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Linux/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

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