Chapter 2.5 : Threads

1. Chapter 2.5 : Threads • Process concept  • Process scheduling  • Interprocess communication  • Deadlocks • Threads Ceng 334 - Operating Systems

2. 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 Ceng 334 - Operating Systems

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) Ceng 334 - Operating Systems

4. ThreadsThe Thread Model (1) (a) Three processes each with one thread (b) One process with three threads Ceng 334 - Operating Systems

5. The Thread Model (2) • Items shared by all threads in a process • Items private to each thread Ceng 334 - Operating Systems

6. The Thread Model (3) Each thread has its own stack Ceng 334 - Operating Systems

7. Code Data Stack TCB1 TCB2 TCB3 PCB Process and Thread Data Structures User Space Kernel Space Ceng 334 - Operating Systems

8. 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 Ceng 334 - Operating Systems

9. 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. Ceng 334 - Operating Systems

10. 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 Ceng 334 - Operating Systems

11. Thread Scheduling (1) Possible scheduling of user-level threads • 50-msec process quantum • threads run 5 msec/CPU burst Ceng 334 - Operating Systems

12. Thread Scheduling (2) Possible scheduling of kernel-level threads • 50-msec process quantum • threads run 5 msec/CPU burst Ceng 334 - Operating Systems

13. Threads of a Task Threads Threads Task Program Counter Data segment Code segment Ceng 334 - Operating Systems

14. Implementing Threads in User Space A user-level threads package Ceng 334 - Operating Systems

15. Implementing Threads in the Kernel A threads package managed by the kernel Ceng 334 - Operating Systems

16. Hybrid Implementations Multiplexing user-level threads onto kernel- level threads Ceng 334 - Operating Systems

17. Start Start Edit Document Print Document Edit Document Print Document End End Single vs. Multiple Threads of Execution Multiple Threads Single Thread Ceng 334 - Operating Systems

18. Thread Usage (1) A word processor with three threads Ceng 334 - Operating Systems

19. Thread Usage (2) A multithreaded Web server Ceng 334 - Operating Systems

20. Thread Usage (3) • Rough outline of code for previous slide (a) Dispatcher thread (b) Worker thread Ceng 334 - Operating Systems

21. Some Benefits of Writing Multithreaded Programs: • Performance gains from multiprocessing hardware (parallelism) • Increased application throughput • Increased application responsiveness • Enhanced process-to-process communications Ceng 334 - Operating Systems

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 Ceng 334 - Operating Systems

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 Ceng 334 - Operating Systems

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 Ceng 334 - Operating Systems

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 Ceng 334 - Operating Systems