Re: CPU load

[malc <av1474@comtv.ru>]

On Wed, 14 Feb 2007, Pavel Machek wrote:

> Hi!

[..snip..]

> > The current situation ought to be documented. Better yet some flag
> > can
>
> It probably _is_ documented, somewhere :-). If you find nice place
> where to document it (top manpage?) go ahead with the patch.


How about this:

<Documentation/load.txt>
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:

<transcript>
$ 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

...
</transcript>

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)
</Documentation/load.txt>

--
vale
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