VB7/Ventura : Issue with clock_gettime() in Ubuntu 22.04 VM

[Denis_Lyon]

I observe a strange behaviour that I’ve never experienced before. This appeared when upgrading from Monterey/VB6 to Ventura/VB7.

When I boot Ubuntu 22.04, the Linux scheduling is fine. But after some CPU load on this VM, I see an issue in the scheduling : For example the usleep() function is not accurate.

The code bellow highlight the issue. It sleeps for 10 seconds and display the difference between 2 calls to clock_gettime Using monotonic time.

Here is an extract of the result of this program :

9.984428045
9.936712734
9.976024270
19.841512940
10.741785027
17.570817783
9.965956486
9.933059723
9.978769179
9.990323837
10.203674177
14.49491216
10.497382894

And the code :

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

#define NSEC_PER_SEC    1000000000
#define SUB_TIMESPEC(t1, t2)\
  do {\
        if (((t1).tv_nsec -= (t2).tv_nsec) < 0) {\
                (t1).tv_nsec += NSEC_PER_SEC;\
                (t1).tv_sec  -= 1;\
        }\
        (t1).tv_sec -= (t2).tv_sec;\
  } while (0)

int main() {
    struct timespec diff, new, old;
    while (1) {
        clock_gettime(CLOCK_MONOTONIC_RAW,&old);
        usleep(10*1000000);
        clock_gettime(CLOCK_MONOTONIC_RAW,&new);
        diff = new;
        SUB_TIMESPEC(diff, old);
        printf("%ld.%ld\n", diff.tv_sec, diff.tv_nsec);
    }
}