clock_gettime(CLOCK_MONOTONIC, ..) occasionally jumping backwards

Question

Possible Duplicate:
Linux clock_gettime(CLOCK_MONOTONIC) strange non-monotonic behavior

After experiencing some problems with Erlang crashing, I've written a program that calls clock_gettime(CLOCK_MONOTONIC, &ts) repeatedly and checks to see if it ever goes backwards, and unfortunately it sometimes does go backwards.

This is the test program I'm using:

#include <time.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
    struct timespec ts_start;
    struct timespec ts_end;
    int i;

    clock_gettime(CLOCK_MONOTONIC, &ts_start);
    clock_gettime(CLOCK_MONOTONIC, &ts_end);

    for(i = 0; i<10000000; i++) {
            if(ts_end.tv_sec <= ts_start.tv_sec
               && ts_end.tv_nsec < ts_start.tv_nsec) {
                    printf("ERROR!\n");
                    return 1;
            }


            ts_start.tv_sec = ts_end.tv_sec;
            ts_start.tv_nsec = ts_end.tv_nsec;
            clock_gettime(CLOCK_MONOTONIC, &ts_end);
    }

    printf("OK\n");
    return 0;
}

On my Hyper-V VM (kernel 2.6.18-238.12.1.e15, have tried others) it occasionally outputs ERROR, but on a physical machine it always outputs OK.

Any idea why CLOCK_MONOTONIC wouldn't be monotonic?