<http://www.opensource.apple.com/source/xnu/xnu-1699.26.8/> Timers can skew for a variety of reasons, including system load, and power management. Considering that 20ms isn't that much more than a scheduling quantum, actual delivery of notifications can vary widely, especially on UP systems. Shantonu On Jul 18, 2012, at 7:21 AM, Paul Albrecht <albrecht@glccom.com> wrote:
Hi,
Doesn't seem like the kqueue timer works correctly on os x. When I use the timer on my mac it consistently runs slow. Here's some test code:
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <errno.h> #include <sys/types.h> #include <sys/event.h> #include <sys/time.h>
int main(void) { int i,msec; int kq,nev; struct kevent inqueue; struct kevent outqueue; struct timeval start,end;
if ((kq = kqueue()) == -1) { fprintf(stderr, "kqueue error!? errno = %s", strerror(errno)); exit(EXIT_FAILURE); } EV_SET(&inqueue, 1, EVFILT_TIMER, EV_ADD | EV_ENABLE, 0, 20, 0);
gettimeofday(&start, 0); for (i = 0; i < 50; i++) { if ((nev = kevent(kq, &inqueue, 1, &outqueue, 1, NULL)) == -1) { fprintf(stderr, "kevent error!? errno = %s", strerror(errno)); exit(EXIT_FAILURE); } else if (outqueue.flags & EV_ERROR) { fprintf(stderr, "EV_ERROR: %s\n", strerror(outqueue.data)); exit(EXIT_FAILURE); } } gettimeofday(&end, 0);
msec = ((end.tv_sec - start.tv_sec) * 1000) + (((1000000 + end.tv_usec - start.tv_usec) / 1000) - 1000);
printf("msec = %d\n", msec);
close(kq); return EXIT_SUCCESS; }
I originally noticed the problem on freebsd. After I posted to the freebsd-hackers mailing list I got a fix, but that doesn't do much good on my mac since I don't have the source.
-- Paul Albrecht
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