Is it Really Busy Waiting If I Thread.Sleep()?

Question

My question is a bit nit-picky on definitions:

Can the code below be described as "busy waiting"? Despite the fact that it uses Thread.Sleep() to allow for context switching?

while (true) {
    if (work_is_ready){
        doWork();
    }
    Thread.Sleep(A_FEW_MILLISECONDS);
}

PS - The current definition for busy waiting in Wikipedia suggests that it is a "less wasteful" form of busy waiting.

Answer 1

Any polling loop, regardless of the time between polling operations, is a busy wait. Granted, sleeping a few milliseconds is a lot less "busy" than no sleep at all, but it still involves processing: thread context switches and some minimal condition checking.

A non-busy wait is a blocking call. The non-busy version of your example would involve waiting on a synchronization primitive such as an event or a condition variable. For example, this pseudocode:

// initialize an event to be set when work is ready
Event word_is_ready;
work_is_ready.Reset();

// in code that processes work items
while (true)
{
    work_is_ready.Wait();  // non-busy wait for work item
    do_work();
}

The difference here is that there is no periodic polling. The Wait call blocks and the thread is never scheduled until the event is set.