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Chapter 2.5 : Threads

Chapter 2.5 : Threads. Process concept  Process scheduling  Interprocess communication  Deadlocks  Threads. Threads. These lecture notes have been adapted from How to program with threads An introduction to multithreaded programming By Bil Lewis and Daniel J. Berg and

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Chapter 2.5 : Threads

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  1. Chapter 2.5 : Threads • Process concept  • Process scheduling  • Interprocess communication  • Deadlocks • Threads

  2. Threads These lecture notes have been adapted from • How to program with threads An introduction to multithreaded programming By Bil Lewis and Daniel J. Berg and • Tanenbaum slides

  3. Processes & Threads • Processes and threads are related concepts • A process is a kernel-level entity • Process structure can only be accessed through system calls • A thread (or a lightweight process) is a user-level entity • The thread structure is in user space • It is accessed directly with the thread library calls, which are just normal user-level functions (threads do not use system calls)

  4. Start Start Edit Document Print Document Edit Document Print Document End End Single vs. Multiple Threads of Execution Multiple Threads Single Thread

  5. Thread Usage (1) A word processor with three threads

  6. Thread Usage (2) A multithreaded Web server

  7. Thread Usage (3) • Rough outline of code for previous slide (a) Dispatcher thread (b) Worker thread

  8. The Classical Thread Model (1) (a) Three processes each with one thread (b) One process with three threads

  9. The Thread Model (2) • (Per process items) Items shared by all threads in a process • (Per thread items) Items private to each thread

  10. The Thread Model (3) Each thread has its own stack

  11. Code Data Stack TCB1 TCB2 TCB3 PCB Process and Thread Data Structures User Space Kernel Space

  12. Characteristics of Threads • The TCB (thread control block) consist of • program counter • register set • stack space Thus the TCB is a reduced PCB • A traditional process is equal to a task with one thread • All threads in a process share the state of that process

  13. Characteristics of Threads (Cont.) • They reside in the exact same memory space (user memory), see the same code and data • When one thread alters a process variable (say, the working directory), all the others will see the change when they next access it • If one thread opens a file to read it, all the other threads can also read from it.

  14. Characteristics of Threads (Cont.) • Because no system calls are involved, threads are fast • There are no kernel structures affected by the existence of threads in a program, so no kernel resources are consumed -- threads are cheap • The kernel doesn't even know that threads exist

  15. Thread Scheduling (1) Possible scheduling of user-level threads • 50-msec process quantum • threads run 5 msec/CPU burst

  16. Thread Scheduling (2) Possible scheduling of kernel-level threads • 50-msec process quantum • threads run 5 msec/CPU burst

  17. Threads of a Task Threads Task Program Counter Data segment Code segment

  18. Implementing Threads in User Space A user-level threads package

  19. Implementing Threads in the Kernel A threads package managed by the kernel

  20. Hybrid Implementations Multiplexing user-level threads onto kernel- level threads

  21. Some Benefits of Writing Multithreaded Programs: • Performance gains from multiprocessing hardware (parallelism) • Increased application throughput • Increased application responsiveness • Enhanced process-to-process communications

  22. Parallellism • Different threads can run on different processors simultaneously with no special input from the user and no effort on the part of the programmer

  23. Throughput • When a traditional, single-threaded program requests a service from the operating system, it must wait for that service to complete, often leaving the CPU idle • Multithreading provides progress even though one or more threads wait for an event as long as other threads are active

  24. Responsiveness • Blocking one part of a process need not block the whole process. Single-threaded applications that do something lengthy when a button is pressed typically display a "please wait" cursor and freeze while the operation is in progress • If such applications were multithreaded, long operations could be done by independent threads, allowing the application to remain active and making the application more responsive to the user

  25. Communications • An application that uses multiple processes to accomplish its tasks can be replaced by an application that uses multiple threads to accomplish those same tasks • Processes-to-process communication through traditional IPC (interprocess communications) facilities (e.g., pipes or sockets) • The threaded application can use the inherently shared memory of the process

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