What is the difference between "green" threads and "native" threads?
Native threads use the operating system's native ability to manage multi-threaded processes - in particular, they use the pthread library. When you run with native threads, the kernel schedules and manages the various threads that make up the process.
Green threads emulate multithreaded environments without relying on any native OS capabilities. They run code in user space that manages and schedules threads; Sun wrote green threads to enable Java to work in environments that do not have native thread support.
Native threads can switch between threads pre-emptively, switching control from a running thread to a non-running thread at any time. Green threads only switch when control is explicitly given up by a thread (Thread.yield(), Object.wait(), etc.) or a thread performs a blocking operation (read(), etc.).
On multi-CPU machines, native threads can run more than one thread simultaneously by assigning different threads to different CPUs. Green threads run on only one CPU.
Native threads create the appearance that many Java processes are running: each thread takes up its own entry in the process table. One clue that these are all threads of the same process is that the memory size is identical for all the threads - they are all using the same memory.
Unfortunately, this behavior limits the scalability of Java on Linux. The process table is not infinitely large, and processes can only create a limited number of threads before running out of system resources or hitting configured limits.