[pron]

Usually, when people say "green threads" they mean 1:N scheduling (i.e. the application consumes one kernel threads, and schedules N threads on top of it). Lightweight threads, on the other hand, employ M:N scheduling, i.e. they use multiple kernel threads and schedule even more fibers on top. That's the approach taken by Erlang, Go, and Quasar. And Erlang has done this well for a long time.

Now, fibers are not perfect: their main shortcoming is integration with legacy code. The optimal approach is scheduler activations, or "user assisted" kernel scheduling.