What is the minimum wait time to use ManualResetEventSlim instead of ManualResetEvent?

Question

From NET 4 I can use the ManualResetEventSlim class that make a little spinning before blocking in order to get a time optimization if the blocking time is little (I have no context switch).

I'd like to measure using a benchmark how little is this time in order to know, more or less, the amount of wait time necessary to prefer using a ManualResetEventSlim instead of a classic ManualResetEvent.

I know that this measure is CPU dependent, it is impossible to know a priori the Spin time, but I'd like to have an order of magnitude.

I wrote a benchmark class in order to get the minimum MillisecondSleep that make ManualResetEventSlim better than ManualResetEvent.

public class ManualResetEventTest
{
    [Params(0, 1, 10)]
    public int MillisecondsSleep;

    [Benchmark]
    public void ManualResetEventSlim()
    {
        using var mres = new ManualResetEventSlim(false);
        var t = Task.Run(() =>
        {
            mres.Wait();
        });

        Thread.Sleep(MillisecondsSleep);
        mres.Set();
        t.Wait();
    }
    
    [Benchmark]
    public void ManualResetEvent()
    {
        using var mres = new ManualResetEvent(false);
        var t = Task.Run(() =>
        {
            mres.WaitOne();
        });

        Thread.Sleep(MillisecondsSleep);
        mres.Set();
        t.Wait();
    }
}

And the result is the following

As you can see I found a improved performance only using a Thread.Sleep(0). Furthermore I see a 15ms mean time with both 1 and 10 ms. Am I missing something?

Is it true that only with the 0 ms wait it is better to use a ManualResetEventSlim instead of ManualResetEvent?

Keratitis

Answer 1

From the excellent C# 9.0 in a Nutshell book:

Waiting or signaling an AutoResetEvent or ManualResetEvent takes about one microsecond (assuming no blocking).

ManualResetEventSlim and CountdownEvent can be up to 50 times faster in short-wait scenarios because of their nonreliance on the OS and judicious use of spinning constructs. In most scenarios, however, the overhead of the signaling classes themselves doesn't create a bottleneck; thus, it is rarely a consideration.

Hopefully that's enough to give you a rough order of magnitude.

Petitionary