kqueue/kevent: EVFILT_TIMER inacurracy

[mpfr]

Given the following code (timertest.c):

#include <err.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>
#include <sys/event.h>
#include <sys/time.h>

int
main(int argc, char *argv[])
{
        long long        timeout, elapsed;
        int              kqfd;
        struct timespec  ts0, ts;
        struct kevent    kev;

        if (argc < 2)
                errx(1, "missing number of seconds");

        timeout = atoll(argv[1]) * 1000;
        if (errno || timeout <= 0)
                err(1, "invalid number of seconds");

        if ((kqfd = kqueue()) == -1)
                err(1, "kqueue");

        EV_SET(&kev, 9999, EVFILT_TIMER, EV_ADD, 0, timeout, NULL);

        if (clock_gettime(CLOCK_REALTIME, &ts0) == -1)
                err(1, "clock_gettime");

        if (kevent(kqfd, &kev, 1, NULL, 0, NULL) == -1)
                err(1, "kevent");
        printf("timer [%lld msec] started ... ", timeout);
        fflush(stdout);

        memset(&kev, 0, sizeof(kev));

        if (kevent(kqfd, NULL, 0, &kev, 1, NULL) == -1)
                err(1, "kevent");

        if (kev.flags & EV_ERROR)
                errx(1, "event error (%llu)", kev.data);

        if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
                err(1, "clock_gettime");
        timespecsub(&ts, &ts0, &ts);
        elapsed = ts.tv_sec * 1000 + ts.tv_nsec / 1000000L;

        printf("done\ntime elapsed: %lld msec\n>>> drift: %lld msec\n",
            elapsed, elapsed - timeout);

        return (0);
}


# cc timertest.c -o timertest

On an older machine, there is a noticeable clock drift:

# ./timertest 600
timer [600000 msec] started ... done
time elapsed: 624008 msec
>>> drift: 24008 msec

On an idle VM, the drift is below noise...

# ./timertest 600
timer [600000 msec] started ... done
time elapsed: 600008 msec
>>> drift: 8 msec

... the same VM under 50% CPU load:

# ./timertest 600
timer [600000 msec] started ... done
time elapsed: 609704 msec
>>> drift: 9704 msec

Is it the way I'm using kevent/EVFILT_TIMER?

Any help is being appreciated.