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| -rw-r--r-- | Documentation/cpu-load.txt | 131 |
1 files changed, 66 insertions, 65 deletions
diff --git a/Documentation/cpu-load.txt b/Documentation/cpu-load.txt index 287224e57cfc..2d01ce43d2a2 100644 --- a/Documentation/cpu-load.txt +++ b/Documentation/cpu-load.txt @@ -1,9 +1,10 @@ +======== 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: +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 @@ -17,7 +18,7 @@ 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 +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. @@ -33,78 +34,78 @@ Example ------- If we imagine the system with one task that periodically burns cycles -in the following manner: +in the following manner:: - time line between two timer interrupts -|--------------------------------------| - ^ ^ - |_ something begins working | - |_ something goes to sleep - (only to be awaken quite soon) + 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 +``/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; -} +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) +- http://lkml.org/lkml/2007/2/12/6 +- Documentation/filesystems/proc.txt (1.8) Thanks |