linux/Documentation/cpu-load.txt
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   1CPU load
   2--------
   3
   4Linux exports various bits of information via `/proc/stat' and
   5`/proc/uptime' that userland tools, such as top(1), use to calculate
   6the average time system spent in a particular state, for example:
   7
   8    $ iostat
   9    Linux 2.6.18.3-exp (linmac)     02/20/2007
  10
  11    avg-cpu:  %user   %nice %system %iowait  %steal   %idle
  12              10.01    0.00    2.92    5.44    0.00   81.63
  13
  14    ...
  15
  16Here the system thinks that over the default sampling period the
  17system spent 10.01% of the time doing work in user space, 2.92% in the
  18kernel, and was overall 81.63% of the time idle.
  19
  20In most cases the `/proc/stat' information reflects the reality quite
  21closely, however due to the nature of how/when the kernel collects
  22this data sometimes it can not be trusted at all.
  23
  24So how is this information collected?  Whenever timer interrupt is
  25signalled the kernel looks what kind of task was running at this
  26moment and increments the counter that corresponds to this tasks
  27kind/state.  The problem with this is that the system could have
  28switched between various states multiple times between two timer
  29interrupts yet the counter is incremented only for the last state.
  30
  31
  32Example
  33-------
  34
  35If we imagine the system with one task that periodically burns cycles
  36in the following manner:
  37
  38 time line between two timer interrupts
  39|--------------------------------------|
  40 ^                                    ^
  41 |_ something begins working          |
  42                                      |_ something goes to sleep
  43                                     (only to be awaken quite soon)
  44
  45In the above situation the system will be 0% loaded according to the
  46`/proc/stat' (since the timer interrupt will always happen when the
  47system is executing the idle handler), but in reality the load is
  48closer to 99%.
  49
  50One can imagine many more situations where this behavior of the kernel
  51will lead to quite erratic information inside `/proc/stat'.
  52
  53
  54/* gcc -o hog smallhog.c */
  55#include <time.h>
  56#include <limits.h>
  57#include <signal.h>
  58#include <sys/time.h>
  59#define HIST 10
  60
  61static volatile sig_atomic_t stop;
  62
  63static void sighandler (int signr)
  64{
  65     (void) signr;
  66     stop = 1;
  67}
  68static unsigned long hog (unsigned long niters)
  69{
  70     stop = 0;
  71     while (!stop && --niters);
  72     return niters;
  73}
  74int main (void)
  75{
  76     int i;
  77     struct itimerval it = { .it_interval = { .tv_sec = 0, .tv_usec = 1 },
  78                             .it_value = { .tv_sec = 0, .tv_usec = 1 } };
  79     sigset_t set;
  80     unsigned long v[HIST];
  81     double tmp = 0.0;
  82     unsigned long n;
  83     signal (SIGALRM, &sighandler);
  84     setitimer (ITIMER_REAL, &it, NULL);
  85
  86     hog (ULONG_MAX);
  87     for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog (ULONG_MAX);
  88     for (i = 0; i < HIST; ++i) tmp += v[i];
  89     tmp /= HIST;
  90     n = tmp - (tmp / 3.0);
  91
  92     sigemptyset (&set);
  93     sigaddset (&set, SIGALRM);
  94
  95     for (;;) {
  96         hog (n);
  97         sigwait (&set, &i);
  98     }
  99     return 0;
 100}
 101
 102
 103References
 104----------
 105
 106http://lkml.org/lkml/2007/2/12/6
 107Documentation/filesystems/proc.txt (1.8)
 108
 109
 110Thanks
 111------
 112
 113Con Kolivas, Pavel Machek
 114
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