does while loop always take full CPU usage? - c++

Question

I need create a server side game loop, the problem is how to limit the loop cpu usage.
In my experience of programming, a busy loop always take maximal CPU usage it could. But I am reading the code of SDL(Simple DirectMedia Layer), it has a function SDL_Delay(UINT32 ms), and it has a while loop, does it take max cpu usage, if not, why?
https://github.com/eddieringle/SDL/blob/master/src/timer/unix/SDL_systimer.c#L137-158
do {
errno = 0;
#if HAVE_NANOSLEEP
tv.tv_sec = elapsed.tv_sec;
tv.tv_nsec = elapsed.tv_nsec;
was_error = nanosleep(&tv, &elapsed);
#else
/* Calculate the time interval left (in case of interrupt) */
now = SDL_GetTicks();
elapsed = (now - then);
then = now;
if (elapsed >= ms) {
break;
}
ms -= elapsed;
tv.tv_sec = ms / 1000;
tv.tv_usec = (ms % 1000) * 1000;
was_error = select(0, NULL, NULL, NULL, &tv);
#endif /* HAVE_NANOSLEEP */
} while (was_error && (errno == EINTR));

Answer 1

This code uses select for a timeout. select usually takes a file descriptor, and makes the caller wait until an IO event occurs on the fd. It also takes a timeout argument for the maximum time to wait. Here the fd is 0, so no events will occur, and the function will always return when the timeout is reached.
The select(3) that you get from the C library is a wrapper around the select(2) system call, which means calling select(3) eventually gets you in the kernel. The kernel then doesn't schedule the process unless an IO event occurs, or the timeout is reached. So the process is not using the CPU while waiting.
Obviously, the jump into the kernel and process scheduling introduce delays. So if you must have very low latency (nanoseconds) you should use busy waiting.

Answer 2

That loop won't take up all CPU. It utilizes one of two different functions to tell the operating system to pause the thread for a given amount of time and letting another thread utilize the CPU:
// First function call - if HAVE_NANOSLEEP is defined.
was_error = nanosleep(&tv, &elapsed);
// Second function call - fallback without nanosleep.
was_error = select(0, NULL, NULL, NULL, &tv);

Answer 3

While the thread is blocked in SDL_Delay, it yields the CPU to other tasks. If the delay is long enough, the operating system will even put the CPU in an idle or halt mode if there is no other work to do. Note that this won't work well if the delay time isn't at least 20 milliseconds or so.
However, this is usually not the right way to do whatever it is you are trying to do. What is your outer problem? Why doesn't your game loop ever finish doing whatever needs to be done at this time and so then need to wait for something to happen so that it has more work to do? How can it always have an infinite amount of work to do immediately?