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authorMichael Ellerman <mpe@ellerman.id.au>2017-07-31 20:20:29 +1000
committerMichael Ellerman <mpe@ellerman.id.au>2017-07-31 20:20:29 +1000
commitbb272221e9db79f13d454e1f3fb6b05013be985e (patch)
tree36f4acc50e3fabac71fadd34c720c0a6011db470 /Documentation/cpu-load.txt
parent253fd51e2f533552ae35a0c661705da6c4842c1b (diff)
parent5771a8c08880cdca3bfb4a3fc6d309d6bba20877 (diff)
downloadop-kernel-dev-bb272221e9db79f13d454e1f3fb6b05013be985e.zip
op-kernel-dev-bb272221e9db79f13d454e1f3fb6b05013be985e.tar.gz
Merge tag 'v4.13-rc1' into fixes
The fixes branch is based off a random pre-rc1 commit, because we had some fixes that needed to go in before rc1 was released. However we now need to fix some code that went in after that point, but before rc1, so merge rc1 to get that code into fixes so we can fix it!
Diffstat (limited to 'Documentation/cpu-load.txt')
-rw-r--r--Documentation/cpu-load.txt131
1 files changed, 66 insertions, 65 deletions
diff --git a/Documentation/cpu-load.txt b/Documentation/cpu-load.txt
index 287224e..2d01ce4 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
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