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  1. 1. 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
  2. 2. Agenda     Introduction Thread Applications Defining Threads Java Threads and States   Accessing Shared Resources   Synchronisation Assignment 1:   Priorities Multi-Threaded Math Server Advanced Issues:   Concurrency Models: master/worker, pipeline, peer processing Multithreading Vs multiprocessing 2
  3. 3. A single threaded program class ABC { …. public void main(..) { … .. } begin body end } 3
  4. 4. A Multithreaded Program Main Thread start Thread A start start Thread B Thread C Threads may switch or exchange data/results 4
  5. 5. Single and Multithreaded Processes threads are light-weight processes within a process Single-threaded Process Multiplethreaded Threads of Process Execution Multiple instruction stream Single instruction stream Common Address Space 5
  6. 6. Multithreaded Server: For Serving Multiple Clients Concurrently Server Process Client 1 Process Server Threads  Internet Client 2 Process 6
  7. 7. Web/Internet Applications: Serving Many Users Simultaneously PC client Internet Server Local Area Network PDA 7
  8. 8. Modern Applications need Threads (ex1): Editing and Printing documents in background. Printing Thread Printing Thread Editing Thread Editing Thread 8
  9. 9. Multithreaded/Parallel File Copy reader() reader() {{ -- -- -- -- -- -- -- -- --lock(buff[i]); lock(buff[i]); read(src,buff[i]); read(src,buff[i]); unlock(buff[i]); unlock(buff[i]); -- -- -- -- -- -- -- -- --}} buff[0] buff[0] buff[1] buff[1] writer() writer() {{ -- -- -- -- -- -- -- -- -- -lock(buff[i]); lock(buff[i]); write(src,buff[i]); write(src,buff[i]); unlock(buff[i]); unlock(buff[i]); -- -- -- -- -- -- -- -- -- -}} Cooperative Parallel Synchronized Cooperative Parallel Synchronized Threads Threads 9
  10. 10. Levels of Parallelism Sockets/ PVM/MPI Threads Compilers CPU Task i-l Task i-l func1 ( () ) func1 {{ .... .... .... .... }} aa( (00) )=.. =.. bb( (00) )=.. =.. + + Task ii Task func2 ( () ) func2 {{ .... .... .... .... }} aa( (11)=.. )=.. bb( (11)=.. )=.. x x Task i+1 Task i+1 func3 ( () ) func3 {{ .... .... .... .... }} aa( (22)=.. )=.. bb( (22)=.. )=.. Load Load Code-Granularity Code-Granularity Code Item Code Item Large grain Large grain (task level) (task level) Program Program Medium grain Medium grain (control level) (control level) Function (thread) Function (thread) Fine grain Fine grain (data level) (data level) Loop (Compiler) Loop (Compiler) Very fine grain Very fine grain (multiple issue) (multiple issue) With hardware With hardware 10
  11. 11. What are Threads?     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
  12. 12. Java Threads     Java has built in thread support for Multithreading Synchronization Thread Scheduling Inter-Thread Communication:      currentThread yield sleep resume start run stop setPriority getPriority suspend Java Garbage Collector is a low-priority thread. 12
  13. 13. Threading Mechanisms...   Create a class that extends the Thread class Create a class that implements the Runnable interface Thread MyThread (objects are threads) [a] Runnable Thread MyClass (objects with run() body) [b] 13
  14. 14. 1st method: Extending Thread class  Create a class by extending Thread class and override run() method: class MyThread extends Thread { public void run() { // thread body of execution } }    Create a thread: MyThread thr1 = new MyThread(); Start Execution of threads: thr1.start(); Create and Execute: new MyThread().start(); 14
  15. 15. An example class MyThread extends Thread { public void run() { System.out.println(" this thread is running ... "); } } class ThreadEx1 { public static void main(String [] args ) { MyThread t = new MyThread(); t.start(); } } 15
  16. 16. 2nd method: Threads by implementing Runnable interface  Create a class that implements the interface Runnable and override run() method: class MyThread implements Runnable { ..... public void run() { // thread body of execution } }  Creating Object: MyThread myObject = new MyThread();  Creating Thread Object: Thread thr1 = new Thread( myObject );  Start Execution: thr1.start(); 16
  17. 17. An example class MyThread implements Runnable { public void run() { System.out.println(" this thread is running ... "); } } class ThreadEx2 { public static void main(String [] args ) { Thread t = new Thread(new MyThread()); t.start(); } } 17
  18. 18. Life Cycle of Thread new start() I/O completed ready notify() waiting wait() resume() Time expired/ interrupted sleeping blocked dispatch sleep() running completion stop() suspend() Block on I/O dead 18
  19. 19. A Program with Three Java Threads  Write a program that creates 3 threads 19
  20. 20. Three threads example                       class A extends Thread { public void run() { for(int i=1;i<=5;i++) { System.out.println("t From ThreadA: 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 ThreadB: j= "+j); } System.out.println("Exit from B"); } } 20
  21. 21.         class C extends Thread { public void run() { for(int k=1;k<=5;k++) { System.out.println("t From ThreadC: k= "+k); }             } } System.out.println("Exit from C"); class ThreadTest { public static void main(String args[]) { new A().start(); new B().start(); new C().start(); } } 21
  22. 22. Run 1 [raj@mundroo] threads [1:76] java ThreadTest From ThreadA: i= 1 From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5 Exit from A From ThreadC: k= 1 From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5 Exit from C From ThreadB: j= 1 From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5 Exit from B  22
  23. 23. Run2 [raj@mundroo] threads [1:77] java ThreadTest From ThreadA: i= 1 From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5 From ThreadC: k= 1 From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5 Exit from C From ThreadB: j= 1 From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5 Exit from B Exit from A  23
  24. 24. Process Parallelism     int add (int a, int b, int & result) // function stuff int sub(int a, int b, int & result) // function stuff Processor IS1 pthread t1, t2; pthread t1, t2; pthread-create(&t1, add, a,b, & r1); pthread-create(&t1, add, a,b, & r1); pthread-create(&t2, sub, c,d, & r2); pthread-create(&t2, sub, c,d, & r2); pthread-par (2, t1, t2); pthread-par (2, t1, t2); add add Processor IS2 sub sub Data a a b b r1 r1 c c d d r2 r2 MISD and MIMD Processing 24
  25. 25. Data Parallelism    Data sort( int *array, int count) //...... //...... pthread-t, thread1, thread2; pthread-t, thread1, thread2; ““ ““ pthread-create(& thread1, sort, array, N/2); pthread-create(& thread1, sort, array, N/2); pthread-create(& thread2, sort, array, N/2); pthread-create(& thread2, sort, array, N/2); pthread-par(2, thread1, thread2); pthread-par(2, thread1, thread2); Processor Sort Sort IS Processor Sort Sort SIMD Processing do “ “ dn/2 dn2/+1 “ “ dn 25
  26. 26. Thread Priority  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.  Java allows users to change priority:  ThreadName.setPriority(intNumber)    MIN_PRIORITY = 1 NORM_PRIORITY=5 MAX_PRIORITY=10 26
  27. 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 ThreadA: 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 ThreadB: j= "+j); } System.out.println("Exit from B"); } } 27
  28. 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 ThreadC: k= "+k); } System.out.println("Exit from C"); } } class ThreadPriority { public static void main(String args[]) { A threadA=new A(); B threadB=new B(); C threadC=new C(); threadC.setPriority(Thread.MAX_PRIORITY); threadB.setPriority(threadA.getPriority()+1); threadA.setPriority(Thread.MIN_PRIORITY); System.out.println("Started Thread A"); threadA.start(); System.out.println("Started Thread B"); threadB.start(); System.out.println("Started Thread C"); threadC.start(); System.out.println("End of main thread"); } } 28
  29. 29. Accessing Shared Resources  Applications Access to Shared Resources need to be coordinated.    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?    Deposit() Withdraw() Enquire() 29
  30. 30. Online Bank: Serving Many Customers and Operations PC client Internet Bank Server Local Area Network Bank Database PDA 30
  31. 31. Shared Resources    If one thread tries to read the data and other thread tries to update the same data, it leads to inconsistent state. This can be prevented by synchronising access to the data. Use “Synchronized” method:   public synchronized void update() {   … } 31
  32. 32. the driver: 3rd Threads sharing the same object class InternetBankingSystem { public static void main(String [] args ) { Account accountObject = new Account (); Thread t1 = new Thread(new MyThread(accountObject)); Thread t2 = new Thread(new YourThread(accountObject)); Thread t3 = new Thread(new HerThread(accountObject)); t1.start(); t2.start(); t3.start(); // DO some other operation } // end main() } 32
  33. 33. Shared account object between 3 threads class MyThread implements Runnable { Account account; public MyThread (Account s) { account = s;} public void run() { account.deposit(); } } // end class MyThread class YourThread implements Runnable { Account account; public YourThread (Account s) { account = s;} public void run() { account.withdraw(); } } // end class YourThread class HerThread implements Runnable { Account account; public HerThread (Account s) { account = s; } public void run() {account.enquire(); } } // end class HerThread account (shared object) 33
  34. 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. } 34
  35. 35. Multithreaded Server Multithreaded Server Client Process Server Process Server Threads Client Process User Mode Kernel Mode Message Passing Facility 35
  36. 36. Assignment 1: Multithreaded MathServer – Demonstrates the use of Sockets and Threads A Client Program What is sqrt(10)? “sqrt 4.0” “2.0” Multithreaded MathServer (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)? 36
  37. 37. A Multithreaded Program MathThreads start MathSin start start MathCos join join MathTan join MathThreads 37
  38. 38. Thread Programming models Thread concurrency/operation models    The master/worker model The peer model A thread pipeline 38
  39. 39. The master/worker model Program Resources Workers taskX taskX Files Databases Master Input (Stream) main ( ( ) main ) taskY taskY Disks taskZ taskZ Special Devices 39
  40. 40. The peer model Program Input Input Resources Workers taskX taskX Files Databases taskY taskY Disks taskZ taskZ Special Devices 40
  41. 41. A thread A thread pipeline pipeline Program Filter Threads Stage 11 Stage Stage 22 Stage Stage 33 Stage Input (Stream) Resources Files Files Files Databases Databases Databases Disks Special Devices Disks Special Devices Disks Special Devices 41
  42. 42. Multithreading and Multiprocessing Deployment issues On Shared and distributed memory systems 42
  43. 43. Multithreading - Multiprocessors Process Parallelism Process Parallelism CPU P1 P1 P2 P2 CPU P3 P3 CPU time time No of execution processes <= the number of CPUs No of execution processes <= the number of CPUs 43
  44. 44. Multithreading on Uni-processor  Concurrency Vs Parallelism  Process Concurrency Process Concurrency P1 P1 P2 P2 CPU P3 P3 time time Number of Simultaneous execution units > number of Number of Simultaneous execution units > number of CPUs CPUs 44
  45. 45. Multi-Processing (clusters & grids) and Multi-Threaded Computing Threaded Libraries, Multi-threaded I/O Application Application Application Application CPU CPU CPU Better Response Times in Multiple Application Environments CPU CPU CPU Higher Throughput for Parallelizeable Applications 45
  46. 46. References   Rajkumar Buyya, Thamarai Selvi, Xingchen Chu, Mastering OOP with Java, McGraw Hill (I) Press, New Delhi, India, 2009. Sun Java Tutorial – Concurrency:  http://java.sun.com/docs/books/tutorial/essentia 46

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