docs: admin-guide: add a series of orphaned documents

[sfrench/cifs-2.6.git] / Documentation / cpu-load.txt

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-========
-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