COP 4020 Programming Languages Parallel Programming in Ada and Java

1. COP 4020Programming LanguagesParallel Programming in Ada and Java Gregory A. Riccardi Department of Computer Science Florida State University December 1, 1998

2. Two Strategies for Parallelism • Ada, task synchronization • Tasks execute independently • Rendezvous provides synchronized execution between 2 tasks • Java, method and object synchronization • Threads execute independently • Synchronized methods are executed as critical sections • only one execution per synch. method • Synchronized statements depend on objects • one execution per object of any statement

3. Tasks in Ada • Ada supports a program unit declaration of tasks • Each task object represents a separate unit of execution • Tasks can share objects through usual global packages and nesting • Task T is . . . end; -- declaration of a single task object • Task type TT is . . . end; -- type for many objects • T1: TT; T2: TT; -- T1 and T2 are task objects • Example using global variables g and h • task type TT ; • task body TT is x: integer; begin x := g; h := x; end; • -- each task of type TT uses global g • T1, T2: TT; g:= 4; g := 5; g:=6; • what is value of h? • this is a race condition. • type ptrT is access T;

4. Synchronization and Communication • Synchronization is required to eliminate race conditions. • Ada tasks synchronize by 'rendezvous' • Two tasks , one is caller, one is receiver • Caller requests a meeting for a specific purpose • Receiver offers to accept a meeting for any of a specific collection of purposes • Meeting takes place: • Caller passes information to receiver and waits • Receiver gets information, executes by itself, passes information back and continues its execution • Caller gets information from receiver and continues • Place (or purpose) is called entry like procedure specification • Call is an entry call, syntax exactly like procedure call • Receive is an accept, very much like a procedure body

5. Task Execution and Termination • declare -- beginning of block • T1: TT; -- T1 gets ready to execute, stack created • begin -- T1 begins its execution here • <statements of block> -- T1 executes in parallel with this code • end; -- block is not completely finished until T1 finishes. • declare -- beginning of block • Tptr: TT * = new TT; -- *Tptr gets ready to execute, and begins • begin • <statements of block> -- T1 executes in parallel with this code • end; -- block does not wait for *Tptr to finish. • Terminate T1; -- immediately terminates execution of T1

6. Storage Management for Tasks • Does each task need its own stack? • Each task has access to other stacks appropriate to its nesting • Guarantees of mutual exclusion • Two tasks that wish to access shared objects • Access only during rendezvous, receiver accesses object and passes value to caller • Use rendezvous to synchronize • Caller references variable, then makes entry call • Receiver waits to reference until it has received a call • The accept statement is guaranteed to execute while the caller is blocked. • Create a third task to provide access to shared data • New task is a monitor. • Create a message passing task to send/receive messages

7. Threads in Java • class MyThread extends Thread { public void run() {// code for execution • MyThread threadVar = new MyThread();threadVar.start(); • class MyRunner implements Runnable { public void run() { // code for execution • MyRunner runVar = new MyRunner();new Thread(runVar).start(); • Other methods • stop, sleep, suspend, resume, yield, destroy

8. Synchronization in Java • Synchronization useslocks and monitors • Objects can be locked • Methods can have exclusive execution • synchronized statement • synchronized (expression) statement • obtain lock for object or array then execute stmt • Example of sorting a shared array • synchronized (a) {//sort array here…} • synchronized modifier • method that is executed exclusively • class MyObject { • public synchronized void update() ...

9. Distributed Objects in Java • Java supports communication between programs • java.net.Socket and Java.net.ServerSocket • To send objects to another program • Socket sock = new Socket(host,port);ObjectOutputStream out = new ObjectOutputStream (sock.getOutputStream();out.writeObject(out); // object is transferred! • To receive object, • create a server socket • open ObjectInputStream • readObject