algorithm - How is CPU usage calculated?

Question

On my desktop, I have a little widget that tells me my current CPU usage. It also shows the usage for each of my two cores.

I always wondered, how does the CPU calculate how much of its processing power is being used? Also, if the CPU is hung up doing some intense calculations, how can it (or whatever handles this activity) examine the usage, without getting hung up as well?

Answer 1

There's a special task called the idle task that runs when no other task can be run. The % usage is just the percentage of the time we're not running the idle task. The OS will keep a running total of the time spent running the idle task:

• when we switch to the idle task, set t = current time
• when we switch away from the idle task, add (current time - t) to the running total

If we take two samples of the running total n seconds apart, we can calculate the percentage of those n seconds spent running the idle task as (second sample - first sample)/n

Note that this is something the OS does, not the CPU. The concept of a task doesn't exist at the CPU level! (In practice, the idle task will put the processor to sleep with a HLT instruction, so the CPU does know when it isn't being used)

As for the second question, modern operating systems are preemptively multi-tasked, which means the OS can switch away from your task at any time. How does the OS actually steal the CPU away from your task? Interrupts: http://en.wikipedia.org/wiki/Interrupt