Reactive Extensions overhead?

Question

I would have expected the following LINQPad code to finish executing in just over 1 second. But on my machine is consistently takes just over 15 seconds to complete. What am I missing here - is this just RX overhead?

int count = 0;
ManualResetEvent done = new ManualResetEvent(false);
Observable.Interval(TimeSpan.FromMilliseconds(1))
    .Take(1000)
    .Subscribe((next) => count++, () => done.Set());
done.WaitOne();
count.Dump("Items observed");

Answer 1

What you are seeing is not the overhead of Rx - it is just that the resolution of .NET timers is 15 milliseconds. Although you specify a 1ms interval, you are going to get 15ms. This is where the 15 seconds is coming from - 1000 x 15ms = 15s.

In constrast, on my laptop the following outputs 0 for the elapsed time:

var subject = new Subject<Unit>();
var stopwatch = new Stopwatch();

subject.Subscribe(_ => {}, () => {
    stopwatch.Stop();
    Console.WriteLine(stopwatch.ElapsedMilliseconds); });

stopwatch.Start();
for(int i = 0; i<1000; i++)
{
    subject.OnNext(Unit.Default);
}
subject.OnCompleted();