How to implement highly accurate timers in Linux Userspace?

Question

I have Angstrom Linux on my BeagleBoard.

I want to implement very accurate timer which can fire at every 500us. I read about hrtimers, but all the implementations I found were for kernel space. I want to implement it in user space.

Is there any API which which can call into these hrtimers, which I can use in userspace or any other way of implementing accurate timer in linux?

I have set the jiffy to a few nanoseconds.

Answer 1

Finally, after some more efforts I found a code which suggested to use timer_create(), clock_gettime() in conjunction with signal handling (handling SIGALRM), similar to what Basile Starynkevitch suggested in his comment.

I tried it on my 1 GHz Beaglebone with clock type CLOCK_MONOTONIC for the interval of 500us.

Out of 10000 times the timer expired, 2% of the time, it was exactly 500us (I ignored the nanosecond difference). and 96.6% of the time it was in the range of 500 +/- 10us. And the rest of the time, average error was not more than +/- 50us.

This is the link for the code

I have posted slightly modified version of code here. I did following modifications to the code:

1. For small intervals ~10us the count was getting infinitely decremented, therefore I added the control on number of tests (count) inside the signal handler itself.

2. Adding a printf in the middle of running timer costs a lot of time. Hence I stored the time difference in an array and then at the end i.e. after last test, I printed everything.

3. I thought calculating time difference in unsigned long (i.e. in nanoseconds) is better than calculating in double (in seconds) as it is more accurate and may be fast. Hence I modified the timerdiff macro to output difference in nanoseconds. Since I am using interval of 500us or less the difference will never overflow the range of unsigned long.

As you can see even after the modifications, only 2% of the results were accurate to <1us. Therefore now I am trying some more modifications like not running unnecessary linux processes, simplifying my program more, etc.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/types.h>
#include <signal.h>
#include <unistd.h>

#define NSEC_PER_SEC 1000000000L

#define MAX_TESTS    10000

#define timerdiff(a,b) (((a)->tv_sec - (b)->tv_sec) * NSEC_PER_SEC + \
(((a)->tv_nsec - (b)->tv_nsec)))

static struct timespec prev = {.tv_sec=0,.tv_nsec=0};
static int count = MAX_TESTS;
unsigned long diff_times[MAX_TESTS];

void handler( int signo )
{
    struct timespec now;
    register int i, correct=0;

    if(count >= 0)
    {
        clock_gettime(CLOCK_MONOTONIC, &now);
        diff_times[count]=timerdiff(&now, &prev);
        prev = now;
        count --;
    }

    else
    {
        for(i=0; i<MAX_TESTS; ++i)
        {
            if(diff_times[i]/1000 < 510 && diff_times[i]/1000 > 490)
            {
                printf("%d->\t", i);
                correct++;
            }
            printf("%lu\n", diff_times[i]);
        }
        printf("-> %d\n", correct);
        exit(0);
    }
}

int main(int argc, char *argv[])
{
    int i = 0;
    timer_t t_id;

    struct itimerspec tim_spec = {.it_interval= {.tv_sec=0,.tv_nsec=500000},
                    .it_value = {.tv_sec=1,.tv_nsec=0}};

    struct sigaction act;
    sigset_t set;

    sigemptyset( &set );
    sigaddset( &set, SIGALRM );

    act.sa_flags = 0;
    act.sa_mask = set;
    act.sa_handler = &handler;

    sigaction( SIGALRM, &act, NULL );

    if (timer_create(CLOCK_MONOTONIC, NULL, &t_id))
        perror("timer_create");

    clock_gettime(CLOCK_MONOTONIC, &prev);

    if (timer_settime(t_id, 0, &tim_spec, NULL))
        perror("timer_settime");

    while(1);

    return 0;
}