Monitors and Priority Waits

Introduction

This is an example of a monitor using priority waits. It implements a simple alarm clock; that is, a process calls alarmclock.wakeme(n), and suspends for n seconds. Note that we are assuming the hardware invokes the procedure tick to update the clock every second.

Algorithm

1	alarmclock: monitor;
2	var now: integer;
3	    wakeup: condition;
4	procedure entry wakeme(n: integer);
5	begin
6		alarmsetting := now + n;
7		while now < alarmsetting do
8			wakeup.wait(alarmsetting);
9		wakeup.signal;
10	end;
11	procedure entry tick;
12	begin
13		now := now + 1;
14		wakeup.signal;
15	end.

Comments

lines 2-3 Here, now is the current time (in seconds) and is updated once a second by the procedure tick. When a process suspends, it will do a wait on the condition wakeup.

line 6 This computes the time at which the process is to be awakened.

lines 7-8 The process now checks that it is to be awakened later, and then suspends itself.

line 9 Once a process has been woken up, it signals the process that is to resume next. That process checks to see if it is time to wake up; if not, it suspends again (hence the while loop above, rather than an if statement). If it is to wake up, it signals the next process...

line 14 This is done once a second (hence the addition of 1 to now). The processes to be woken up are queued in order of remaining time to wait with the next one to wake up first. So, when tick signals, the next one to wake up determines if it is in fact time to wake up. If not, it suspends itself; if so, it proceeds.