Question

Currently I think that processor only has two states: run and not run. If it’s running, it will use its full power to process a task. If there are multiple processes, processes will be shared by a portion of CPU.

How can the computing power can be divided into “portions”? So, suppose a CPU has 1 million transistors, only half of the transistors are used if the CPU is only at 50%?

Or is this related to allocated processing time for each process? i.e. assume “100%” means a process seizes a CPU for 200 milliseconds, if a process with a default nice value (priority value) 0, which means the process will receive 50% computing power or, in other word, 100 milliseconds. What is the correct idea?

Answer 1

Let me explain this on the example of Intel x86 CPUs and Windows NT (and its derivatives). One of the built-in system processes on these OSes is the System Idle Process. This process represents how much CPU time is utilized by the operating system’s “idle loop”. That idle loop does nothing else but executes the HLT instruction of the CPU. That instruction, in turn, commands the CPU to do nothing until the next interrupt arrives.

Therefore, if the scheduler decides that there are no processes that require CPU time at the given moment, it is given to the System Idle Process. If, say, 99% of the time in the last n seconds was spent by “executing” that process, it means that the CPU was really utilized only in 1% in these n seconds.

I believe it is totally analogous with Linux, only that it doesn’t have a separate process to model the “idleness” of the CPU.

On a side note : it is, of course, possible, to have a OS that doesn’t execute the HLT instruction at all. That was the case with Windows 98 and earlier (including, obviously, MS-DOS), whose idle loop simply consisted of a jmp $. That caused the CPU to use much more power.