Multithreaded Programming using Java Threads Prof Rajkumar Buyya

Multithreaded Programming using Java Threads Prof. Rajkumar Buyya Cloud Computing and Distributed Systems (CLOUDS) Laboratory Dept. of Computer Science and Software Engineering University of Melbourne, Australia http: //www. cloudbus. org/~raj or http: //www. buyya. com 1

Agenda n n Introduction Thread Applications Defining Threads Java Threads and States n n Accessing Shared Resources n n Synchronisation Assignment 1: n n Priorities Multi-Threaded Math Server Advanced Issues: n n Concurrency Models: master/worker, pipeline, peer processing Multithreading Vs multiprocessing 2

A single threaded program class ABC { …. public void main(. . ) { …. . } begin body end } 3

A Multithreaded Program Main Thread start Thread A start Thread B Thread C Threads may switch or exchange data/results 4

Single and Multithreaded Processes threads are light-weight processes within a process Single-threaded Process Multiplethreaded Process Threads of Execution Multiple instruction stream Single instruction stream Common Address Space 5

Multithreaded Server: For Serving Multiple Clients Concurrently Server Process Client 1 Process Server Threads n Internet Client 2 Process 6

Web/Internet Applications: Serving Many Users Simultaneously PC client Internet Server Local Area Network PDA 7

Modern Applications need Threads (ex 1): Editing and Printing documents in background. Printing Thread Editing Thread 8

Multithreaded/Parallel File Copy reader() { - - - - lock(buff[i]); read(src, buff[i]); unlock(buff[i]); - - - - } buff[0] buff[1] writer() { - - - - - lock(buff[i]); write(src, buff[i]); unlock(buff[i]); - - - - - } Cooperative Parallel Synchronized Threads 9

Levels of Parallelism Sockets/ PVM/MPI Threads Compilers CPU Task i-l func 1 ( ) {. . . . } a ( 0 ) =. . b ( 0 ) =. . + Task i func 2 ( ) {. . . . } a ( 1 )=. . b ( 1 )=. . x Task i+1 func 3 ( ) {. . . . } a ( 2 )=. . b ( 2 )=. . Load Code-Granularity Code Item Large grain (task level) Program Medium grain (control level) Function (thread) Fine grain (data level) Loop (Compiler) Very fine grain (multiple issue) With hardware 10

What are Threads? n n A piece of code that run in concurrent with other threads. Each thread is a statically ordered sequence of instructions. Threads are being extensively used express concurrency on both single and multiprocessors machines. Programming a task having multiple threads of control – Multithreading or Multithreaded Programming. 11

Java Threads n n Java has built in thread support for Multithreading Synchronization Thread Scheduling Inter-Thread Communication: n n n current. Thread yield sleep resume start run stop set. Priority get. Priority suspend Java Garbage Collector is a low-priority thread. 12

Threading Mechanisms. . . n n Create a class that extends the Thread class Create a class that implements the Runnable interface Thread My. Thread (objects are threads) [a] Runnable Thread My. Class (objects with run() body) [b] 13

1 st method: Extending Thread class n Create a class by extending Thread class and override run() method: class My. Thread extends Thread { public void run() { // thread body of execution } } n n n Create a thread: My. Thread thr 1 = new My. Thread(); Start Execution of threads: thr 1. start(); Create and Execute: new My. Thread(). start(); 14

An example class My. Thread extends Thread { public void run() { System. out. println(" this thread is running. . . "); } } class Thread. Ex 1 { public static void main(String [] args ) { My. Thread t = new My. Thread(); t. start(); } } 15

2 nd method: Threads by implementing Runnable interface n Create a class that implements the interface Runnable and override run() method: class My. Thread implements Runnable {. . . public void run() { // thread body of execution } } n Creating Object: My. Thread my. Object = new My. Thread(); n Creating Thread Object: Thread thr 1 = new Thread( my. Object ); n Start Execution: thr 1. start(); 16

An example class My. Thread implements Runnable { public void run() { System. out. println(" this thread is running. . . "); } } class Thread. Ex 2 { public static void main(String [] args ) { Thread t = new Thread(new My. Thread()); t. start(); } } 17

Life Cycle of Thread new start() I/O completed ready notify() waiting wait() resume() Time expired/ interrupted sleeping blocked dispatch sleep() running suspend() Block on I/O completion stop() dead 18

A Program with Three Java Threads n Write a program that creates 3 threads 19

Three threads example n n n n n n class A extends Thread { public void run() { for(int i=1; i<=5; i++) { System. out. println("t From Thread. A: i= "+i); } System. out. println("Exit from A"); } } class B extends Thread { public void run() { for(int j=1; j<=5; j++) { System. out. println("t From Thread. B: j= "+j); } System. out. println("Exit from B"); } } 20

n n n n class C extends Thread { public void run() { for(int k=1; k<=5; k++) { System. out. println("t From Thread. C: k= "+k); } n n n } } System. out. println("Exit from C"); class Thread. Test { public static void main(String args[]) { new A(). start(); new B(). start(); new C(). start(); } } 21

Run 1 [raj@mundroo] threads [1: 76] java Thread. Test From Thread. A: i= 1 From Thread. A: i= 2 From Thread. A: i= 3 From Thread. A: i= 4 From Thread. A: i= 5 Exit from A From Thread. C: k= 1 From Thread. C: k= 2 From Thread. C: k= 3 From Thread. C: k= 4 From Thread. C: k= 5 Exit from C From Thread. B: j= 1 From Thread. B: j= 2 From Thread. B: j= 3 From Thread. B: j= 4 From Thread. B: j= 5 Exit from B n 22

Run 2 [raj@mundroo] threads [1: 77] java Thread. Test From Thread. A: i= 1 From Thread. A: i= 2 From Thread. A: i= 3 From Thread. A: i= 4 From Thread. A: i= 5 From Thread. C: k= 1 From Thread. C: k= 2 From Thread. C: k= 3 From Thread. C: k= 4 From Thread. C: k= 5 Exit from C From Thread. B: j= 1 From Thread. B: j= 2 From Thread. B: j= 3 From Thread. B: j= 4 From Thread. B: j= 5 Exit from B Exit from A n 23

Process Parallelism n n int add (int a, int b, int & result) // function stuff int sub(int a, int b, int & result) // function stuff Processor IS 1 pthread t 1, t 2; pthread-create(&t 1, add, a, b, & r 1); pthread-create(&t 2, sub, c, d, & r 2); pthread-par (2, t 1, t 2); add Processor IS 2 sub Data a b r 1 c d r 2 MISD and MIMD Processing 24

Data Parallelism n n n Data sort( int *array, int count) //. . . pthread-t, thread 1, thread 2; “ “ pthread-create(& thread 1, sort, array, N/2); pthread-create(& thread 2, sort, array, N/2); pthread-par(2, thread 1, thread 2); Processor Sort IS Processor Sort SIMD Processing do “ “ dn/2 dn 2/+1 “ “ dn 25

Thread Priority n In Java, each thread is assigned priority, which affects the order in which it is scheduled for running. The threads so far had same default priority (NORM_PRIORITY) and they are served using FCFS policy. n Java allows users to change priority: n Thread. Name. set. Priority(int. Number) n n n MIN_PRIORITY = 1 NORM_PRIORITY=5 MAX_PRIORITY=10 26

Thread Priority Example class A extends Thread { public void run() { System. out. println("Thread A started"); for(int i=1; i<=4; i++) { System. out. println("t From Thread. A: i= "+i); } System. out. println("Exit from A"); } } class B extends Thread { public void run() { System. out. println("Thread B started"); for(int j=1; j<=4; j++) { System. out. println("t From Thread. B: j= "+j); } System. out. println("Exit from B"); } } 27

Thread Priority Example class C extends Thread { public void run() { System. out. println("Thread C started"); for(int k=1; k<=4; k++) { System. out. println("t From Thread. C: k= "+k); } System. out. println("Exit from C"); } } class Thread. Priority { public static void main(String args[]) { A thread. A=new A(); B thread. B=new B(); C thread. C=new C(); thread. C. set. Priority(Thread. MAX_PRIORITY); thread. B. set. Priority(thread. A. get. Priority()+1); thread. A. set. Priority(Thread. MIN_PRIORITY); System. out. println("Started Thread A"); thread. A. start(); System. out. println("Started Thread B"); thread. B. start(); System. out. println("Started Thread C"); thread. C. start(); System. out. println("End of main thread"); } } 28

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Multithreaded Server Client Process Server Threads Client Process User Mode Kernel Mode Message Passing Facility 30

Assignment 1: Multithreaded Math. Server – Demonstrates the use of Sockets and Threads A Client Program What is sqrt(10)? “sqrt 4. 0” “ 2. 0” Multithreaded Math. Server (sin, cos, sqrt, etc. ) A Client Program What is sin(10)? A Client Program in “C” What is sin(10)? A Client Program in “C++” What is sin(10)? 31

A Multithreaded Program Math. Threads start Math. Sin start Math. Cos join Math. Tan join Math. Threads 32

References n n Rajkumar Buyya, Thamarai Selvi, Xingchen Chu, Mastering OOP with Java, Mc. Graw Hill (I) Press, New Delhi, India, 2009. Sun Java Tutorial – Concurrency: n http: //java. sun. com/docs/books/tutorial/esse ntial/concurrency/ 33