[dragonwriter]

IIRC, Java's original threads were N:1 green threads, while the lightweight threads in Haskell discussed above are M:N green threads. N:1 is easier to implement, and fine for single core systems, but doesn't give you real parallelism. M:N, like 1:1 native threading, gives you real parallelism, but also, like N:1, cheap concurrency (at the expense of being the hardest to implement well.)

Multicore processors and the fact that single-threaded performance basically hit a wall explains why an N:1 threading models in something with the use cases of Java fell out of favor. M:N, while still "green threads", has a somewhat different set of trade-offs versus 1:1 native threads than N:1 does, though.