[dullgiulio]

Languages that to green threads don't do them for memory savings, but to save on context switches when a thread is blocked and cannot run. System threads are scheduled by the OS, green threads my the language runtime, which saves a context switch.

[tijsvd]

Green threads are scheduled by the language runtime and by the OS. If the OS switches from one thread to another in the same process, there is no context switch, really, apart from the syscall itself which was happening anyway (the recv that blocks and causes the switch). At least not on Linux, where I've measured the difference.