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POLinux, timerfd accuracy
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copied!<p>I have a system that needs at least 10 mseconds of accuracy for timers.<br> I went for timerfd as it suits me perfectly, but found that even for times up to 15 milliseconds it is not accurate at all, either that or I don't understand how it works. </p> <p>The times I have measured were up to 21 mseconds on a 10 mseconds timer.<br> I have put together a quick test that shows my problem.<br> Here a test:</p> <pre><code>#include <sys/timerfd.h> #include <time.h> #include <string.h> #include <stdint.h> int main(int argc, char *argv[]){ int timerfd = timerfd_create(CLOCK_MONOTONIC,0); int milliseconds = atoi(argv[1]); struct itimerspec timspec; bzero(&timspec, sizeof(timspec)); timspec.it_interval.tv_sec = 0; timspec.it_interval.tv_nsec = milliseconds * 1000000; timspec.it_value.tv_sec = 0; timspec.it_value.tv_nsec = 1; int res = timerfd_settime(timerfd, 0, &timspec, 0); if(res < 0){ perror("timerfd_settime:"); } uint64_t expirations = 0; int iterations = 0; while( res = read(timerfd, &expirations, sizeof(expirations))){ if(res < 0){ perror("read:"); continue; } if(expirations > 1){ printf("%lld expirations, %d iterations\n", expirations, iterations); break; } iterations++; } } </code></pre> <p>And executed like this:</p> <pre><code>Zack ~$ for i in 2 4 8 10 15; do echo "intervals of $i milliseconds"; ./test $i;done intervals of 2 milliseconds 2 expirations, 1 iterations intervals of 4 milliseconds 2 expirations, 6381 iterations intervals of 8 milliseconds 2 expirations, 21764 iterations intervals of 10 milliseconds 2 expirations, 1089 iterations intervals of 15 milliseconds 2 expirations, 3085 iterations </code></pre> <p>Even assuming some possible delays, 15 milliseconds delays sounds too much for me.</p>

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