How is CPU usage computed?

Question

The Windows Task Manager shows CPU usage in percentage. What's the formula behind this? Is it this:

% CPU usage for process A = (Sum of all time slices given to A till now)/ Total time since the machine booted

Or is it something else?

Answer 1

I am not 100% sure what is uses, but I think you are a bit off on the CPU calculation.

I believe they are doing something like.

Process A CPU Usage = (Cycles for A over last X seconds)/(Total cycles for last X seconds)

I believe it is tied to the "update interval" set in task manager.

While doing a bit of research for you though I found this MSDN article that shows a microsoft recommended way of calculating the CPU time of a set of instructions, this might point you a bit towards their calculation as well.

Answer 2

No, it's not "since boot time" - it's far more time-sensitive than that.

It's "proportion of time during which a CPU was actively running a thread in that process since the last refresh". (Where the refresh rate is typically about a second.) In task manager I believe it's then divided by the number of CPUs, so the total ends up being 100% (i.e. on a dual core machine, a single-threaded CPU hog will show as 50%). Other similar programs sometimes don't do this, giving a total of 100% * cores.