5 years ago I showed a way to fight with "all-threads-busy" problem while writing Hopac code. The same problem exists on JVM as well. The root is in the cooperative concurrency model, which is based on thread pools, async/futures/etc. are scheduled to run on them. So, if an async is blocked by an IO call of if it's just executing some computation, the OS thread is not able to do anything else: the scheduler is unable to "pause" such a blocked async and execute anothe one. So, asynchronous code on .NET, JVM[...]