1CMSC 330: Organization of Programming LanguagesThreadsCMSC 330 2Reminders• Homework 2 due on Oct. 30• Project 3 due Oct. 31• Midterm 2 on Nov. 1• Done with OCaml… now onto Threads…CMSC 330 3Computation AbstractionsCPU 1 CPU 2p3p1 p2 p4t1t2t1t2t3t1t4t5A computerProcesses(e.g., JVM’s)ThreadsCMSC 330 4Processes vs. Threadsint x;foo() {…x…}int x;foo() {…x…}int x;foo() {…x…}foo() {…x…}Processes do notshare dataThreads share datawithin a processCMSC 330 5So, What Is a Thread?• Conceptually: it is a parallel computation occurring within a process• Implementation view: it’s a program counter and a stack. The heap and static area are shared among all threads• All programs have at least one thread (main)CMSC 330 6Implementation View• Per-thread stack and instruction pointer– Saved in memory when thread suspended– Put in hardware esp/eip when thread resumeseipeipeipespespesp2CMSC 330 7Tradeoffs• Threads can increase performance– Parallelism on multiprocessors– Concurrency of computation and I/O• Natural fit for some programming patterns– Event processing– Simulations• But increased complexity– Need to worry about safety, liveness, composition• And higher resource usageCMSC 330 8Programming Threads• Threads are available in many languages– C, C++, Objective Caml, Java, SmallTalk …• In many languages (e.g., C and C++), threads are a platform specific add-on– Not part of the language specification• They're part of the Java language specificationCMSC 330 9Java Threads• Every application has at least one thread– The “main” thread, started by the JVM to run the application’s main() method•main()can create other threads– Explicitly, using the Thread class– Implicitly, by calling libraries that create threads as a consequence• RMI, AWT/Swing, Applets, etc.CMSC 330 10Thread Creationexecution (time)main threadthread startsthread startsthread endsthreadjoinCMSC 330 11Thread Creation in Java• To explicitly create a thread:– Instantiate a Thread object• An object of class Thread or a subclass of Thread– Invoke the object’s start() method• This will start executing the Thread’s run() method concurrently with the current thread – Thread terminates when its run() method returnsCMSC 330 12Running Example: Alarms• Goal: let's set alarms which will be triggered in the future– Input: time t (seconds) and message m– Result: we’ll see m printed after t seconds3CMSC 330 13Example: Synchronous alarmswhile (true) {System.out.print("Alarm> ");// read user inputString line = b.readLine();parseInput(line); // sets timeout// wait (in secs)try {Thread.sleep(timeout * 1000);} catch (InterruptedException e) { }System.out.println("("+timeout+") "+msg);}like phone callsthrown when another thread calls interruptCMSC 330 14Making It Threaded (1)public class AlarmThread extends Thread {private String msg = null;private int timeout = 0;public AlarmThread(String msg, int time) {this.msg = msg;this.timeout = time;}public void run() {try {Thread.sleep(timeout * 1000);} catch (InterruptedException e) { }System.out.println("("+timeout+") "+msg);}}CMSC 330 15Making It Threaded (2)while (true) {System.out.print("Alarm> ");// read user inputString line = b.readLine();parseInput(line);if (m != null) {// start alarm threadThread t = new AlarmThread(m,tm);t.start();}}CMSC 330 16Alternative: The Runnable Interface• Extending Thread prohibits a different parent• Instead implement Runnable– Declares that the class has a void run() method• Construct a Thread from the Runnable– Constructor Thread(Runnable target)– Constructor Thread(Runnable target, String name)CMSC 330 17Thread Example Revisitedpublic class AlarmRunnable implements Runnable {private String msg = null;private int timeout = 0;public AlarmRunnable(String msg, int time) {this.msg = msg;this.timeout = time;}public void run() {try {Thread.sleep(timeout * 1000);} catch (InterruptedException e) { }System.out.println("("+timeout+") "+msg);}}CMSC 330 18Thread Example Revisited (2)while (true) {System.out.print("Alarm> ");// read user inputString line = b.readLine();parseInput(line);if (m != null) {// start alarm threadThread t = new Thread(new AlarmRunnable(m,tm));t.start();}}4CMSC 330 19Notes: Passing Parameters• run() doesn’t take parameters• We “pass parameters” to the new thread by storing them as private fields– In the extended class– Or the Runnable object– Example: the time to wait and the message to print in the AlarmThread classCMSC 330 20Concurrency•A concurrent program is one that has multiple threads that may be active at the same time– Might run on one CPU• The CPU alternates between running different threads•The scheduler takes care of the details– Switching between threads might happen at any time– Might run in parallel on a multiprocessor machine• One with more than one CPU• May have multiple threads per CPU• Multiprocessor machines are becoming more common– Multi-CPU machines aren't that expensive any more– Dual-core CPUs are available nowCMSC 330 21Scheduling Example (1)CPU 1CPU 2p1p2p1p2One process per CPUp2 threads:p1 threads:CMSC 330 22Scheduling Example (2)CPU 1CPU 2p1p2p1p2Threads shared between CPUsp2 threads:p1 threads:CMSC 330 23Concurrency and Shared Data• Concurrency is easy if threads don’t interact– Each thread does its own thing, ignoring other threads– Typically, however, threads need to communicate with each other• Communication is done by sharing data– In Java, different threads may access the heap simultaneously– But the scheduler might interleave threads arbitrarily– Problems can occur if we’re not careful.CMSC 330 24Data Race Examplepublic class Example extends Thread {private static int cnt = 0; // shared statepublic void run() {int y = cnt;cnt=y+1;}public static void main(String args[]) {Thread t1 = new Example();Thread t2 = new Example();t1.start();t2.start();}}5CMSC 330 25Data Race Examplestatic int cnt = 0;t1.run() {int y = cnt;cnt = y + 1;}t2.run() {int y = cnt;cnt = y + 1;}cnt = 0Start: both threads ready torun. Each will increment theglobal cnt. Shared stateCMSC 330 26Data Race Examplestatic int cnt = 0;t1.run() {int y = cnt;cnt = y + 1;}t2.run() {int y = cnt;cnt = y + 1;}cnt = 0T1 executes, grabbingthe global counter value intoits own y.Shared statey = 0CMSC 330 27Data Race Examplestatic int cnt = 0;t1.run() {int y = cnt;cnt = y + 1;}t2.run() {int y =
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