1. 1 OS Support for Building Distributed Applications: Multithreaded Programming using Java Threads

2. 2 Outline Introduction Thread Applications Defining Threads Java Threads and States Architecture of Multithreaded servers Threads Synchronization Thread Concurrency Models Summary

3. 3 Introduction We discuss how Middleware is supported by the Operating System (OS) facilities at the nodes of a distributed system. The OS facilitates: Encapsulation and protection of resources inside servers; It supports invocation of mechanisms required to access those resources including concurrent access/processing.

4. 4 Middleware and Network Operating System (NOS) Many DOS (Distributed OS) have been investigated, but there are none in general/wide use. But NOS are in wide use for various reasons both technical and non- technical. Users have much invested in their application software; they will not adopt to new OS that will not run their applications. The 2 nd reason against the adoption of DOS is that users tend to prefer to have a degree of autonomy of their machines, even in a closely knit organisation. A combination of middleware and NOSs provides an acceptable balance between the requirement of autonomy and network transparency. NOS allows users to run their favorite word processor. Middleware enables users to take advantage of services that become available in their distributed system.

5. 5 Operating system layers and Middleware Unix and Windows are two examples of Network Operating Systems – have a networking capability built into them and so can be used to access remote resources using basic services such as rlogin, telnet.

6. 6 Core OS components and functionality

7. 7 A single threaded program class ABC { …. public void main(..) { ….. } begin body end

8. 8 A Multithreaded Program Main Thread Thread A Thread BThread C start Threads may switch or exchange data/results

9. 9 Single and Multithreaded Processes Single-threaded Process Single instruction stream Multiple instruction stream Multiplethreaded Process Threads of Execution Common Address Space threads are light-weight processes within a process

10. 10 Application CPU Better Response Times in Multiple Application Environments Higher Throughput for Parallelizeable Applications CPU Threaded Libraries, Multi-threaded I/O Multi-Processing (clusters & grids) and Multi-Threaded Computing

11. 11 Web/Internet Applications: Serving Many Users Simultaneously Internet Server PC client Local Area Network PD A

12. 12 Server Threads Server Process Client 1 Process Client 2 Process Multithreaded Server: For Serving Multiple Clients Concurrently Internet

13. 13 Printing Thread Editing Thread Modern Applications need Threads (ex1): Editing and Printing documents in background.

14. 14 Multithreaded/Parallel File Copy reader() { - - - - - lock(buff[i]); read(src,buff[i]); unlock(buff[i]); - - - - - } reader() { - - - - - lock(buff[i]); read(src,buff[i]); unlock(buff[i]); - - - - - } writer() { - - - - - lock(buff[i]); write(src,buff[i]); unlock(buff[i]); - - - - - } writer() { - - - - - lock(buff[i]); write(src,buff[i]); unlock(buff[i]); - - - - - } buff[0] buff[1] Cooperative Parallel Synchronized Threads

15. 15 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 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 Task i-l Task i Task i+1 func1 ( ) {.... } func1 ( ) {.... } func2 ( ) {.... } func2 ( ) {.... } func3 ( ) {.... } func3 ( ) {.... } a ( 0 ) =.. b ( 0 ) =.. a ( 0 ) =.. b ( 0 ) =.. a ( 1 )=.. b ( 1 )=.. a ( 1 )=.. b ( 1 )=.. a ( 2 )=.. b ( 2 )=.. a ( 2 )=.. b ( 2 )=.. + + x x Load Sockets/ PVM/MPI Threads Compilers CPU Levels of Parallelism

16. 16 Process Parallelism P1 P2 P3 time No of executing process ≥ the number of CPUs CPU Multithreading - Multiprocessors

17. 17 Multithreading on Uni-processor Concurrency Vs Parallelism K Process Concurrency Number of Simultaneous execution units > number of CPUs P1 P2 P3 time CPU

18. 18 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 to express concurrency on both single and multiprocessor machines. Programming a task having multiple threads of control – Multithreading or Multithreaded Programming.

19. 19 Java Threads Java has built in thread support for Multithreading Synchronization Thread Scheduling Inter-Thread Communication: currentThreadstartsetPriority yieldrungetPriority sleepstopsuspend resume Java Garbage Collector is a low-priority thread.

20. 20 Threading Mechanisms... Create a class that extends the Thread class Create a class that implements the Runnable interface

21. 21 1st method: Extending Thread class Threads are implemented as objects that contains a method called run() 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();

22. 22 An example class MyThread extends Thread { // the thread public void run() { System.out.println(" this thread is running... "); } } // end class MyThread class ThreadEx1 { // a program that utilizes the thread public static void main(String [] args ) { MyThread t = new MyThread(); MyThread t2 = new MyThread(); // due to extending the Thread class (above) // I can call start(), and this will call // run(). start() is a method in class Thread. t2.start(); t.start(); } // end main() } // end class ThreadEx1

23. 23 threads programming More about threads programming Using interfaces Multiple threads in a program

24. 24 2nd method: Threads by implementing Runnable interface class MyThread extends ABC 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();

25. 25 An example class MyThread implements Runnable { public void run() { System.out.println(" this thread is running... "); } } // end class MyThread class ThreadEx2 { public static void main(String [] args ) { Thread t = new Thread(new MyThread()); // due to implementing the Runnable interface // I can call start(), and this will call run(). t.start(); } // end main() } // end class ThreadEx2

26. 26 Life Cycle of Thread new runnable non-runnable dead wait() sleep() suspend() blocked notify() slept resume() unblocked start() stop()

27. 27 A Program with Three Java Threads Write a program that creates 3 threads

28. 28 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"); }

29. 29 Three threads example 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(); }

30. 30 Run 1 [user@speedy] 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

31. 31 Run2 [user@speedy] 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

32. 32 Server Threads Message Passing Facility Server Process Client Process User Mode Kernel Mode Multithreaded Server

33. 33 Architecture for Multithread Servers Multithreading enables servers to maximize their throughput, measured as the number of requests processed per second. Threads may need to treat requests with varying priorities: A corporate server could prioritize request processing according to class of customers. Architectures: Worker pool Thread-per-request Thread-per-connection Thread-per-object

34. 34 Client and server with threads In worker-pool architecture, the server creates a fixed pool of worker threads to process requests. The module “receipt and queuing” receives requests from sockets/ports and places them on a shared request queue for retrieval by the workers. Server N threads Input-output Client Thread 2 makes T1 Thread 1 requests to server generates results Requests Receipt & queuing

35. 35 Alternative server threading architectures

36. 36 Application 1: Multithreaded Dictionary Server – Demonstrate the use of Sockets and Threads Multithreaded Dictionary Server (try, dictionary.com) A Client Program What is the Meaning of (“Love”)? A Client Program What is the Meaning of (“Love”)? A Client Program in “C++” A Client Program in “C” “Love” “A deep, tender, ineffable feeling of affection and solicitude toward a person ” “care or concern, as for the well-being of another” “Solicitude”

37. 37 Synchronisation & Priorities Thread Synchronisation Thread Priorities

38. 38 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()

39. 39 Online Bank: Serving Many Customers and Operations Internet Bank Server PC client Local Area Network PD A Bank Database

40. 40 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() { … }

41. 41 the driver: 3 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() }

42. 42 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 accoun t (shared object)

43. 43 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. }

44. 44 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

45. 45 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"); }

46. 46 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"); }

47. 47 Thread Programming models Thread concurrency/operation models The master/worker model The peer model A thread pipeline

48. 48 taskX taskY taskZ main ( ) Workers Program Files Resources Databases Disks Special Devices Master Input (Stream) The master/worker model

49. 49 Example main() /* the master */ { forever { get a request; switch( request ) case X: pthread_create(....,taskX); case Y: pthread_create(....,taskY);.... } taskX() /* worker */ { perform the task, sync if accessing shared resources } taskY() /* 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

50. 50 The peer model taskX taskY Workers Program Files Resources Databases Disks Special Devices taskZ Input

51. 51 Example main() { pthread_create(....,thread1...taskX); pthread_create(....,thread2...taskY);.... signal all workers to start wait for all workers to finish do any cleanup } taskX() /* worker */ { wait for start perform the task, sync if accessing shared resources } taskY() /* worker */ { wait for start perform the task, sync if accessing shared resources }

52. 52 A thread pipeline Resources Files Databases Disks Special Devices Files Databases Disks Special Devices Files Databases Disks Special Devices Stage 1 Stage 2 Stage 3 Program Filter Threads Input (Stream) A thread pipeline

53. 53 Example main() { pthread_create(....,stage1); pthread_create(....,stage2);.... wait for all pipeline threads to finish do any cleanup } stage1() { get next input for the program do stage 1 processing of the input pass result to next thread in pipeline } stage2(){ get input from previous thread in pipeline do stage 2 processing of the input pass result to next thread in pipeline } stageN() { get input from previous thread in pipeline do stage N processing of the input pass result to program output. }

54. 54 Java thread constructor and management methods Thread(ThreadGroup group, Runnable target, String name) Creates a new thread in the SUSPENDED state, which will belong to group and be identified as name; the thread will execute the run() method of target. setPriority(int newPriority), getPriority() Set and return the thread’s priority. run() A thread executes the run() method of its target object, if it has one, and otherwise its own run() method (Thread implements Runnable). start() Change the state of the thread from SUSPENDED to RUNNABLE. sleep(int millisecs) Cause the thread to enter the SUSPENDED state for the specified time. yield() Enter the READY state and invoke the scheduler. destroy() Destroy the thread.

55. 55 Java thread synchronization calls thread.join(int millisecs) Blocks the calling thread for up to the specified time until thread has terminated. thread.interrupt() Interrupts thread: causes it to return from a blocking method call such as sleep(). object.wait(long millisecs, int nanosecs) Blocks the calling thread until a call made to notify() or notifyAll() on object wakes the thread, or the thread is interrupted, or the specified time has elapsed. object.notify(), object.notifyAll() Wakes, respectively, one or all of any threads that have called wait() on object.

56. 56 RPC delay against parameter size

57. 57 A lightweight remote procedure call

58. 58 Times for serialized and concurrent invocations

59. 59 Summary Operating system provides various types of facilities to support middleware for distributed system: encapsulation, protection, and concurrent access and management of node resources. Multithreading enables servers to maximize their throughput, measured as the number of requests processed per second. Threads support treating of requests with varying priorities. Various types of architectures can be used in current processing: Worker pool Thread-per-request Thread-per-connection Thread-per-object Threads need to be synchronized when accessing and manipulating shared resources. New OS designs provide flexibility in terms of separating mechanisms from policies.