CODEWITHSUNDEEP
c++cpu
frkn
frkn

Reputation: 324

why is empty while loop using more cpu?

I have two programs that are supposed to do the same thing with slight differences. Both have infinite game loops that runs forever unless user stops the game somehow. One of these programs' game loop is implemented and rendering something, the other game loop is empty and does nothing(just listens for user to stop).

When i opened the task manager to see resource usage, i have discovered that the program with the empty loop uses 14% CPU and the program that actually draws something to screen uses about 1-2%.

My guess on the subject is as follows:

I compared the code of the both programs and looked for differences and there was not much. Then it occurred to me that the loop that renders to screen might be bound by other factors(like sending pixels to the screen, refresh rate maybe?) So after CPU does its thing, it puts that thread to sleep until other stuff is completed. But since other program does pretty much nothing and doing nothing is really easy, CPU never puts that thread to sleep and just keeps going. I lack the knowledge to confirm that if this is the reason, so i am asking you. Is this the reason this is happening? (Bonus question) And if so, why does the CPU stop at about 14% and not going all the way up to 100% ?

Thank you.

Upvotes: 1

Views: 941

Answers (1)

Oliver Matthews
Oliver Matthews

Reputation: 7813

Hard to say for certain without seeing the code, but drawing to the screen will, inevitably involve some wait on IO; how much depends on many factors including sync + buffering options.

As for the 14% cpu usage - I'm guessing that your machine has 8 processing units (either cores or cores * hyperthreading) and your code is singlethreaded - i.e. it is maxing out one processing unit.

Upvotes: 3

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