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
70 static unsigned long hog (unsigned long niters)
71 {
72 stop = 0;
73 while (!stop && --niters);
74 return niters;
75 }
76
77 int main (void)
78 {
79 int i;
80 struct itimerval it = {
81 .it_interval = { .tv_sec = 0, .tv_usec = 1 },
82 .it_value = { .tv_sec = 0, .tv_usec = 1 } };
83 sigset_t set;
84 unsigned long v[HIST];
85 double tmp = 0.0;
86 unsigned long n;
87 signal(SIGALRM, &sighandler);
88 setitimer(ITIMER_REAL, &it, NULL);
89
90 hog (ULONG_MAX);
91 for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog(ULONG_MAX);
92 for (i = 0; i < HIST; ++i) tmp += v[i];
93 tmp /= HIST;
94 n = tmp - (tmp / 3.0);
95
96 sigemptyset(&set);
97 sigaddset(&set, SIGALRM);
98
99 for (;;) {
100 hog(n);
101 sigwait(&set, &i);
102 }
103 return 0;
104 }
105
106
107 References
108 ----------
109
110 - https://lore.kernel.org/r/loom.20070212T063225-663@post.gmane.org
111 - Documentation/filesystems/proc.rst (1.8)
112
113
114 Thanks
115 ------
116
117 Con Kolivas, Pavel Machek
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