[PATCH] Documentation: CPU load calculation description
Describes how/when the information exported to `/proc/stat' is calculated,
and possible problems with this approach.
Signed-off-by: Vassili Karpov <av1474@comtv.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/cpu-load.txt b/Documentation/cpu-load.txt
new file mode 100644
index 0000000..287224e
--- /dev/null
+++ b/Documentation/cpu-load.txt
@@ -0,0 +1,113 @@
+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
