How to sleep for a few microseconds

Question

Consider the following code:

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

// Compile with gcc -lrt -lm -o test_clock test_clock.c

#define CLOCK CLOCK_MONOTONIC

int main(int argc, char** argv) {
    double temp, elapsed;
    int j;
    struct timespec requestStart, requestEnd, req;

    // Pseudo-sleep
    clock_gettime(CLOCK, &requestStart);
    temp = 0;
    for(j=0; j < 40; j++)
        temp += sin(j);
    clock_gettime(CLOCK, &requestEnd);
    elapsed = ( requestEnd.tv_sec - requestStart.tv_sec ) / 1e-6
                 + ( requestEnd.tv_nsec - requestStart.tv_nsec ) / 1e3;
    printf("Elapsed: %lf us\n", elapsed);

    // Nanosleep
    clock_gettime(CLOCK, &requestStart);
    req.tv_nsec = 5000;
    req.tv_sec = 0;
    clock_nanosleep(CLOCK, 0, &req, NULL);
    clock_gettime(CLOCK, &requestEnd);
    elapsed = ( requestEnd.tv_sec - requestStart.tv_sec ) / 1e-6
                 + ( requestEnd.tv_nsec - requestStart.tv_nsec ) / 1e3;

    printf("Elapsed: %lf us\n", elapsed);

}

On my 2.6.32 system, the result is

Elapsed: 5.308000 us
Elapsed: 69.142000 us

I agree that this is most likely because nanosleep() asks the kernel to reschedule the process. How can I avoid this? I want to keep ownership of the CPU and just idle around for a precise amount of time.

Answer 1

If you want your application to be able to "sleep" as precisely as possible, first put your application in realtime conditions

• use a realtime scheduler class for your program / thread : SCHED_FIFO or SCHED_RR
• elevate your program / thread priority
• and if you're going to "sleep" for less than the minimum amount the kernel is going to handle, manually busywait

Have a look at http://www.drdobbs.com/184402031

And this other question: nanosleep high cpu usage?