9.4 Preemptive Context Switching

Most implementations of POSIX threads perform time-slicing: when a thread has run for a while, it automatically gives up the CPU to another thread. Our implementation is currently nonpreemptive (no time-slicing). Threads give up control at two points:

• If they block (eg, at a mutex).
• If they call pthread_yield().

Usually, the first rule is sufficient to make most programs work. However, a few programs (particularly, those that busy-wait) may need explicit insertion of yields.