COMP 346 WINTER Tutorial 5 MONITORS

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1 COMP 346 WINTER Tutorial 5 MONITORS

2 WHY DO WE NEED MONITORS? Semaphores are very useful for solving concurrency problems But it s easy to make mistakes! If proper usage of semaphores is failed by even one process, the entire system could break down. Solution? We need something better. 2

3 MONITOR CONCEPT o Monitors are other mechanism of concurrent programming. o One can think of a monitor as another Abstract Data Type like a structure or class with functions and private data. o The key attribute of a monitor is that it can be accessed only by one thread at a time. 3

4 It's a higher level mechanism than semaphores. A monitor is an instance of a class that can be used safely by several threads. All the methods of a monitor are executed with mutual exclusion. So at most one thread can execute a method of the monitor at the same time. This mutual exclusion policy makes it easier to work with monitor and to develop the method content of the monitor. Monitors have another feature, the possibility to make a thread waiting for a condition. During the wait time, the thread temporarily gives up its exclusive access and must reacquire it after the condition has been met. 4

5 MONITOR ANALOGY A monitor object can be thought of as an object where each access to it is protected by a mutex: mutex.wait(); mymonitor.exm(); mutex.signal(); 5

6 MONITOR EXAMPLE monitor SharedBalance { private int balance; public SharedBalance(int amount) {balance = amount; public credit(int amount) {balance += amount; public debit(int amount) {balance -= amount; 6 6

7 SHOW ME THE MONEY Let s say we only allow someone to complete a debit transaction when there is enough money to take: SharedBalance::debit(int amount) { while(balance < amount) { // wait for the cash balance -= amount; 7 7

8 WHOOPS! DEADLOCK!!! This won t work if a debit() call occurs when there is not enough money: debit() waits for someone to credit the account credit() can t run because debit is already executing in the monitor! 8 8

9 MONITOR WITH CONDITION VARIABLES Condition variables are used when: a thread is running in a monitor. encounters a condition that is not satisfied, which can only be satisfied by another thread 2 operations: wait() - block on a variable, give up monitor signal()- wake up a thread blocked on this 9 Monitors 9

10 You wait on a condition variable(s) when you want to get another thread to do something for you. This is what happens when you wait(): It temporarily blocks you, Hands over ownership of the monitor you are running in to another thread, Gives you back the ownership of the monitor later on 10 Monitor s 10

11 SHOW ME THE MONEY ALREADY! Using a new member variable condvar: SharedBalance::credit(int amount) { balance += amount; condvar.signal(); SharedBalance::debit(int amount) { while(balance < amount) { condvar.wait(); balance -= amount; 11 Monitors 11

12 MONITORS IN JAVA Every object of a class that has a synchronized method has a monitor associated with it Any such method is guaranteed by the Java Virtual Machine execution model to execute mutually exclusively from any other synchronized methods for that object Access to individual objects such as arrays can also be synchronized wait() releases a lock i.e enters holding area notify() signals a process to be allowed to continue notifyall() allows all waiting processes to continue 12 Operating Systems: Monitors

13 no way to notify a particular thread (but threads can have a priority) synchronized methods can call other synchronized and nonsynchronized methods monitor can be re-entrant i.e. can be re-entered recursively example: class DataBase { public synchronized void write (... ) {... public synchronized read (... ) {... public void getversion() {... once a thread enters either of the read or write methods, JVM ensures that the other is not concurrently entered for the same object getversion could be entered by another thread since not synchronized code could still access a database safely by locking the call rather than by using synchronized methods: Operating Systems: Monitors DataBase db = new DataBase(); synchronized(db) { db.write(... ); 13

14 (BARBER PROBLEM) one barber, one barber s chair, n customer chairs barber sleeps until a customer appears first customer to appear wakes barber up subsequent customers sit down until all chairs are occupied, otherwise they leave how to program the barber and customers without race conditions using Monitor? 14

15 monitor SB { condition : customers, barbers; int waiting = 0; entry barber {.. cut hair entry customer {.. get haircut 15

16 monitor SB { condition : customers, barbers; int waiting = 0; entry barber { if(waiting==0) wait(customers); waiting = waiting -1; signal(barbers); cut hair entry customer { if (waiting < n) { waiting = waiting+1; if(waiting==1) signal(customers); wait(barbers); get haircut 16

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