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Documentation / cpu-load.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:54 EST.

1	========
2	CPU load
3	========
4	
5	Linux exports various bits of information via ``/proc/stat`` and
6	``/proc/uptime`` that userland tools, such as top(1), use to calculate
7	the average time system spent in a particular state, for example::
8	
9	    $ iostat
10	    Linux 2.6.18.3-exp (linmac)     02/20/2007
11	
12	    avg-cpu:  %user   %nice %system %iowait  %steal   %idle
13	              10.01    0.00    2.92    5.44    0.00   81.63
14	
15	    ...
16	
17	Here the system thinks that over the default sampling period the
18	system spent 10.01% of the time doing work in user space, 2.92% in the
19	kernel, and was overall 81.63% of the time idle.
20	
21	In most cases the ``/proc/stat``	 information reflects the reality quite
22	closely, however due to the nature of how/when the kernel collects
23	this data sometimes it can not be trusted at all.
24	
25	So how is this information collected?  Whenever timer interrupt is
26	signalled the kernel looks what kind of task was running at this
27	moment and increments the counter that corresponds to this tasks
28	kind/state.  The problem with this is that the system could have
29	switched between various states multiple times between two timer
30	interrupts yet the counter is incremented only for the last state.
31	
32	
33	Example
34	-------
35	
36	If we imagine the system with one task that periodically burns cycles
37	in the following manner::
38	
39	     time line between two timer interrupts
40	    |--------------------------------------|
41	     ^                                    ^
42	     |_ something begins working          |
43	                                          |_ something goes to sleep
44	                                         (only to be awaken quite soon)
45	
46	In the above situation the system will be 0% loaded according to the
47	``/proc/stat`` (since the timer interrupt will always happen when the
48	system is executing the idle handler), but in reality the load is
49	closer to 99%.
50	
51	One can imagine many more situations where this behavior of the kernel
52	will lead to quite erratic information inside ``/proc/stat``::
53	
54	
55		/* gcc -o hog smallhog.c */
56		#include <time.h>
57		#include <limits.h>
58		#include <signal.h>
59		#include <sys/time.h>
60		#define HIST 10
61	
62		static volatile sig_atomic_t stop;
63	
64		static void sighandler (int signr)
65		{
66		(void) signr;
67		stop = 1;
68		}
69		static unsigned long hog (unsigned long niters)
70		{
71		stop = 0;
72		while (!stop && --niters);
73		return niters;
74		}
75		int main (void)
76		{
77		int i;
78		struct itimerval it = { .it_interval = { .tv_sec = 0, .tv_usec = 1 },
79					.it_value = { .tv_sec = 0, .tv_usec = 1 } };
80		sigset_t set;
81		unsigned long v[HIST];
82		double tmp = 0.0;
83		unsigned long n;
84		signal (SIGALRM, &sighandler);
85		setitimer (ITIMER_REAL, &it, NULL);
86	
87		hog (ULONG_MAX);
88		for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog (ULONG_MAX);
89		for (i = 0; i < HIST; ++i) tmp += v[i];
90		tmp /= HIST;
91		n = tmp - (tmp / 3.0);
92	
93		sigemptyset (&set);
94		sigaddset (&set, SIGALRM);
95	
96		for (;;) {
97			hog (n);
98			sigwait (&set, &i);
99		}
100		return 0;
101		}
102	
103	
104	References
105	----------
106	
107	- http://lkml.org/lkml/2007/2/12/6
108	- Documentation/filesystems/proc.txt (1.8)
109	
110	
111	Thanks
112	------
113	
114	Con Kolivas, Pavel Machek
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