1======== 2CPU load 3======== 4 5Linux exports various bits of information via ``/proc/stat`` and 6``/proc/uptime`` that userland tools, such as top(1), use to calculate 7the 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 17Here the system thinks that over the default sampling period the 18system spent 10.01% of the time doing work in user space, 2.92% in the 19kernel, and was overall 81.63% of the time idle. 20 21In most cases the ``/proc/stat`` information reflects the reality quite 22closely, however due to the nature of how/when the kernel collects 23this data sometimes it can not be trusted at all. 24 25So how is this information collected? Whenever timer interrupt is 26signalled the kernel looks what kind of task was running at this 27moment and increments the counter that corresponds to this tasks 28kind/state. The problem with this is that the system could have 29switched between various states multiple times between two timer 30interrupts yet the counter is incremented only for the last state. 31 32 33Example 34------- 35 36If we imagine the system with one task that periodically burns cycles 37in 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 46In the above situation the system will be 0% loaded according to the 47``/proc/stat`` (since the timer interrupt will always happen when the 48system is executing the idle handler), but in reality the load is 49closer to 99%. 50 51One can imagine many more situations where this behavior of the kernel 52will 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 107References 108---------- 109 110- http://lkml.org/lkml/2007/2/12/6 111- Documentation/filesystems/proc.rst (1.8) 112 113 114Thanks 115------ 116 117Con Kolivas, Pavel Machek 118