1. Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition, Chapter 4: Multithreaded Programming

2. 4.2 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Chapter 4: Multithreaded Programming 4.1 Overview 4.2 Multicore Progarmming 4.3 Multithreading Models 4.4 Thread Libraries 4.5 Implicit Threading 4.6 Threading Issues 4.7 Operating System Examples

3. 4.3 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Objectives To introduce the notion of a thread — a fundamental unit of CPU utilization that forms the basis of multithreaded computer systems To discuss the APIs for the Pthreads, Win32, and Java thread libraries To explore several strategies that provide implicit threading To examine issues related to multithreaded programming To cover operating system support for threads in Windows and Linux

4. 4.4 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 4.1 Overview Most modern applications are multithreaded Multiple threads run within an application Multiple tasks with the application can be implemented by separate threads Threads in a web browser:  Responding to users  Fetching remote files  Displaying graphics  Spelling checking Process creation is heavy-weight while thread creation is light-weight Multithreaded programming can simplify code, increase efficiency Kernels are generally multithreaded

5. 4.5 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Single and Multithreaded Processes

6. 4.6 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Multithreaded Server Architecture

7. 4.7 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Benefits of Multitheaded Programming Responsiveness – may allow continued execution if part of process is blocked, especially important for user interfaces Resource sharing – threads share resources of process, easier than shared memory or message passing between processes Economy – cheaper than process creation, thread switching lower overhead than context switching Scalability – process can take advantage of multiprocessor architectures

8. 4.8 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 4.2 Multicore Programming Concurrency supports more than one task making progress Concurrent execution on a single-core system  Single processor (core), scheduler providing concurrency Parallelism implies a system can perform more than one task simultaneously Parallel execution on a multicore dystem Software thread vs. hardware thread As number of threads grows, so does architectural support for threading CPUs have cores as well as hardware threads Consider Oracle SPARC T4 with 8 cores, and 8 hardware threads per core

9. 4.9 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Multicore Programming Challenges Multicore systems putting pressure on programmers, challenges include Identifying/Dividing activities Balance Data splitting Data dependency Testing and debugging Parallel programming must be emphasized on the software development of multicore systems Types of parallelism Data parallelism – distributes subsets of the same data across multiple cores, same operation on each Task parallelism – distributing threads across cores, each thread performing unique operation

10. 4.10 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Amdahl’s Law Identifies performance gains from adding additional cores to an application that has both serial and parallel components S is serial portion N processing cores I.e. if application is 75% parallel / 25% serial, moving from 1 to 2 cores results in speedup of 1.6 times As N approaches infinity, speedup approaches 1 / S Serial portion of an application has disproportionate effect on performance gained by adding additional cores The law considers only system speedup, but does not take into account system throughput.

11. 4.11 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Software Threads User Threads Thread management done by user-level threads library Three primary thread libraries: POSIX Pthreads Win32 threads Java threads Kernel Threads Supported by the kernel-level threads library Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X

12. 4.12 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 4.3 Multithreading Models Many-to-One Model One-to-One Model Many-to-Many Model Two-level Model

13. 4.13 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Many-to-One Model Many user-level threads mapped to single kernel thread Examples: Solaris Green Threads GNU Portable Threads

14. 4.14 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition One-to-One Model Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux Solaris 9 and later

15. 4.15 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 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 Examples: Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

16. 4.16 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Two-level Model Similar to Many-to-Many, except that it allows a user thread to be bound to kernel thread Examples IRIX HP-UX Tru64 UNIX Solaris 8 and earlier

17. 4.17 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 4.4 Thread Libraries Thread library provides programmer with API for creating and managing threads Two primary ways of implementing Library entirely in user space Kernel-level library supported by the OS Three main thread libraries POSIX Pthread Windows Java Strategies for creating multiple threads Asynchronous threading Synchronous threading

18. 4.18 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Pthreads May be provided either as user-level or kernel-level 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

19. 4.19 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Win32 API Multithreaded C Program Windows threads library is a kernel-level library

20. 4.20 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Java Threads Java threads are managed by the JVM Typically implemented using the threads model provided by underlying OS:  POSIX Pthread  Windows thread library Java threads may be created by: Extending Thread class Implementing the Runnable interface

21. 4.21 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Java Multithreaded Program

22. 4.22 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Java Multithreaded Program (Cont.)

23. 4.23 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 4.5 Implicit Threading Growing in popularity as numbers of threads increase, program correctness more difficult with explicit threads Creation and management of threads done by compilers and run- time libraries rather than programmers Three methods explored Thread Pools OpenMP Grand Central Dispatch Other methods include Microsoft Threading Building Blocks (TBB) java.util.concurrent package

24. 4.24 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 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 to be bound to the size of the pool Separating task to be performed from mechanics of creating task allows different strategies for running task  i.e. Tasks could be scheduled to run periodically Windows API supports thread pools:

25. 4.25 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition OpenMP Provides a set of compiler directives and an API for C, C++, FORTRAN Provides support for parallel programming in shared- memory environments Identifies parallel regions – blocks of code that can run in parallel #pragma omp parallel Create as many threads as there are cores #pragma omp parallel for for(i=0;i<N;i++) { c[i] = a[i] + b[i]; } // Run for loop in parallel

26. 4.26 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Grand Central Dispatch Apple technology for Mac OS X and iOS operating systems Extensions to C, C++ languages, API, and run-time library Allows identification of parallel sections Manages most of the details of threading Block is in “^{ }” - ˆ{ printf("I am a block"); } Blocks placed in dispatch queue  Assigned to available thread in thread pool when removed from queue Two types of dispatch queues: serial – blocks removed in FIFO order, queue is per process, called main queue concurrent – removed in FIFO order but several may be removed at a time

27. 4.27 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 4.6 Threading Issues Semantics of fork() and exec() system calls Thread cancellation of target thread Asynchronous or deferred Signal handling Thread-local storage Scheduler activations

28. 4.28 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Semantics of fork() and exec() Does fork() duplicate only the calling thread or all threads? Results of exec()?

29. 4.29 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 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

30. 4.30 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 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 thread to receive all signals for the process

31. 4.31 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Thread-Local Storage 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 ©2013 Operating System Concepts – 9 th Edition Scheduler Activations Both many-to-many and two-level models require communication to maintain the appropriate number of kernel threads allocated to the application Lighweight process (LWP) sits between user and kernel threads  Also called virtual processor Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library

33. 4.33 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 4.7 Operating System Examples Windows XP Threads Linux Threads

34. 4.34 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 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 primary data structures of a thread: ETHREAD (executive thread block) KTHREAD (kernel thread block) TEB (thread environment block)

35. 4.35 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition 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)