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diff --git a/Documentation/admin-guide/cpu-load.rst b/Documentation/admin-guide/cpu-load.rst new file mode 100644 index 000000000..f3ada90e9 --- /dev/null +++ b/Documentation/admin-guide/cpu-load.rst @@ -0,0 +1,117 @@ +======== +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.rst (1.8) + + +Thanks +------ + +Con Kolivas, Pavel Machek |