispatch_main();
  59         return 0;
  60 }
  61