Green threads aren't just a substitute for when you don't have system level threads. Instead, they're a way of structuring code that allows you to express highly concurrent programs without requiring the heavy overhead of launching and switching between operating system threads.
Linux switches between threads at some frequency, I think it used to be 100 Hz. It involves swapping out the process registers, doing some kernel bookkeeping, etc—this is called a "context switch" and it's quite costly. Also, Linux threads allocate at least one memory page (4 KB) for the stack. [If I'm wrong about these details, please correct me!]
Basically, the cost associated with an operating system thread comes from the fact that it has to be isolated from other system threads on a low level... whereas language runtimes that offer green threads impose their own safety via language construction, e.g., Erlang processes can't reach in and mess with other processes memory (without C hacks).
So green threads can be much more efficient, but they require some care in the implementation, especially to support I/O, and to have fair and efficient load balancing, etc. Then you run N operating system threads to get balancing across cores, and distribute green thread work.
Linux switches between threads at some frequency, I think it used to be 100 Hz. It involves swapping out the process registers, doing some kernel bookkeeping, etc—this is called a "context switch" and it's quite costly. Also, Linux threads allocate at least one memory page (4 KB) for the stack. [If I'm wrong about these details, please correct me!]
Basically, the cost associated with an operating system thread comes from the fact that it has to be isolated from other system threads on a low level... whereas language runtimes that offer green threads impose their own safety via language construction, e.g., Erlang processes can't reach in and mess with other processes memory (without C hacks).
So green threads can be much more efficient, but they require some care in the implementation, especially to support I/O, and to have fair and efficient load balancing, etc. Then you run N operating system threads to get balancing across cores, and distribute green thread work.