[Lukasa]

Honestly, I think the distinction between M:N threading and straight OS threading is pretty minor. It grants some advantages to the language runtime: it can control the stack size, for example. But in terms of how it affects the development style and what kinds of bugs it encourages/discourages I don't think it dramatically differs from the OS threading model.

[sagichmal]

It is categorically different. OS threads are orders of magnitude more expensive, which makes them a nonstarter for most problems that are a good fit for lightweight conceptual concurrency.

[Lukasa]

It is not categorically different.

As I said above, green threading has advantages over OS threading, but they behave exactly the same in terms of design patterns and potential bugs.

This is what I was getting at when I said "not that different": compared to the difference between event-loop concurrency and threaded concurrency, M:N green threading is basically just a subcategory of threading.

[sagichmal]

No. Green threads enable entirely new classes of design patterns that are categorically infeasible with OS threads.