5.1 About multitasking

5.1.2 Process scheduling

An alive process is one that is either active or inactive.

An active process is one the scheduler is aware of; if a process is alive, it is generally active. An active process can be run under certain circumstances.
An inactive process is one that is alive but that cannot be run. You can make a process inactive by calling the functiondeactivate-process; the functionactivate-process makes an inactive process active again.

A dead process is one that has been killed or has run to completion. An active process is runnable unless it is associated with a wait function. A wait function is a function that is evaluated by the scheduler to determine whether a process can be run. You can use the functionsprocess-wait,process-wait-forever, orprocess-wait-with-timeout to associate a wait function with a process. You can also create a process in a waiting state by using the:wait-function option to the functionmake-process. The functionprocess-wait-function returns the wait function of a process, and the functionprocess-wait-arguments returns its arguments.

Every process has a priority. Active processes are kept on a queue that is sorted by priority. Processes with higher priority appear at the front of the queue.

The scheduler simulates concurrent execution of processes by periodically passing control from one process to another. It examines each process in order of priority and takes the appropriate action:

If the process is runnable, the scheduler passes control to it, and it executes until the scheduler is next invoked.
If the process is waiting, its wait function is applied to its wait arguments.
- If the wait function returns a non-nil value, the process becomes runnable, its wait function is discarded, and the scheduler immediately passes control to it. No other wait functions are evaluated.
- If the wait function returnsnil, the scheduler examines the next process.
If no processes are runnable, the scheduler runs the idle process.

Before a process is run, it is moved back in the queue beyond any other process with the same priority. In other words, processes with the same priority run with round-robin scheduling.

The wait function is evaluated in the dynamic context of the process that is current when the scheduler is entered. Scheduling is inhibited while the wait function runs to prevent recursive invocations of the scheduler. If the wait function fails to terminate when the scheduler invokes it, Lisp will hang.

The scheduler is called automatically at the end of the scheduling quantum, or you can invoke it with the following functions:

process-allow-schedule simply starts the scheduler.
process-wait,process-wait-forever, andprocess-wait-with-timeout associate a wait function with a process, mark the current process as waiting, and call the scheduler.

The Advanced User's Guide - 9 SEP 1996

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