Difference between CPU usage in top and /proc/stat

Question

I'm trying to write a little script that, among other things, calculates the cpu usage by reading /proc/stat, and I can't seem to get consistent results. I'm basically doing a

grep cpu /proc/stat

and then processing the first line that is supposed to accumulate data from all the threads. In my case I've got 4 cores/8 threads. I've also written a program that constantly occupies one thread just to test stuff (it's basically a while(1) { 1 }). In this context if I'm doing nothing else I would expect 12.5% cpu usage, and if I run top, I get that:

%Cpu(s): 12,7 usuario,  0,1 sist,  0,0 adecuado, 87,1 inact,  0,1 en espera,  0,

(sorry that it's in another language). The first line of /proc/stat, though, is as follows:

cpu  1490042 169968 416799 32056116 21179 0 19504 0 0 0

And if I calculate the percentage from here:

(1490042 + 169968 + 416799)/(1490042 + 169968 + 416799 + 32056116) = 6%

Why the inconsistency? I guess I'm not interpreting /proc/stat properly?

Answer 1

I'll have to answer my question since finding an answer wasn't all that hard, I just misread the documentation on /proc/stat

/proc/stat shows aggregated data since bootup, therefore to calculate average cpu usage during a given time interval, you have to calculate the differences of the values given in /proc/stat at the beginning and end of that time interval.