Re: kqueue timer interval fence post error?
Re: kqueue timer interval fence post error?
- Subject: Re: kqueue timer interval fence post error?
- From: Shantonu Sen <email@hidden>
- Date: Wed, 18 Jul 2012 10:40:44 -0700
<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 <email@hidden> 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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