If your implementation is interpreted, or at least can act indistinguishably from an interpreter, it's possible to interrupt a green thread every N operations.

If your implementation is interpreted, or at least can act indistinguishably from an interpreter, it's possible to interrupt a green thread every N operations. Obviously if you block in the OS for IO, your interpreter won't have a chance to preempt you, which is one of several issues with M:N threading models.