Monitors: An Operating System Structuring Concept

Paper by C. A. R. Hoare

Presentation by Emerson Murphy-Hill

The Problem

• An OS’s main task is to control access to a machine’s resources (data, devices, time, space)

• If resources are not tightly controlled, “chaos will ensue”- race conditions

The Solution

• Monitors provide control by allowing only one process to access a critical resource at a time– A class/module/package– Contains procedures and data

An Abstract Monitor

name : monitor

… some local declarations… initialize local dataprocedure name(…arguments)

… do some work … other procedures

Monitor Rules

• Any process can access any monitor procedure at any time

• Only one process may enter a monitor procedure

• No process may directly access a monitor’s local variables

• A monitor may only access it’s local variables

Things Needed to Enforce Monitor

• “wait” operation– Forces running process to sleep

• “signal” operation– Wakes up a sleeping process

• A condition– Something to store who’s waiting

for a particular reason– Implemented as a queue

A Running Example – Emerson’s Kitchen

kitchen : monitor

occupied : Boolean; occupied := false;nonOccupied : condition;

procedure enterKitchen

if occupied then nonOccupied.wait;occupied = true;

procedure exitKitchenoccupied = false;nonOccupied.signal;

Monitor Declaration

Declarations / Initialization

Procedure

Procedure

Multiple Conditions

• Sometimes desirable to be able to wait on multiple things

• Can be implemented with multiple conditions

• Example: Two reasons to enter kitchen - cook (remove clean dishes) or clean (add clean dishes)

• Two reasons to wait:– Going to cook, but no clean dishes– Going to clean, no dirty dishes

Emerson’s Kitchenkitchen : monitor

cleanDishes, dirtyDishes : condition; dishes, sink : stack; dishes := stack of 10 dishes

sink := stack of 0 dishes

procedure cookif dishes.isEmpty then cleanDishes.waitsink.push ( dishes.pop );dirtyDishes.signal;

procedure cleanDishif sink.isEmpty then dirtyDishes.waitdishes.push (sink.pop)cleanDishes.signal

Condition Queue

• Checking if any process is waiting on a condition:– “condition.queue” returns true if a

process is waiting on condition

• Example: Doing dishes only if someone is waiting for them

Emerson’s Kitchenkitchen : monitor

cleanDishes, dirtyDishes : condition; dishes, sink : stack; dishes := stack of 10 dishes

sink := stack of 0 dishes

procedure cookif dishes.isEmpty then cleanDishes.waitsink.push ( dishes.pop );dirtyDishes.signal;

procedure cleanDishesif sink.isEmpty then dirtyDishes.waitif cleanDishes.queue

dishes.push (sink.pop);cleanDishes.signal;

Scheduled Waits

• Gives a waiting thread a number• Lowest number gets woken up

first (inverse priority)• Example: People who want to

cook are prioritized:– Highest: Me!– Medium: Family– Lowest: Vagrants/Friends

Emerson’s Kitchenkitchen : monitor

cleanDishes, dirtyDishes : condition; dishes, sink : stack; dishes := stack of 10 dishes

sink := stack of 0 dishes

procedure cook( p : Person )if dishes.isEmpty then

if p.isEmerson then cleanDishes.wait(1);if p.isFamily then cleanDishes.wait(2);if p.isFriendAndOrVagrant then

cleanDishes.wait(3);sink.push ( dishes.pop );dirtyDishes.signal;

procedure cleanDishesif sink.isEmpty then dirtyDishes.wait;while(cleanDishes.queue)

dishes.push (sink.pop);cleanDishes.signal;

Summary

• Advantages– Data access synchronization simplified

(vs. semaphores or locks)– Better encapsulation

• Disadvantages:– Deadlock still possible (in monitor code)– Programmer can still botch use of

monitors– No provision for information exchange

between machines

Other Issues Discussed

• Power of Monitors– SemaphoreMonitor– MonitorSemaphore

• Proof Rules– I (b.wait) I & B– I & B (b.signal) I

• OS Examples– Bounded Buffer– Alarm clock– Buffer allocation– Disk-head Scheduler– Reader/Writer

Mutual Exclusion: ImplementationDisabling Interruptskitchen : monitor

occupied : Boolean; occupied := false;

nonOccupied : condition;

procedure enterKitchen

disableInterrupts();

if occupied then nonOccupied.wait;

occupied = true;

enableInterrupts();

procedure exitKitchen

disableInterrupts();

occupied = false;

nonOccupied.signal;

enableInterrupts();

Mutex Insertionkitchen : monitor

occupied : Boolean; occupied := false;

nonOccupied : condition;

lock1 : lock;

procedure enterKitchen

lock1.acquire();

if occupied then nonOccupied.wait;

occupied = true;

lock1.release();

procedure exitKitchen

lock1.acquire();

occupied = false;

nonOccupied.signal;

lock1.release();

Monitors In Context

• Multithreaded code could be implemented with monitors (w/language support)

• Enforcement of mutex locking conventions

• Abstraction: deadlock can be avoided, but of course, only if the monitor is written correctly!

Conclusion

• Monitors are a synchronization mechanism

• A higher level, easier to use abstraction, better encapsulation vs. semaphores/locks

• Monitors still suffer from various afflictions

References

• “Monitors: An Operating System Structuring Concept,” Hoare.

• Modern Operating Systems, Second Edition, Tannenbaum, pp. 115-119.

• Jon Walpole, correspondence.

Questions

1. What is the essential implementation difference between monitors and semaphores?

2. What problems can still arise with the use of monitors?

3. What is the most specialized compiler mechanism required to implement monitors?

4. Generally, monitors allow us to encapsulate mutex use and avoid the spread of mutex across code. Can you think of a case where this is not possible?

Possible Answers

1. Atomicity of blocked code-reentrance in monitors

2. Deadlock, inconsistent use (eg, acquire without release)

3. Mutual exclusion in monitor methods

4. When we need uninterrupted access to 2 or more resources located in different monitor proceedures.