问题
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.
回答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:
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.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.I thought calculating time difference in
unsigned long
(i.e. in nanoseconds) is better than calculating indouble
(in seconds) as it is more accurate and may be fast. Hence I modified thetimerdiff
macro to output difference in nanoseconds. Since I am using interval of 500us or less the difference will never overflow the range ofunsigned 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;
}
来源:https://stackoverflow.com/questions/24051863/how-to-implement-highly-accurate-timers-in-linux-userspace