8.1 Multiplexing

Xv6 multiplexes by switching each CPU from one process to another in two situations. First, xv6 switches when a process makes a system call that blocks (has to wait), for example read or wait. Second, xv6 periodically forces a switch to cope with processes that compute for long periods without blocking. The former are called voluntary switches, the latter involuntary.

Implementing multiplexing poses a few challenges. First, how to switch from one process to another? The basic idea is to save and restore CPU registers, though the fact that this cannot be expressed in C makes it tricky. Second, how to force switches in a way that is transparent to user processes? Xv6 uses the standard technique in which a hardware timer’s interrupts drive context switches. Third, all of the CPUs switch among the same set of processes, so a locking plan is necessary to avoid mistakes such as two CPUs deciding to run the same process at the same time. Fourth, a process’s memory and other resources must be freed when the process exits, but it cannot finish all of this itself. Fifth, each CPU of a multi-core machine must remember which process it is executing so that system calls affect the correct process’s kernel state.