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author | Mauro Carvalho Chehab <mchehab+samsung@kernel.org> | 2019-06-27 14:56:51 -0300 |
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committer | Mauro Carvalho Chehab <mchehab+samsung@kernel.org> | 2019-07-15 11:03:02 -0300 |
commit | 4f4cfa6c560c93ba180c30675cf845e1597de44c (patch) | |
tree | 0bbe2ec9e6ef62ed2a347504dda50c6bdbe43703 /Documentation/cpu-load.txt | |
parent | da82c92f1150f66afabf78d2c85ef9ac18dc6d38 (diff) | |
download | linux-4f4cfa6c560c93ba180c30675cf845e1597de44c.tar.bz2 |
docs: admin-guide: add a series of orphaned documents
There are lots of documents that belong to the admin-guide but
are on random places (most under Documentation root dir).
Move them to the admin guide.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Diffstat (limited to 'Documentation/cpu-load.txt')
-rw-r--r-- | Documentation/cpu-load.txt | 114 |
1 files changed, 0 insertions, 114 deletions
diff --git a/Documentation/cpu-load.txt b/Documentation/cpu-load.txt deleted file mode 100644 index 2d01ce43d2a2..000000000000 --- a/Documentation/cpu-load.txt +++ /dev/null @@ -1,114 +0,0 @@ -======== -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 |