SpinWait vs Sleep waiting. Which one to use?

Solution 1:

In .NET 4 SpinWait performs CPU-intensive spinning for 10 iterations before yielding. But it does not return to the caller immediately after each of those cycles; instead, it calls Thread.SpinWait to spin via the CLR (essentially the OS) for a set time period. This time period is initially a few tens of nano-seconds but doubles with each iteration until the 10 iterations are complete. This enables clarity/predictability in the total time spent spinning (CPU-intensive) phase, which the system can tune according to conditions (number of cores etc.). If SpinWait remains in the spin-yielding phase for too long it will periodically sleep to allow other threads to proceed (see J. Albahari's blog for more information). This process is guaranteed to keep a core busy...

So, SpinWait limits the CPU-intensive spinning to a set number of iterations, after which it yields its time slice on every spin (by actually calling Thread.Yield and Thread.Sleep), lowering its resource consumption. It will also detect if the user is running a single core machine and yield on every cycle if that is the case.

With Thread.Sleep the thread is blocked. But this process will not be as expensive as the above in terms of CPU.

Solution 2:

The best approach is to have some mechanism to actively detect the thing becoming true (rather than passively polling for it having become true); this could be any kind of wait-handle, or maybe a Task with Wait, or maybe an event that you can subscribe to to unstick yourself. Of course, if you do that kind of "wait until something happens", that is still not as efficient as simply having the next bit of work done as a callback, meaning: you don't need to use a thread to wait. Task has ContinueWith for this, or you can just do the work in an event when it gets fired. The event is probably the simplest approach, depending on the context. Task, however, already provides most-everything you are talking about here, including both "wait with timeout" and "callback" mechanisms.

And yes, spinning for 10 seconds is not great. If you want to use something like your current code, and if you have reason to expect a short delay, but need to allow for a longer one - maybe SpinWait for (say) 20ms, and use Sleep for the rest?

Re the comment; here's how I'd hook an "is it full" mechanism:

private readonly object syncLock = new object();
public bool WaitUntilFull(int timeout) {
    if(CollectionIsFull) return true; // I'm assuming we can call this safely
    lock(syncLock) {
        if(CollectionIsFull) return true;
        return Monitor.Wait(syncLock, timeout);
    }
}

with, in the "put back into the collection" code:

if(CollectionIsFull) {
    lock(syncLock) {
        if(CollectionIsFull) { // double-check with the lock
            Monitor.PulseAll(syncLock);
        }
    }
}