Here's a working race condition with async: https://gist.github.com/39409838626b22433f8bf1878276b3c1 https://gist.github.com/39409838626b22433f8bf1878276b3c1

Here's a working race condition with async: https://gist.github.com/39409838626b22433f8bf1878276b3c1 https://gist.github.com/39409838626b22433f8bf1878276b3c1 Prints 1 instead of 1000 If you remove the sleep there's no race because the fiber scheduler never hits a blocking call anywhere, but the sleep stands in for some kind of i/o. And then there's the terrible global variable. (Conversely this does show you how badly you have to write code to get race conditions with async and the sleep there is important to the generation of the race, which would not be necessary with threads) And I think if you remove the sleep it doesn't race due to the GIL and single-threadedness of ruby for pure userspace computations?