idle -- Device

Device that returns system's idle time
/dev/idle

/dev/idle is the device from which you can read the system's idle time.  It
has major  device 0,  the same  as /dev/null and  /dev/cmos; and  has minor
number 11.  This non-portable  device node is used exclusively for tracking
system load.   Its driver recognizes the system  calls open(), ioctl(), and
close(), but not read() or write().

The  only available  ioctl() for  /dev/idle writes  a pair  of longs  to an
address that you supply.  The long at the lower address contains the number
of system idle clock ticks (or,  more precisely, the number of ticks at the
end of which the system was  idle) that have occurred since system startup.
The long  at the higher  address contains the  total number of  clock ticks
that have occurred since system  startup.  To estimate system load during a
specific interval  of time, perform the ioctl() for  /dev/idle at the start
and end of an interval.

Example

The following  program prints system load over  a five-second interval.  To
see a  nonzero load  percentage, run  it concurrently with  a CPU-intensive
process.

#include <sys/null.h>

main()
{
    long x[2];  /* tick values at start of interval */
    long y[2];  /* tick values at end of interval */

    long delta_idle, delta_lbolt;
    int fd;

    /* We need to open a device before we can ioctl it. */
    fd = open("/dev/idle", 0);

    /* Get tick values at start of interval. */
    ioctl(fd, NLIDLE, x);

    /* Sleep during the interval. */
    sleep(5);

    /* Get tick values at end of interval. */
    ioctl(fd, NLIDLE, y);

    /* Compute number of system idle ticks during the interval. */
    delta_idle = y[0] - x[0];

    /* Compute total number of clock ticks during the interval. */
    delta_lbolt = y[1] - x[1];

    /* System is loaded when it isn't idle, so system load factor
     * is 100% minus the percentage of system idle time.
     */
    printf("system load = %ld%%\n",
        100L - (100L * delta_idle)/delta_lbolt);
    close(fd);
}

See Also

device drivers,
ioctl(),
null

© 2012 by Stephen A. Ness