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Chapter 2.5 : Threads. Process concept Process scheduling Interprocess communication Deadlocks 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.
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Chapter 2.5 : Threads • Process concept • Process scheduling • Interprocess communication • Deadlocks • Threads Ceng 334 - Operating Systems
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
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
ThreadsThe Thread Model (1) (a) Three processes each with one thread (b) One process with three threads Ceng 334 - Operating Systems
The Thread Model (2) • Items shared by all threads in a process • Items private to each thread Ceng 334 - Operating Systems
The Thread Model (3) Each thread has its own stack Ceng 334 - Operating Systems
Code Data Stack TCB1 TCB2 TCB3 PCB Process and Thread Data Structures User Space Kernel Space Ceng 334 - Operating Systems
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
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
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
Thread Scheduling (1) Possible scheduling of user-level threads • 50-msec process quantum • threads run 5 msec/CPU burst Ceng 334 - Operating Systems
Thread Scheduling (2) Possible scheduling of kernel-level threads • 50-msec process quantum • threads run 5 msec/CPU burst Ceng 334 - Operating Systems
Threads of a Task Threads Threads Task Program Counter Data segment Code segment Ceng 334 - Operating Systems
Implementing Threads in User Space A user-level threads package Ceng 334 - Operating Systems
Implementing Threads in the Kernel A threads package managed by the kernel Ceng 334 - Operating Systems
Hybrid Implementations Multiplexing user-level threads onto kernel- level threads Ceng 334 - Operating Systems
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
Thread Usage (1) A word processor with three threads Ceng 334 - Operating Systems
Thread Usage (2) A multithreaded Web server Ceng 334 - Operating Systems
Thread Usage (3) • Rough outline of code for previous slide (a) Dispatcher thread (b) Worker thread Ceng 334 - Operating Systems
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
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
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
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
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