COE518 Lecture Notes Week 7 (Oct 17, 2011)

Transcription

1 coe518 (Operating Systems) Lecture Notes: Week 7 Page 1 of 10 COE518 Lecture Notes Week 7 (Oct 17, 2011) Topics multithreading in Java Note: Much of this material is based on Textbook sections Stallings (7 th edition): Tanenbaum (3 rd edition): Summary (mini-review) Threads in Java Multithreading has been built in to Java from the very beginning. This involves: adding the keyword synchronized to the language adding the type qualifier volatile to the language adding 5 methods to Object (which all objects ultimately inherit: notify() notifyall() wait() wait(long timeoutmillisecs) wait(long timeoutmillisecs, int nanosecs)

2 coe518 (Operating Systems) Lecture Notes: Week 7 Page 2 of 10 You did not encounter any of these aspects of Java in your introductory course; you will in this one! Synchronization is the key primitive operation build into the language to handle concurrency. Other ways of controlling concurrency (such as messages or semaphores) can be implemented in pure Java by building on top of this mechanism. (For example, there is a Semaphore class in the standard library.) The Thread API is available at SimpleThreads example This example illustrates: The Thread static method sleep(long time); Thread instance methods: isalive() join(long timeout) and join() waits for the thread to die (or until the timeout has elapsed.) Example: Thread t1, t2; //create t1 and t2 t1.start(); t2.start(); t2.join(1000);//wait till t2 dead or 1 second if(t2.isalive()) { t2.interrupt(); t2.join(); //happens REAL FAST //Now wait indefinitely for t1 death t1.join(); public class SimpleThreads { //Display a message, preceded by the name of the current thread static void threadmessage(string message) { String threadname = Thread.currentThread().getName();

3 coe518 (Operating Systems) Lecture Notes: Week 7 Page 3 of 10 System.out.format("%s: %s%n", threadname, message); private static class MessageLoop implements Runnable public void run() { String importantinfo[] = { "Mares eat oats", "Does eat oats", "Little lambs eat ivy", "A kid will eat ivy too" ; try { for (int i = 0; i < importantinfo.length; i++) { //Pause for 4 seconds Thread.sleep(4000); //Print a message threadmessage(importantinfo[i]); catch (InterruptedException e) { threadmessage("i wasn't done!"); public static void main(string args[]) throws InterruptedException { //Delay, in milliseconds before we interrupt MessageLoop //thread (default one hour). long patience = 1000 * 60 * 60; //patience = 1000 * 2; //If command line argument present, gives patience in seconds. if (args.length > 0) { try { patience = Long.parseLong(args[0]) * 1000; catch (NumberFormatException e) { System.err.println("Argument must be an integer."); System.exit(1);

4 coe518 (Operating Systems) Lecture Notes: Week 7 Page 4 of 10 threadmessage("starting MessageLoop thread"); long starttime = System.currentTimeMillis(); Thread t = new Thread(new MessageLoop()); t.start(); threadmessage("waiting for MessageLoop thread to finish"); //loop until MessageLoop thread exits while (t.isalive()) { threadmessage("still waiting..."); //Wait maximum of 1 second for MessageLoop thread to //finish. t.join(1000); if (((System.currentTimeMillis() - starttime) > patience) && t.isalive()) { threadmessage("tired of waiting!"); t.interrupt(); //Shouldn't be long now -- wait indefinitely t.join(); threadmessage("finally!"); Memory consistency errors and synchronized methods. The happens-before concept. Example: Bank and synchronized methods import java.util.logging.level; import java.util.logging.logger; public class Bank { private int balance;

5 coe518 (Operating Systems) Lecture Notes: Week 7 Page 5 of 10 public int getbalance() { return balance; //If this method not synchronized, problems! public void deposit(int b) { int bb = balance; //Waste some time (increase chance of context switch) for (int i = 0; i < ; i++) ; balance = bb + b; //Change this for synchronizations problems! public static final int NUM_THREADS = 100; public static void main(string[] args) throws InterruptedException { Bank bank = new Bank(); Thread[] threads = new Deposit[NUM_THREADS]; for (int i = 0; i < NUM_THREADS; i++) { threads[i] = new Deposit(bank, 1000 / NUM_THREADS); for (Thread t : threads) { System.out.println("" + t); t.start(); for (Thread t : threads) { t.join(); System.out.println("Balance: " + bank.getbalance()); class Deposit extends Thread { private Bank bank; private int n; public Deposit(Bank b, int n) { bank = b; this.n = n;

6 coe518 (Operating Systems) Lecture Notes: Week 7 Page 6 of public void run() { for (int i = 0; i < n; i++) { System.out.println("" + Thread.currentThread().getName()); bank.deposit(1000); The wait()and notify() methods Example: Queue public class Cell { Cell next; Object ob; public Cell(Object ob) { this.ob = ob; class Queue { private Cell head; private Cell tail; public synchronized void add(object ob) { Cell toadd = new Cell(ob); if (tail == null) {

7 coe518 (Operating Systems) Lecture Notes: Week 7 Page 7 of 10 head = toadd; else { tail.next = toadd; toadd.next = null; tail = toadd; notifyall(); public synchronized Object take() throws InterruptedException { while (head == null) { wait(); Cell gone = head; head = head.next; if (head == null) { tail = null; return gone.ob; Volatile Consider: int value; value = 5; while(true) { System.out.println(value);

8 coe518 (Operating Systems) Lecture Notes: Week 7 Page 8 of 10 Deadlock Example public class Deadlock { static class Friend { private final String name; public Friend(String name) { this.name = name; public String getname() { return this.name; public synchronized void bow(friend bower) { System.out.format( "%s: %s has bowed to me!%n", this.name, bower.getname()); bower.bowback(this); public synchronized void bowback( Friend bower) { System.out.format( "%s: %s has bowed back to me!%n", this.name, bower.getname()); public static void main(string[] args) { final Friend alphonse = new Friend( "Alphonse"); final Friend gaston = new Friend("Gaston"); new Thread(new Runnable() { public void run() { alphonse.bow(gaston); ).start();

9 coe518 (Operating Systems) Lecture Notes: Week 7 Page 9 of 10 new Thread(new Runnable() { public void run() { gaston.bow(alphonse); ).start(); Questions 1. Answers 1. a. The sequence a requests and obtains P a requests and obtains Q b requests Q and blocks a releases Q b unblocks a releases P and exits successfully b requests and obtains P b does its work, releases P and Q and exits successfully b The sequence: a requests and obtains P b requests and obtains Q a requests Q and blocks b requests P and blocks ---> Deadlock!

10 coe518 (Operating Systems) Lecture Notes: Week 7 Page 10 of Simply protect the body of f as follows: void f() { semaphorewwait(my_special); //critical section of f... semaphoresignal(my_special);