Monitoring execution times

QNX Neutrino includes some special CPU-time clocks that you can use to monitor the execution times for processes and threads.

This mechanism is implemented following the POSIX “Execution Time Monitoring” option, which defines special clock IDs that represent the execution time of a thread or a process:

To obtain: Call: Specifying: Classification
A process CPU-time clock ID clock_getcpuclockid() A process ID, or 0 to get the clock ID for the calling process POSIX
A thread CPU-time clock ID pthread_getcpuclockid() A thread ID in the calling process POSIX
Either of the above ClockId() A process ID and a thread ID QNX Neutrino

A process has permission to get the CPU-time clock ID of any process.

To get the execution time for the process or thread, call clock_gettime() or ClockTime(), passing a process or thread CPU-time clock ID. POSIX also defines the following, which you can use instead of calling clock_getcpuclockid() or pthread_getcpuclockid():

CLOCK_PROCESS_CPUTIME_ID
The CPU-time clock ID for the calling process.
CLOCK_THREAD_CPUTIME_ID
The CPU-time clock ID for the calling thread.

QNX Neutrino calculates the execution times for all threads and processes on every clock tick. The full tick interval is added to the running time of the active threads, so the times are approximate.

Here's an example:

int process_clock_id, ret;
struct timespec process_time;

ret = clock_getcpuclockid(0, &process_clock_id);
if (ret != 0) {
    perror ("clock_getcpuid()");
    return (EXIT_FAILURE);
}

printf ("Process clock ID: %d\n", process_clock_id);
  
ret = clock_gettime (process_clock_id, &process_time);
if (ret != 0) {
    perror ("clock_gettime()");
    return (EXIT_FAILURE);
}
printf ("Process clock: %ld sec, %ld nsec\n", process_time.tv_sec,
        process_time.tv_nsec);

Given that we're getting the execution time for the calling process, we could use CLOCK_PROCESS_CPUTIME_ID instead of clock_getcpuclockid():

int process_clock_id, ret;
struct timespec process_time;

ret = clock_gettime (CLOCK_PROCESS_CPUTIME_ID, &process_time);
if (ret != 0) {
    perror ("clock_gettime()");
    return (EXIT_FAILURE);
}
printf ("Process clock: %ld sec, %ld nsec\n", process_time.tv_sec,
        process_time.tv_nsec);
Note: You can't use clock_settime() or ClockTime() to set a process or thread CPU-time clock.

Timers using a thread CPU-time clock

In QNX Neutrino 7.0.n or later, you can arrange to be notified when a thread uses more than a given amount of CPU time. You can't currently do this for a process.

Setting up this type of notification is similar to setting up a timer:

  1. Get a thread CPU-time clock ID, as described above.
  2. Set up a sigevent that specifies how you want to be notified.
  3. Call timer_create(), passing to it the thread CPU-time clock ID and the sigevent.
    Note:
    • There's currently a limit of one timer per thread. If the thread already has a timer, timer_create() gives an error of EAGAIN.
    • If you try to create a timer for a process CPU-time clock, timer_create() gives an error of EINVAL.
  4. Set up the appropriate data structure (e.g., an itimerspec), specifying the execution time as the timeout.
  5. Call timer_settime(), passing to it the timer ID and the timeout.

Here's an example:

struct sigevent event;
timer_t timer_id;
struct itimerspec cpu_usage;

/* Set up the method of notification. */
SIGEV_SIGNAL_INIT(&event, SIGALRM);
signal( SIGALRM, sig_handler );

/* Create a timer. */
if (timer_create(CLOCK_THREAD_CPUTIME_ID, &event, &timer_id)) {
   perror( "timer_create" );
   return EXIT_FAILURE;
}

/* Set up the amount of CPU time and then set the timer. */
cpu_usage.it_value.tv_sec = 0;
cpu_usage.it_value.tv_nsec = nsec; 
cpu_usage.it_interval.tv_sec = 0;
cpu_usage.it_interval.tv_nsec = period_nsec; 

if (timer_settime(timer_id, 0, &cpu_usage, NULL)) {
   perror( "timer_settime" );
   exit(EXIT_FAILURE);
}

/* If the thread exceeds the specified CPU time, we'll get a SIGALRM, and
   our sig_handler() function will be called. */

The accuracy of this mechanism depends on the clock period; if the thread exceeds the given execution time, the actual amount of time used is at least the amount you specified but less than the same amount plus the clock period.