linux:greenthread

http://www.jguru.com/faq/view.jsp?EID=143462

greenthread:
run on only one CPU even there are multi cpu
only yield() or waiting on something will make
the thread to release the CPU

In linux, thread like process, all the threads of a process have
the same memory. They sit inside the process table. It
is a scalability problem because there are limitation on the number
of processes/threads allocated in the system.