[jameshart]

In a managed platform like .NET or Java there's no reason in principle why any code should be able to tell whether it's in an OS or a green thread - the decision of how to actually accomplish having multiple simultaneously active callstacks with pre-emptive scheduling is made by the runtime. Early versions of Java used green threads within the JVM rather than relying on native threads, presumably as part of an attempt to provide consistent write-once-run-anywhere behaviors, so this is not just theoretical.

I think the async/await model arrives as a reasonable compromise in terms of supporting an alternative to OS threads in that it allows you to get the benefits of green threads (i.e. that you can process a lot of different stack contexts simultaneously without needing to spin up a huge amount of OS threads) without getting into the messy situation of having a user-space scheduler pre-emptively switching green-thread stacks (which probably needs the scheduler itself to run on another OS thread and definitely needs to be done at a 'runtime' level) - instead, you just rely on co-operative scheduling, which can be done at a 'library' level.