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Source at commit fbf123cd4cc0c097fe9a99c90109ebb2a5e94a50 created 10 years 3 months ago. By Lars-Peter Clausen, dma: jz4740: Dequeue descriptor from active list before completing it | |
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1 | CPU load |
2 | -------- |
3 | |
4 | Linux exports various bits of information via `/proc/stat' and |
5 | `/proc/uptime' that userland tools, such as top(1), use to calculate |
6 | the average time system spent in a particular state, for example: |
7 | |
8 | $ iostat |
9 | Linux 2.6.18.3-exp (linmac) 02/20/2007 |
10 | |
11 | avg-cpu: %user %nice %system %iowait %steal %idle |
12 | 10.01 0.00 2.92 5.44 0.00 81.63 |
13 | |
14 | ... |
15 | |
16 | Here the system thinks that over the default sampling period the |
17 | system spent 10.01% of the time doing work in user space, 2.92% in the |
18 | kernel, and was overall 81.63% of the time idle. |
19 | |
20 | In most cases the `/proc/stat' information reflects the reality quite |
21 | closely, however due to the nature of how/when the kernel collects |
22 | this data sometimes it can not be trusted at all. |
23 | |
24 | So how is this information collected? Whenever timer interrupt is |
25 | signalled the kernel looks what kind of task was running at this |
26 | moment and increments the counter that corresponds to this tasks |
27 | kind/state. The problem with this is that the system could have |
28 | switched between various states multiple times between two timer |
29 | interrupts yet the counter is incremented only for the last state. |
30 | |
31 | |
32 | Example |
33 | ------- |
34 | |
35 | If we imagine the system with one task that periodically burns cycles |
36 | in the following manner: |
37 | |
38 | time line between two timer interrupts |
39 | |--------------------------------------| |
40 | ^ ^ |
41 | |_ something begins working | |
42 | |_ something goes to sleep |
43 | (only to be awaken quite soon) |
44 | |
45 | In the above situation the system will be 0% loaded according to the |
46 | `/proc/stat' (since the timer interrupt will always happen when the |
47 | system is executing the idle handler), but in reality the load is |
48 | closer to 99%. |
49 | |
50 | One can imagine many more situations where this behavior of the kernel |
51 | will lead to quite erratic information inside `/proc/stat'. |
52 | |
53 | |
54 | /* gcc -o hog smallhog.c */ |
55 | #include <time.h> |
56 | #include <limits.h> |
57 | #include <signal.h> |
58 | #include <sys/time.h> |
59 | #define HIST 10 |
60 | |
61 | static volatile sig_atomic_t stop; |
62 | |
63 | static void sighandler (int signr) |
64 | { |
65 | (void) signr; |
66 | stop = 1; |
67 | } |
68 | static unsigned long hog (unsigned long niters) |
69 | { |
70 | stop = 0; |
71 | while (!stop && --niters); |
72 | return niters; |
73 | } |
74 | int main (void) |
75 | { |
76 | int i; |
77 | struct itimerval it = { .it_interval = { .tv_sec = 0, .tv_usec = 1 }, |
78 | .it_value = { .tv_sec = 0, .tv_usec = 1 } }; |
79 | sigset_t set; |
80 | unsigned long v[HIST]; |
81 | double tmp = 0.0; |
82 | unsigned long n; |
83 | signal (SIGALRM, &sighandler); |
84 | setitimer (ITIMER_REAL, &it, NULL); |
85 | |
86 | hog (ULONG_MAX); |
87 | for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog (ULONG_MAX); |
88 | for (i = 0; i < HIST; ++i) tmp += v[i]; |
89 | tmp /= HIST; |
90 | n = tmp - (tmp / 3.0); |
91 | |
92 | sigemptyset (&set); |
93 | sigaddset (&set, SIGALRM); |
94 | |
95 | for (;;) { |
96 | hog (n); |
97 | sigwait (&set, &i); |
98 | } |
99 | return 0; |
100 | } |
101 | |
102 | |
103 | References |
104 | ---------- |
105 | |
106 | http://lkml.org/lkml/2007/2/12/6 |
107 | Documentation/filesystems/proc.txt (1.8) |
108 | |
109 | |
110 | Thanks |
111 | ------ |
112 | |
113 | Con Kolivas, Pavel Machek |
114 |
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