// gcc -O2 -o tenp tenp.c -lrt
// code from interbench.c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
/*
 * Start $forks processes that run for 10% cpu time each. Set this to
 * 15 * number of cpus for best effect.
 */
int forks = 15;

unsigned long run_us = 1000000000, sleep_us;
unsigned long loops_per_ms;

void terminal_error(const char *name)
{
	fprintf(stderr, "\n");
	perror(name);
	exit (1);
}

unsigned long long get_nsecs(struct timespec *myts)
{
	if (clock_gettime(CLOCK_REALTIME, myts))
		terminal_error("clock_gettime");
	return (myts->tv_sec * 1000000000 + myts->tv_nsec );
}

void burn_loops(unsigned long loops)
{
	unsigned long i;

	/*
	 * We need some magic here to prevent the compiler from optimising
	 * this loop away. Otherwise trying to emulate a fixed cpu load
	 * with this loop will not work.
	 */
	for (i = 0 ; i < loops ; i++)
	     asm volatile("" : : : "memory");
}

/* Use this many usecs of cpu time */
void burn_usecs(unsigned long usecs)
{
	unsigned long ms_loops;

	ms_loops = loops_per_ms / 1000 * usecs;
	burn_loops(ms_loops);
}

void microsleep(unsigned long long usecs)
{
	struct timespec req, rem;

	rem.tv_sec = rem.tv_nsec = 0;

	req.tv_sec = usecs / 1000000;
	req.tv_nsec = (usecs - (req.tv_sec * 1000000)) * 1000;
continue_sleep:
	if ((nanosleep(&req, &rem)) == -1) {
		if (errno == EINTR) {
			if (rem.tv_sec || rem.tv_nsec) {
				req.tv_sec = rem.tv_sec;
				req.tv_nsec = rem.tv_nsec;
				goto continue_sleep;
			}
			goto out;
		}
		terminal_error("nanosleep");
	}
out:
	return;
}

/*
 * In an unoptimised loop we try to benchmark how many meaningless loops
 * per second we can perform on this hardware to fairly accurately
 * reproduce certain percentage cpu usage
 */
void calibrate_loop(void)
{
	unsigned long long start_time, loops_per_msec, run_time = 0,
		min_run_us = run_us;
	unsigned long loops;
	struct timespec myts;
	int i;

	printf("Calibrating loop\n");
	loops_per_msec = 1000000;
redo:
	/* Calibrate to within 1% accuracy */
	while (run_time > 1010000 || run_time < 990000) {
		loops = loops_per_msec;
		start_time = get_nsecs(&myts);
		burn_loops(loops);
		run_time = get_nsecs(&myts) - start_time;
		loops_per_msec = (1000000 * loops_per_msec / run_time ? :
			loops_per_msec);
	}

	/* Rechecking after a pause increases reproducibility */
	microsleep(1);
	loops = loops_per_msec;
	start_time = get_nsecs(&myts);
	burn_loops(loops);
	run_time = get_nsecs(&myts) - start_time;

	/* Tolerate 5% difference on checking */
	if (run_time > 1050000 || run_time < 950000)
		goto redo;
	loops_per_ms=loops_per_msec;
	printf("Calibrating sleep interval\n");
	microsleep(1);
	/* Find the smallest time interval close to 1ms that we can sleep */
	for (i = 0; i < 100; i++) {
		start_time=get_nsecs(&myts);
		microsleep(1000);
		run_time=get_nsecs(&myts)-start_time;
		run_time /= 1000;
		if (run_time < run_us && run_us > 1000)
			run_us = run_time;
	}
	/* Then set run_us to that duration and sleep_us to 9 x that */
	sleep_us = run_us * 9;
	printf("Calibrating run interval\n");
	microsleep(1);
	/* Do a few runs to see what really gets us run_us runtime */
	for (i = 0; i < 100; i++) {
		start_time=get_nsecs(&myts);
		burn_usecs(run_us);
		run_time=get_nsecs(&myts)-start_time;
		run_time /= 1000;
		if (run_time < min_run_us && run_time > run_us)
			min_run_us = run_time;
	}
	if (min_run_us < run_us)
		run_us = run_us * run_us / min_run_us;
	printf("Each fork will run for %lu usecs and sleep for %lu usecs\n",
		run_us, sleep_us);
}

int main(void){
	int i;

	calibrate_loop();
	printf("starting %d forks\n", forks);
	for(i = 1; i < forks; i++){
		if(!fork())
			break;
	}
	while(1){
		burn_usecs(run_us);
		microsleep(sleep_us);
	}
	return 0;
}