I think there's a naming confusion here. You said "M:N threads, aka green threads". However, your linked document clarifies that "green threads" are the M:1 threads that "[do] not exploit multiprocessors" (see the "Green Threads" parenthetical in the first subheading). It is interesting that JDK 1.1 also had an M:N threading model, but that's not what people usually mean by green threads.