Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
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
