Why does an empty loop use so much processor time?

Question

If I have an empty while loop in my code such as:

while(true);

It will drive the processor usage up to about 25%. However if I do the followin

Answer 1

With the former, the condition true must be checked by the processor as often as the application can possibly get focus. As soon as the application gets processor attention, it checks true, just like any other loop condition, to see if the next instruction in the loop can be executed. Of course, the next instruction in the loop is also true. Basically, you are forcing the processor to constantly determine whether or not true is true, which, although trivial, when performed constantly bogs the processor down.

In the latter, the processor checks true once, and then executes the next statement. In this case, that is a 1ms wait. Since 1 ms is far greater than the amount of time required to check true, and the processor knows it can do other things during this wait, you free up a lot of power.

Answer 2

A cpu can do some billion operations per second. This means the empty loop runs mybe one million times per second. The loop with the sleep statement runs only 1000 times per second. In this case the cpu has some operations per second left to do other things.

Say we have a 3GHz cpu. 3Ghz = 3 000 000 000Hz - The cpu can run the loop three bilion times a second (simplyfied).

With the sleep statement the loop is executed 1000 times a second. This means the cpu load is

1000 / 3 000 000 000 * 100 = 0.0001%

Answer 3

The first continuously uses CPU operations. The latter switches the context of the currently running thread, putting is in sleep mode, thus allowing for other processes to be scheduled.

Answer 4

Basically, you've got several "process scheduler" states. I'll name three of them. One: Ready Two: Running Three: Blocked

These states / queues only exist because of the limited number of cores on your processor. In Ready, processes are scheduled that are totally ready for execution. They don't have to wait for input, time, or whatever. On Running, processes actually "have" the processor and thus ARE running. State Blocked means your process is waiting for an event to happen before queueing for the processor.

When you keep on testing for while(true) you keep your process in the "ready" queue. Your process scheduler gives it a certain amount of time, and after a while, removes it from the processor (placing it on the back of the "ready" queue). And thus your process will keep coming back "on" the processor, keeping it busy.

When you execute a "sleep" statement, your process will not be scheduled on process until the prerequisity is fulfilled - in this particular case, as long as the time passed after the "sleep" command <= 1000 ms.

Answer 5

Sleep() is not really doing anything during the period that the thread is sleeping. It hands its time over to other processes to use. A loop on the other hand is continuously checking to see if the condition is true or false.

Answer 6

The sleep in the second one is kinda like a 'yield' to the OS's process scheduler.