Green threads (today) aren't a singular thing, the definition is that they're in user space not kernel space. They are implemented in a variety of ways: https:

Green threads (today) aren't a singular thing, the definition is that they're in user space not kernel space. They are implemented in a variety of ways: https://en.wikipedia.org/wiki/Green_thread https://en.wikipedia.org/wiki/Green_thread Do you imitate a more traditional OS-thread style with preemption, do you use cooperating tasks, coroutines, what? Since there is no singular best or consensus model, there is little reason for an OS to adopt wholesale one of these variations at this time. The original green threads (from that page) shared one OS thread and used cooperative multitasking (most coroutine approaches would be analogous to this). But today, like with Go and BEAM languages, they're distributed across real OS threads to get parallelism. Which approach should an OS adopt? And if it did, would other languages/runtimes abandon their own models if it were significantly different?