1. Chapter 4: Threads

2. 4.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Chapter 4: Threads Overview Multithreading Models Threading Issues Pthreads Windows XP Threads Linux Threads Java Threads

3. 4.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Threads A thread (or lightweight process) is a basic unit of CPU utilization; it consists of: program counter register set stack space A thread shares with its peer threads its: code section data section operating-system resources collectively know as a task. A traditional or heavyweight process is equal to a task with one thread

4. 4.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Threads (Cont.) In a multiple threaded task, while one server thread is blocked and waiting, a second thread in the same task can run. Cooperation of multiple threads in same job confers higher throughput and improved performance. Applications that require sharing a common buffer (i.e., producer- consumer) benefit from thread utilization. Threads provide a mechanism that allows sequential processes to make blocking system calls while also achieving parallelism. Kernel-supported threads (Mach and OS/2). User-level threads; supported above the kernel, via a set of library calls at the user level (Project Andrew from CMU). Hybrid approach implements both user-level and kernel-supported threads (Solaris 2).

5. 4.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multiple Threads within a Task

6. 4.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Single and Multithreaded Processes

7. 4.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Benefits Responsiveness Resource Sharing Economy Utilization of MP Architectures

8. 4.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts User Threads Thread management done by user-level threads library Three primary thread libraries: POSIX Pthreads Java threads Win32 threads

9. 4.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Kernel Threads Supported by the Kernel Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X

10. 4.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multithreading Models Many-to-One One-to-One Many-to-Many

11. 4.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-One Many user-level threads mapped to single kernel thread Examples Solaris Green Threads GNU Portable Threads

12. 4.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-One Model

13. 4.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts One-to-One Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux Solaris 9 and later

14. 4.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts One-to-one Model

15. 4.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-Many Model Allows many user level threads to be mapped to many kernel threads Allows the operating system to create a sufficient number of kernel threads Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

16. 4.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-Many Model

17. 4.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Two-level Model Similar to M:M, except that it allows a user thread to be bound to kernel thread Examples IRIX HP-UX Tru64 UNIX Solaris 8 and earlier

18. 4.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Two-level Model

19. 4.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Libraries Pthreads A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization API specifies behavior of the thread library, implementation is up to development of the library Common in UNIX operating systems (Solaris, Linux, Mac OS X)

20. 4.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Pthreads int sum; /* this data is shared by the thread(s) */ void *runner(void *param); /* the thread */ main(int argc, char *argv[]) { pthread_t tid; /* the thread identifier */ pthread_attr_t attr; /* set of attributes for the thread */ /* get the default attributes */ pthread_attr_init(&attr); /* create the thread */ pthread_create(&tid,&attr,runner,argv[1]); /* now wait for the thread to exit */ pthread_join(tid,NULL); printf("sum = %d\n",sum); } void *runner(void *param) { int upper = atoi(param); int i; sum = 0; if (upper > 0) { for (i = 1; i <= upper; i++) sum += i; } pthread_exit(0); }

21. 4.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Threads Java threads are managed by the JVM Java threads may be created by: Extending Thread class Implementing the Runnable interface

22. 4.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Extending the Thread Class class Worker1 extends Thread { public void run() { System.out.println("I Am a Worker Thread"); } public class First { public static void main(String args[]) { Worker1 runner = new Worker1(); runner.start(); System.out.println("I Am The Main Thread"); }

23. 4.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts The Runnable Interface public interface Runnable { public abstract void run(); }

24. 4.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Implementing the Runnable Interface class Worker2 implements Runnable { public void run() { System.out.println("I Am a Worker Thread "); } public class Second { public static void main(String args[]) { Runnable runner = new Worker2(); Thread thrd = new Thread(runner); thrd.start(); System.out.println("I Am The Main Thread"); }

25. 4.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Thread States

26. 4.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Threading Issues Semantics of fork() and exec() system calls Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations

27. 4.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Semantics of fork() and exec() Does fork() duplicate only the calling thread or all threads?

28. 4.28 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Cancellation Terminating a thread before it has finished Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled

29. 4.29 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Signal Handling Signals are used in UNIX systems to notify a process that a particular event has occurred A signal handler is used to process signals  Signal is generated by particular event  Signal is delivered to a process  Signal is handled Options: Deliver the signal to the thread to which the signal applies Deliver the signal to every thread in the process Deliver the signal to certain threads in the process Assign a specific threa to receive all signals for the process

30. 4.30 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Pools Create a number of threads in a pool where they await work Advantages: Usually slightly faster to service a request with an existing thread than create a new thread Allows the number of threads in the application(s) to be bound to the size of the pool

31. 4.31 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Specific Data Allows each thread to have its own copy of data Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

32. 4.32 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Scheduler Activations Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library This communication allows an application to maintain the correct number kernel threads

33. 4.33 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Lightweight Process (LWP)

34. 4.34 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Threads Support in Solaris 2 Solaris 2 is a version of UNIX with support for threads at the kernel and user levels, symmetric multiprocessing, and real-time scheduling. LWP – intermediate level between user-level threads and kernel-level threads. Resource needs of thread types: Kernel thread: small data structure and a stack; thread switching does not require changing memory access information – relatively fast. LWP: PCB with register data, accounting and memory information,; switching between LWPs is relatively slow. User-level thread: only need stack and program counter; no kernel involvement means fast switching. Kernel only sees the LWPs that support user-level threads.

35. 4.35 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Solaris 2 Threads

36. 4.36 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Windows XP Threads Implements the one-to-one mapping Each thread contains A thread id Register set Separate user and kernel stacks Private data storage area The register set, stacks, and private storage area are known as the context of the threads The primary data structures of a thread include: ETHREAD (executive thread block) KTHREAD (kernel thread block) TEB (thread environment block)

37. 4.37 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Data Structures of a Windows XP thread

38. 4.38 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Linux Threads Linux refers to them as tasks rather than threads Thread creation is done through clone() system call clone() allows a child task to share the address space of the parent task (process) How much sharing is determined by a set of passed flags No flags, no sharing; acts like fork() system call

39. 4.39 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts clone() flags in Linux Threads

40. 4.40 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts More on Java Threads: Joining Threads class JoinableWorker implements Runnable { public void run() { System.out.println("Worker working"); } public class JoinExample { public static void main(String[] args) { Thread task = new Thread(new JoinableWorker()); task.start(); try { task.join(); } catch (InterruptedException ie) { } System.out.println("Worker done"); }

41. 4.41 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Cancellation Thread thrd = new Thread (new InterruptibleThread()); Thrd.start();... // now interrupt it Thrd.interrupt();

42. 4.42 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Cancellation public class InterruptibleThread implements Runnable { public void run() { while (true) { /** * do some work for awhile */ if (Thread.currentThread().isInterrupted()) { System.out.println("I'm interrupted!"); break; } // clean up and terminate }

43. 4.43 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Specific Data class Service { private static ThreadLocal errorCode = new ThreadLocal(); public static void transaction() { try { /** * some operation where an error may occur */ catch (Exception e) { errorCode.set(e); } /** * get the error code for this transaction */ public static Object getErrorCode() { return errorCode.get(); }

44. 4.44 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Specific Data class Worker implements Runnable { private static Service provider; public void run() { provider.transaction(); System.out.println(provider.getErrorCode()); }

45. 4.45 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Producer-Consumer Problem public class Factory { public Factory() { // first create the message buffer Channel mailBox = new MessageQueue(); // now create the producer and consumer threads Thread producerThread = new Thread(new Producer(mailBox)); Thread consumerThread = new Thread(new Consumer(mailBox)); producerThread.start(); consumerThread.start(); } public static void main(String args[]) { Factory server = new Factory(); }

46. 4.46 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Producer Thread class Producer implements Runnable { private Channel mbox; public Producer(Channel mbox) { this.mbox = mbox; } public void run() { Date message; while (true) { SleepUtilities.nap(); message = new Date(); System.out.println("Producer produced " + message); // produce an item & enter it into the buffer mbox.send(message); }

47. 4.47 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Consumer Thread class Consumer implements Runnable { private Channel mbox; public Consumer(Channel mbox) { this.mbox = mbox; } public void run() { Date message; while (true) { SleepUtilities.nap(); // consume an item from the buffer System.out.println("Consumer wants to consume."); message = (Date)mbox.receive(); if (message != null) System.out.println("Consumer consumed " + message); }

48. End of Chapter 4