CMSC 330: Organization of Programming LanguagesMultithreaded Programming in Java2Multiprocessors• Description – Multiple processing units (multiprocessor)– From single microprocessor to large compute clusters– Can perform multiple tasks in parallel simultaneously106K processor IBM BlueGene/L32 processor Pentium XeonDual-core AMD AthlonX23Computation AbstractionsCPU 1 CPU 2p3p1 p2 p4t1t2t1t2t3t1t4t5A computerProcesses(e.g., JVM’s)Threads4Processes vs. Threadsint x;foo() {…x…}int x;foo() {…x…}int x;foo() {…x…}foo() {…x…}Processes do notshare dataThreads share datawithin a process5So, What Is a Thread?• Conceptually– Parallel computation occurring within a process• Implementation view– Program counter and stack– Heap and static area shared among all threads• All programs have at least one thread (main)6Implementation View• Per-thread stack and instruction pointer– Saved in memory when thread suspended– Put in hardware esp/eip when thread resumeseipeipeipespespesp7Tradeoffs• 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 usage8Programming Threads• Threads are available in many languages– C, C++, OCaml, Java, Ruby, …• In many languages (e.g., C and C++), threads are a platform specific add-on– Not part of the language specification– Implemented as code libraries (e.g., pthreads)• They're part of the Java language specification9Java 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.10Thread Creationexecution (time)main threadthread startsthread startsthread endsthreadjoin11Thread 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 returns12Running 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 seconds13Example: 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);}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);}}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();}}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)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);}}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();}}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 class20Concurrency• 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 now21Scheduling Example (1)CPU 1CPU 2p1p2p1p2One process per CPUp2 threads:p1 threads:22Scheduling Example (2)CPU 1CPU 2p1p2p1p2Threads shared between CPU’sp2 threads:p1 threads:23Concurrency and Shared Data• Concurrency is easy if threads don’t interact– Each thread does its own thing, ignoring other threads– Typically, however, threads must communicate• In Java, communication via shared data– Static and heap data can be accessed by all threads– Called shared memory concurrency• Potential pitfalls– Different threads may access the heap simultaneously– But the scheduler might interleave threads arbitrarily– Problems can occur if we’re not careful.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();}}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 state26Data 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 = 027Data Race Examplestatic int cnt = 0;t1.run() {int y = cnt;cnt = y + 1;}t2.run() {int y =
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