1. Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8 th Edition Chapter 4: Threads

2. 4.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Chapter 4: Threads Overview Multithreading Models Thread Libraries Threading Issues Operating System Examples Windows XP Threads Linux Threads

3. 4.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 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 examine issues related to multithreaded programming

4. 4.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Overview A thread is a basic unit of CPU utilization. It compromises a : Thread ID, Program counter Register set Stack Threads that belongs to the same process shared: Code section Data section Other OS resources as open files and signals.

5. 4.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Motivation Threads run within application. A traditional process has a single task thread of control.(performs one task at a time). A process of Multi-threads of controls, can perform multi-tasks at a time. Multiple tasks with the application can be implemented by separate threads Update display Fetch data Spell checking Answer a network request Process creation is heavy-weight while thread creation is light-weight Can simplify code, increase efficiency Kernels are generally multithreaded

6. 4.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Single and Multithreaded Processes

7. 4.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multithreaded Server Architecture

8. 4.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Benefits Responsiveness: allowing a program to continue running even if part of it is blocked or perform another operation, which increases responsiveness to the user. Resource Sharing: process threads can share the memory and other resources of memory by default, no need to shared memory or message passing techniques. So an application will have several different threads within the same address space. Economy: Allocating memory and resources for process creation is costly Scalability: In multiprocessor systems multi-threads can be run in parallel on different processors.

9. 4.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multicore Programming Multithreaded programming provides a mechanism for more efficient use of multiple-cores and improving concurrency. Multi-core systems putting pressure on programmers and designers to make better use of the multiple computing cores, this trend have many challenges resulted from the need to modifying the existing program, those challenges are: Dividing activities into separate, concurrent tasks to run in parallel. Balance, each task must perform equal task of equal value. Data Splitting, by dividing data in order to run tasks on separate cores Data Dependency, in case that some data is shared between some tasks Testing and debugging concurrent programs is more difficult than debugging single-threaded applications

10. 4.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Concurrent Execution on a Single-core System

11. 4.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Parallel Execution on a Multicore System

12. 4.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multithreading Models User-Threads: Support for those threads provided at the user level, Kernel-Threads: Support for those threads provided at the kernel level,

13. 4.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition User Threads Thread management done by user-level threads library. Without kernel support. Three primary thread libraries: POSIX Pthreads Win32 threads Java threads

14. 4.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Kernel Threads Supported and managed directly by the OS. Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X

15. 4.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multithreading Models There are 3-common ways for establishing a relationship that must exist between user threads and kernel threads: 1. Many-to-One 2. One-to-One 3. Many-to-Many

16. 4.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Many-to-One Many user-level threads mapped to single kernel thread Thread management is done by the thread library in the user space. The entire process block if any thread makes a blocking system call Multiple threads are unable to run in parallel because only one thread can access the kernel at a time. Examples: Solaris Green Threads GNU Portable Threads

17. 4.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Many-to-One Model

18. 4.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition One-to-One Each user-level thread maps to kernel thread. It provides more concurrency than the many-to-one model. By allowing another thread to run when a running thread makes a blocking system call. Multiple threads are able to run in parallel on multiprocessors. -ve: The overhead that results from creating multi-kernel threads for each user- thread. Examples Windows NT/XP/2000 Linux Solaris 9 and later

19. 4.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition One-to-one Model

20. 4.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 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. Kernel threads can run in parallel on a multiprocessor. It provides more concurrency than the many-to-one model. By allowing another thread to run when a running thread makes a blocking system call. Examples Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

21. 4.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Many-to-Many Model

22. 4.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

23. 4.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Two-level Model

24. 4.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Thread Libraries Thread library provides programmer with API for creating and managing threads Two primary ways of implementing a threaded library: 1. Provide a Library entirely in user space with no kernel support. All code and data structure for the library exists in the user space. So it enables local function call, not a system call. 2. Kernel-level library supported by the OS. All code and data structure for the library exists in the kernel space. So it enables a system call, not a local function call

25. 4.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Thread Libraries cont… Three main thread libraries are in use today: 1. POSIX Pthreads 2. Win32 3. Java

26. 4.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 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)

27. 4.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Pthreads Example

28. 4.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Pthreads Example (Cont.)

29. 4.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Win32 Is a kernel-level library available on windows system.

30. 4.30 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Win32 API Multithreaded C Program

31. 4.31 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Win32 API Multithreaded C Program (Cont.)

32. 4.32 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Java Threads Allows threads to be created and damaged directly in a Java program Java threads are managed by the JVM Typically implemented using the threads model provided by underlying OS. So on windows sys. Java threads are implementing using Win32. Java threads may be created by: Extending Thread class Implementing the Runnable interface

33. 4.33 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Java Multithreaded Program

34. 4.34 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Java Multithreaded Program (Cont.)

35. 4.35 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Threading Issues Semantics of fork() and exec() system calls Thread cancellation of target thread Asynchronous or deferred Signal handling Synchronous and asynchronous

36. 4.36 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Threading Issues (Cont.) Thread pools Thread-specific data Create Facility needed for data private to thread Scheduler activations

37. 4.37 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Semantics of fork() and exec() Does fork() duplicate only the calling thread or all threads?

38. 4.38 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 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.

39. 4.39 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 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

40. 4.40 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 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(s) to be bound to the size of the pool

41. 4.41 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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)

42. 4.42 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

43. 4.43 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Lightweight Processes

44. 4.44 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Operating System Examples Windows XP Threads Linux Thread

45. 4.45 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Windows XP Threads Data Structures

46. 4.46 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Windows XP Threads Implements the one-to-one mapping, kernel-level 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)

47. 4.47 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 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) n struct task_struct points to process data structures (shared or unique)

48. 4.48 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linux Threads fork() and clone() system calls Doesn’t distinguish between process and thread Uses term task rather than thread n clone() takes options to determine sharing on process create n struct task_struct points to process data structures (shared or unique)

49. Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8 th Edition End of Chapter 4