Ceng Operating Systems Chapter 2.5 : Threads Process concept  Process scheduling  Interprocess communication  Deadlocks  Threads

Ceng 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 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 Operating Systems Threads The Thread Model (1) (a) Three processes each with one thread (b) One process with three threads

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

Ceng Operating Systems The Thread Model (3) Each thread has its own stack

Ceng Operating Systems Process and Thread Data Structures Kernel Space User Space PCB CodeDataStack TCB1TCB2TCB3

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

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

Ceng Operating Systems Threads of a Task Threads Code segment Data segment Program Counter Threads Task

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

Ceng Operating Systems Implementing Threads in the Kernel A threads package managed by the kernel

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

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

Ceng Operating Systems Thread Usage (1) A word processor with three threads

Ceng Operating Systems Thread Usage (2) A multithreaded Web server

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

Ceng 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 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 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 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 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