Chapter 4: Threads

1. Chapter 4: Threads

2. Chapter 4: Threads Topics: • Overview • Multithreading Models • Thread Libraries • Threading Issues • Operating System Examples • Windows XP Threads • Linux Threads 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

3. Threads Motivation • Threads run within application • 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

4. Benefits • Responsiveness • Resource Sharing • Economy • Scalability

5. Question • Which pieces of information from a process can be shared by all threads of a process and which ones need to be unique? Process info: • State • Program counter • Registers • Open files • Stack • Text section (code) • Data

6. Single and Multithreaded Processes

7. Multithreaded Server Architecture

8. Multicore Programming • Multicore systems putting pressure on programmers, challenges include: • Dividing activities • Balance • Data splitting • Data dependency • Testing and debugging

9. Concurrency Single-core System Multicore System

10. Multithreading Models

11. User Threads • Thread management done by user-level threads library • Three primary thread libraries: • POSIX Pthreads • Win32 threads • Java threads

12. Kernel Threads • Supported by the Kernel • Examples • Windows XP/2000 • Solaris • Linux • Tru64 UNIX • Mac OS X

13. Multithreading Models • Many-to-One • One-to-One • Many-to-Many • Two-level • Main considerations: • Limited number of threads? • Concurrency: What if a thread makes a blocking system call? • Allows for parallel execution on multiprocessor machines?

14. Many-to-One • Many user-level threads mapped to single kernel thread • Examples: • Solaris Green Threads • GNU Portable Threads

15. One-to-One • Each user-level thread maps to kernel thread • Examples: Windows NT/XP/2000, Linux, Solaris 9 and later

16. 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

17. 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. Thread Libraries

19. Thread Libraries • Thread libraryprovides 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

20. 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 • Policy vs. mechanism • Common in UNIX operating systems (Solaris, Linux, Mac OS X & in Windows via shareware implementations)

21. Examples • Pthreads • http://www.cs.mtsu.edu/~hcarroll/3250/private/code/chapter04/pthreadsExample.c • Win32 API • http://www.cs.mtsu.edu/~hcarroll/3250/private/code/chapter04/win32ThreadsExample.c • Java • http://www.cs.mtsu.edu/~hcarroll/3250/private/code/chapter04/Driver.java

22. Java Threads • Java threads are managed by the JVM • Typically implemented using the threads model provided by underlying OS • Java threads may be created by: • Extending Thread class • Implementing the Runnable interface

23. Tuesday, January 31, 2012 • lab1 – graded copies pushed out today @ 4:00 PM • lab2 – due tomorrow • hwk04 – due Thursday

24. Threading Issues

25. Threading Issues • Scheduler activations • Semantics of fork() and exec() system calls • Thread cancellationof target thread • Asynchronous or deferred • Signal handling • Synchronous and asynchronous • Thread pools • Thread-specific data • Create Facility needed for data private to thread

26. Scheduler Activations • Both many-to-one and many-to-many 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

27. Lightweight Processes • Virtual processor (LWP) between thread library (user thread(s)) and kernel • Not present in all OSes • In some contexts, LWP refers to the kernel thread

28. Tracking Threads on ranger • ps –eLf command: $ ps -eLf | head -n 1; ps -eLf | grep $USER UID PIDPPIDLWP C NLWP STIME TTY TIME CMD hcarroll 13291 14715 13291 0 2 13:23 pts/22 00:00:00 ./pthreadsExample 55555555555 hcarroll 13291 14715 13292 98 2 13:23 pts/22 00:00:31 ./pthreadsExample 55555555555 hcarroll 13458 14715 13458 0 1 13:24 pts/22 00:00:00 ps -eLf hcarroll 13459 14715 13459 0 1 13:24 pts/22 00:00:00 grep hcarroll root 14712 1 14712 0 1 12:21 ? 00:00:00 sshd: hcarroll [priv] hcarroll 14714 14712 14714 0 1 12:21 ? 00:00:00 sshd: hcarroll@pts/22 hcarroll 14715 14714 14715 0 1 12:21 pts/22 00:00:00 -bash hcarroll 30081 14715 30081 0 1 12:45 pts/22 00:00:00 emacs

29. Semantics of fork() and exec() • What does fork() do? • What does exec() do? • Does fork() duplicate only the calling thread or all threads for that process? • Discuss with your neighbor(s) how can you test this?

30. Multi-processes with Multi-threads • http://www.cs.mtsu.edu/~hcarroll/3250/private/code/chapter04/webServerSkeleton.c • http://www.cs.mtsu.edu/~hcarroll/3250/private/code/chapter04/webServerSkeleton-withFork.c • ps -eLf | head -n 1; ps -eLf | grep $USER

31. 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.

32. Signal Handling • Signals are used in UNIX systems to notify a process that a particular event has occurred. • A signal handleris used to process signals • Signal is generated by particular event • Signal is delivered to a process • Signal is handled • Options for multi-threaded processes: • 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

33. 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

34. 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)

35. Operating System Examples

36. 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 contextof the threads • The primary data structures of a thread include: • ETHREAD (executive thread block) • KTHREAD (kernel thread block) • TEB (thread environment block)

37. Windows XP Threads Data Structures

38. Linux Threads • fork() and clone() system calls • Doesn’t distinguish between process and thread • Uses term task rather than thread • clone() takes options to determine sharing on process create • struct task_struct points to process data structures (shared or unique)

39. Recap • Provide 2 programming example in which multithreading provides better performance than a single-threaded solution. • What are at least 2 differences between a kernel thread and a user thread? • What is the difference between creating and managing processes and threads (e.g., what do they share)? • Which of the following components of program state are shared across threads in a multithreaded process? (Exercise 4.10) Register values, Heap memory, Global variables, Stack memory • Consider a multiprocessor system and a multithreaded program written using the many-to-many threading model. Let the number of user-level threads in the program be more than the number of processors in the system. Discuss the performance implications of the following scenarios: (Exercise 4.14) • Number of kernel threads allocated to the program is less than the number of cores. • Number of kernel threads allocated to the program is equal to the number of cores. • Number of kernel threads allocated to the program is greater than the number of cores but less than the number of user-level threads.

40. Recap • Provide 2 programming example in which multithreading provides better performance than a single-threaded solution. • A Web server that services each request in a separate thread. • A parallelized application such as matrix multiplication where different parts of the matrix may be worked on in parallel. • An interactive GUI program such as a debugger where a thread is used to monitor user input, another thread represents the running application, and a third thread monitors performance. • What are at least 2 differences between a kernel thread and a user thread? • User-level threads are unknown by the kernel, whereas the kernel is aware of kernel threads. • On systems using either M:1 or M:N mapping, user threads are scheduled by the thread library and the kernel schedules kernel threads. • Kernel threads need not be associated with a process whereas every user thread belongs to a process. Kernel threads are generally more expensive to maintain than user threads as they must be represented with a kernel data structure.

41. Recap • What is the difference between creating and managing processes and threads (e.g., what do they share)? • Process (using fork()): • Makes a COPY of the process (global & local variables) • Shares open files (e.g., pipes) • Execution starts directly after fork() • Threads (using pthread_create() / CreateThread()): • Shares global variables, memory space (e.g., things on the heap), open files, etc. • Execution starts in functions • <Timing example: timing-*.c> • Which of the following components of program state are shared across threads in a multithreaded process? (Exercise 4.10) Register values, Heap memory, Global variables, Stack memory

42. Recap • Consider a multiprocessor system and a multithreaded program written using the many-to-many threading model. Let the number of user-level threads in the program be more than the number of processors in the system. Discuss the performance implications of the following scenarios: (Exercise 4.10) • The number of kernel threads allocated to the program is less than the number of processors. • Not all of the processors will be utilized • The number of kernel threads allocated to the program is equal to the number of processors. • Whenever a kernel thread is blocked, a processor will go unutilized • The number of kernel threads allocated to the program is greater than the number of processors but less than the number of user-level threads. • Allows for the greatest utilization of processors and the kernel threads will be scheduled on the processors

43. End of Chapter 4