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Chapter 4: Threads
Operating System Concepts – 8th Edition
Silberschatz, Galvin and Gagne ©2009
Chapter 4: Threads

Overview

Motivation

Benefits
Operating System Concepts – 8th Edition
4.2
Silberschatz, Galvin and Gagne ©2009
Objectives

To introduce the notion of a thread — a fundamental unit of CPU utilization that
forms the basis of multithreaded computer systems.
Operating System Concepts – 8th Edition
4.3
Silberschatz, Galvin and Gagne ©2009
Overview

A thread is a basic unit of CPU utilization (i.e. It is the smallest sequence of
programmed instructions that can be managed independently)

Thread comprises :

ID, a program counter, a register set and a stack.

It shares with other threads belonging to the same process its code section,
data section and other operating system resources.

A traditional (or heavyweight) process has a single thread of control.

A thread could be considered a ‘lightweight’ process

If a process has multiple threads of control, it can perform more than one task
at a time.

Threads provide a way to improve application performance through parallelism.

Each thread belongs to exactly one process and no thread can exist outside a
process.
Operating System Concepts – 8th Edition
4.4
Silberschatz, Galvin and Gagne ©2009
Single and Multithreaded Processes

Traditional ( heavyweight ) process has a single thread of control - There is one program
counter, and one sequence of instructions that can be carried out at any given time.

Multi-threaded applications have multiple threads within a single process, each having their
own program counter, stack and set of registers, but sharing common code, data, and certain
structures such as open files.
Operating System Concepts – 8th Edition
4.5
Silberschatz, Galvin and Gagne ©2009
Motivation

Many software packages that run on modern desktop PCs are multithreaded. An
application typically is implemented as a separate process with several threads of
control.

Examples :


E.G. (1) : A word processor may have a thread for displaying graphics, another
thread for responding to keystrokes from the user, and a third thread for
performing spelling and grammar checking in the background.

E.G. (2) : A web server - Multiple threads allow for multiple requests to be
satisfied simultaneously, without having to service requests sequentially or to
fork off separate processes for every incoming request….(creating a new thread
belongs to an existing process is less overhead of creating a new process)
Most operating system kernels are now multithreaded >>>> several threads operate
in the kernel, and each thread performs a specific task, such as managing devices or
interrupt handling.
Operating System Concepts – 8th Edition
4.6
Silberschatz, Galvin and Gagne ©2009
Benefits

There are four major categories of benefits to multi-threading:

Responsiveness - One thread may provide rapid response while other threads
are blocked or slowed down doing intensive calculations.

Resource sharing –

Processes may only share resources through techniques (e.g. shared
memory or message passing) …Such techniques must be explicitly arranged
by the programmer.

By default threads share common code, data, and other resources, which
allows multiple tasks to be performed simultaneously in a single address
space.
Operating System Concepts – 8th Edition
4.7
Silberschatz, Galvin and Gagne ©2009
Benefits (Cont.)

Economy - Creating and managing threads is much faster than performing the
same tasks for processes.

Scalability

The benefits of multithreading can be greatly increased in a multiprocessor
architecture, where threads may be running in parallel on different
processors.

A single-threaded process can only run on one processor, regardless how
many CPUs are available. whereas the execution of a multi-threaded
application may be split amongst available processors. …….( Note that
single threaded processes can still benefit from multi-processor architectures
when there are multiple processes contending for the CPU)
Operating System Concepts – 8th Edition
4.8
Silberschatz, Galvin and Gagne ©2009
End of Chapter 4
Operating System Concepts – 8th Edition
Silberschatz, Galvin and Gagne ©2009