========

  CPU load

  ========

  Linux exports various bits of information via ``/proc/stat`` and
  ``/proc/uptime`` that userland tools, such as top(1), use to calculate
  the average time system spent in a particular state, for example::

  
      $ iostat
      Linux 2.6.18.3-exp (linmac)     02/20/2007
  
      avg-cpu:  %user   %nice %system %iowait  %steal   %idle
                10.01    0.00    2.92    5.44    0.00   81.63
  
      ...
  
  Here the system thinks that over the default sampling period the
  system spent 10.01% of the time doing work in user space, 2.92% in the
  kernel, and was overall 81.63% of the time idle.

  In most cases the ``/proc/stat``	 information reflects the reality quite

  closely, however due to the nature of how/when the kernel collects
  this data sometimes it can not be trusted at all.
  
  So how is this information collected?  Whenever timer interrupt is
  signalled the kernel looks what kind of task was running at this
  moment and increments the counter that corresponds to this tasks
  kind/state.  The problem with this is that the system could have
  switched between various states multiple times between two timer
  interrupts yet the counter is incremented only for the last state.
  
  
  Example
  -------
  
  If we imagine the system with one task that periodically burns cycles

  in the following manner::

       time line between two timer interrupts
      |--------------------------------------|
       ^                                    ^
       |_ something begins working          |
                                            |_ something goes to sleep
                                           (only to be awaken quite soon)

  
  In the above situation the system will be 0% loaded according to the

  ``/proc/stat`` (since the timer interrupt will always happen when the

  system is executing the idle handler), but in reality the load is
  closer to 99%.
  
  One can imagine many more situations where this behavior of the kernel

  will lead to quite erratic information inside ``/proc/stat``::
  
  
  	/* gcc -o hog smallhog.c */
  	#include <time.h>
  	#include <limits.h>
  	#include <signal.h>
  	#include <sys/time.h>
  	#define HIST 10
  
  	static volatile sig_atomic_t stop;
  
  	static void sighandler (int signr)
  	{
  	(void) signr;
  	stop = 1;
  	}
  	static unsigned long hog (unsigned long niters)
  	{
  	stop = 0;
  	while (!stop && --niters);
  	return niters;
  	}
  	int main (void)
  	{
  	int i;
  	struct itimerval it = { .it_interval = { .tv_sec = 0, .tv_usec = 1 },
  				.it_value = { .tv_sec = 0, .tv_usec = 1 } };
  	sigset_t set;
  	unsigned long v[HIST];
  	double tmp = 0.0;
  	unsigned long n;
  	signal (SIGALRM, &sighandler);
  	setitimer (ITIMER_REAL, &it, NULL);
  
  	hog (ULONG_MAX);
  	for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog (ULONG_MAX);
  	for (i = 0; i < HIST; ++i) tmp += v[i];
  	tmp /= HIST;
  	n = tmp - (tmp / 3.0);
  
  	sigemptyset (&set);
  	sigaddset (&set, SIGALRM);
  
  	for (;;) {
  		hog (n);
  		sigwait (&set, &i);
  	}
  	return 0;
  	}

  
  
  References
  ----------

  - http://lkml.org/lkml/2007/2/12/6
  - Documentation/filesystems/proc.txt (1.8)

  
  
  Thanks
  ------
  
  Con Kolivas, Pavel Machek