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
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