Documentation/cpu-load.txt

   1 CPU load
   2 --------
   3
   4 Linux exports various bits of information via `/proc/stat' and
   5 `/proc/uptime' that userland tools, such as top(1), use to calculate
   6 the 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
  16 Here the system thinks that over the default sampling period the
  17 system spent 10.01% of the time doing work in user space, 2.92% in the
  18 kernel, and was overall 81.63% of the time idle.
  19
  20 In most cases the `/proc/stat' information reflects the reality quite
  21 closely, however due to the nature of how/when the kernel collects
  22 this data sometimes it can not be trusted at all.
  23
  24 So how is this information collected?  Whenever timer interrupt is
  25 signalled the kernel looks what kind of task was running at this
  26 moment and increments the counter that corresponds to this tasks
  27 kind/state.  The problem with this is that the system could have
  28 switched between various states multiple times between two timer
  29 interrupts yet the counter is incremented only for the last state.
  30
  31
  32 Example
  33 -------
  34
  35 If we imagine the system with one task that periodically burns cycles
  36 in 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
  45 In the above situation the system will be 0% loaded according to the
  46 `/proc/stat' (since the timer interrupt will always happen when the
  47 system is executing the idle handler), but in reality the load is
  48 closer to 99%.
  49
  50 One can imagine many more situations where this behavior of the kernel
  51 will 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
  61 static volatile sig_atomic_t stop;
  62
  63 static void sighandler (int signr)
  64 {
  65      (void) signr;
  66      stop = 1;
  67 }
  68 static unsigned long hog (unsigned long niters)
  69 {
  70      stop = 0;
  71      while (!stop && --niters);
  72      return niters;
  73 }
  74 int 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
 103 References
 104 ----------
 105
 106 http://lkml.org/lkml/2007/2/12/6
 107 Documentation/filesystems/proc.txt (1.8)
 108
 109
 110 Thanks
 111 ------
 112
 113 Con Kolivas, Pavel Machek