/* 
   RealTimeInterrupt.c

   This program is a work-arround of the "time slice problem" for Linux
   system, which may improve performance of an MPI application using
   several processes on a single CPU system.

   This program requires that Real Time Clock be enabled in the Linux
   kernel (see linux/Documentation/rtc.txt).

   Usage:

   Compile this program and start it in background (at a higher priority)
   before running MPI applications.
*/

#include <stdio.h>
#include <linux/mc146818rtc.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>

/* The frequency must be within the range 2-8192 in increments of power of 2.
   Only root can use frequencies >= 128 */

#define frequency	64L

static void sig_term(int signum);

static int fd=-1;

void main(void) {

    int retval;
    unsigned long data;

#ifdef DEBUG
    fprintf(stderr,"Real Time Clock frequency is set to %ld Hz\n", frequency);
#endif

    fd = open ("/dev/rtc", O_RDONLY);

    if (fd ==  -1) {
    	perror("/dev/rtc");
    	exit(errno);
    }

    signal(SIGHUP, SIG_IGN);
    signal(SIGTERM, sig_term);
    signal(SIGABRT, sig_term);
    signal(SIGINT, sig_term);

    retval = ioctl(fd, RTC_IRQP_SET, frequency);
    if (retval == -1) {
    	perror("ioctl");
    	exit(errno);
    }

    /* Enable periodic interrupts */
    retval = ioctl(fd, RTC_PIE_ON, 0);
    if (retval == -1) {
    	perror("ioctl");
    	exit(errno);
    }

    while ( 1 ) {
    	/* This blocks */
    	retval = read(fd, &data, sizeof(unsigned long));
    	if (retval == -1) {
    		perror("read");
    		exit(errno);
    	}
#ifdef DEBUG
    	fprintf(stderr, "+");
    	fflush(stderr);
#endif
    }

    /* Disable periodic interrupts */
    retval = ioctl(fd, RTC_PIE_OFF, 0);
    if (retval == -1) {
    	perror("ioctl");
    	exit(errno);
    }

    close(fd);

} /* end main */

static void sig_term(int signum)
{
    if ( fd != -1 ) {
  	ioctl(fd, RTC_PIE_OFF, 0);
	close(fd);
    }

#ifdef DEBUG 
    fprintf(stderr, "Signal %d received, exit!\n", signum);
#endif
    exit(0);
}