Multithreaded Programming using Java Threads Rajkumar Buyya Grid

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

Agenda n n n Introduction Thread Applications Defining Threads Java Threads and States Examples 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 13

1 st method: Extending Thread class n Threads are implemented as objects that contains a method called run() 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 { // the thread public void run() { System. out. println(" this thread is running. . . "); } } // end class My. Thread class Thread. Ex 1 { // a program that utilizes the thread public static void main(String [] args ) { My. Thread t = new My. Thread(); // due to extending the Thread class (above) // I can call start(), and this will call // run(). start() is a method in class Thread. t. start(); } // end main() } // end class Thread. Ex 1 15

2 nd method: Threads by implementing Runnable interface 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. . . "); } } // end class My. Thread class Thread. Ex 2 { public static void main(String [] args ) { Thread t = new Thread(new My. Thread()); // due to implementing the Runnable interface // I can call start(), and this will call run(). t. start(); } // end main() } // end class Thread. Ex 2 17

Life Cycle of Thread new start() wait() sleep() suspend() blocked runnable stop() dead non-runnable notify() slept resume() unblocked 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

Next Class n n Thread Priorities Thread Synchronisation 26

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 27

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"); } } 28

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"); } } 29

Accessing Shared Resources n Applications Access to Shared Resources need to be coordinated. n n n Printer (two person jobs cannot be printed at the same time) Simultaneous operations on your bank account. Can the following operations be done at the same time on the same account? n n n Deposit() Withdraw() Enquire() 30

Online Bank: Serving Many Customers and Operations PC client Internet Bank Server Local Area Network Bank Database PDA 31

Shared Resources n n n If one thread tries to read the data and other thread tries to update the same date, it leads to inconsistent state. This can be prevented by synchronising access to the data. Use “Synchronized” method: n n public synchronized void update() { n n … } 32

the driver: 3 rd Threads sharing the same object class Internet. Banking. System { public static void main(String [] args ) { Account account. Object = new Account (); Thread t 1 = new Thread(new My. Thread(account. Object)); Thread t 2 = new Thread(new Your. Thread(account. Object)); Thread t 3 = new Thread(new Her. Thread(account. Object)); t 1. start(); t 2. start(); t 3. start(); // DO some other operation } // end main() } 33

Shared account object between 3 threads class My. Thread implements Runnable { Account account; public My. Thread (Account s) { account = s; } public void run() { account. deposit(); } } // end class My. Thread class Your. Thread implements Runnable { Account account; public Your. Thread (Account s) { account = s; } public void run() { account. withdraw(); } } // end class Your. Thread class Her. Thread implements Runnable { Account account; public Her. Thread (Account s) { account = s; } public void run() {account. enquire(); } } // end class Her. Thread account (shared object) 34

Monitor (shared object access): serializes operation on shared object class Account { // the 'monitor' int balance; // if 'synchronized' is removed, the outcome is unpredictable public synchronized void deposit( ) { // METHOD BODY : balance += deposit_amount; } } public synchronized void withdraw( ) { // METHOD BODY: balance -= deposit_amount; } public synchronized void enquire( ) { // METHOD BODY: display balance. } 35

Multithreaded Server Client Process Server Threads Client Process User Mode Kernel Mode Message Passing Facility 36

Assignment 1: Multithreaded Math. Server – Demonstrate the use 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)? 37

Thread Programming models Thread concurrency/operation n The master/worker model The peer model A thread pipeline 38

The master/worker model Program Resources Workers task. X Files Databases Master task. Y Input (Stream) main ( ) Disks task. Z Special Devices 39

Example n n n n n n main() /* the master */ { forever { get a request; switch( request ) case X: pthread_create(. . , task. X); case Y: pthread_create(. . , task. Y); . . } } task. X() /* worker */ { perform the task, sync if accessing shared resources } task. Y() /* worker */ { perform the task, sync if accessing shared resources }. . --Above runtime overhead of creating thread can be solved by thread pool * the master thread creates all worker thread at program initialization and each worker thread suspends itself immediately for a wakeup call from the master 40

The peer model Program Input Resources Workers task. X Files Databases task. Y Disks task. Z Special Devices 41

Example n n n n n main() { pthread_create(. . , thread 1. . . task. X); pthread_create(. . , thread 2. . . task. Y); . . signal all workers to start wait for all workers to finish do any cleanup } } task. X() /* worker */ { wait for start perform the task, sync if accessing shared resources } task. Y() /* worker */ { wait for start perform the task, sync if accessing shared resources } 42

A thread pipeline Program Filter Threads Stage 1 Stage 2 Stage 3 Input (Stream) Resources Files Databases Disks Special Devices 43

Example main() { pthread_create(. . , stage 1); pthread_create(. . , stage 2); . . wait for all pipeline threads to finish do any cleanup } stage 1() { get next input for the program do stage 1 processing of the input pass result to next thread in pipeline } stage 2(){ get input from previous thread in pipeline do stage 2 processing of the input pass result to next thread in pipeline } stage. N() { get input from previous thread in pipeline do stage N processing of the input pass result to program output. } 44

Multithreading and Multiprocessing Deployment issues n On Shared and distributed memory systems 45

Multithreading - Multiprocessors Process Parallelism CPU P 1 P 2 CPU P 3 CPU time No of execution process more the number of CPUs 46

Multithreading on Uni-processor n Concurrency Vs Parallelism K Process Concurrency P 1 P 2 CPU P 3 time Number of Simultaneous execution units > number of CPUs 47

Multi-Processing (clusters & grids) and Multi-Threaded Computing Threaded Libraries, Multi-threaded I/O Application CPU CPU Better Response Times in Multiple Application Environments CPU CPU Higher Throughput for Parallelizeable Applications 48