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Chapter 4: Threads
Operating System Concepts Essentials – 8th Edition
Silberschatz, Galvin and Gagne ©2011
Chapter 4: Threads
 Overview
 Multithreading Models
 Thread Libraries
 Threading Issues
 Operating System Examples
 Windows XP Threads
 Linux Threads
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Objectives
 To introduce the notion of a thread — a fundamental
unit of CPU utilization that forms the basis of
multithreaded computer systems
 To discuss the APIs for the Pthreads, Win32, and
Java thread libraries
 To examine issues related to multithreaded
programming
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Motivation
 Threads run within application
 Multiple tasks with the application can be implemented by
separate threads

Update display

Fetch data

Spell checking

Answer a network request
 Process creation is heavy-weight while thread creation is light-
weight
 Can simplify code, increase efficiency
 Kernels are generally multithreaded
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Single and Multithreaded Processes
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Benefits of Multithreaded Programming
 Responsiveness

Allows a program to continue running even if part of it is
blocked or is performing a lengthy operation.

Thereby increasing responsiveness to the user.
 Resource Sharing:

Sharing code and data allows an application to have several
different threads of activity within the same address space.
 Economy:

It’s more economical to create and context-switch threads.
 Scalability:

Threads may be running in parallel on different processors.
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Multicore Programming
 Multicore systems putting pressure on programmers,
challenges include:

Dividing activities

Balance

Data splitting

Data dependency

Testing and debugging
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Multithreaded Server Architecture
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Concurrent Execution on a Single-core System
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Parallel Execution on a Multicore System
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User Threads
 Thread management done by user-level threads library
 Three primary thread libraries:

POSIX Pthreads

Win32 threads

Java threads
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Kernel Threads
 Supported by the Kernel
 Examples

Windows XP/2000

Solaris

Linux

Tru64 UNIX

Mac OS X
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Multithreading Models
 Many-to-One
 One-to-One
 Many-to-Many
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Many-to-One
 Many user-level threads mapped to single kernel
thread
 Thread management is done by the thread library in
user space, so it’s efficient.
 The entire process will block if a thread makes a
blocking system call
 Because only one thread can access the kernel at a
time, threads are not able to run in parallel
 Examples:

Solaris Green Threads

GNU Portable Threads
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Many-to-One Model
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One-to-One
 Each user-level thread maps to kernel thread
 No blocking system call problems
 Allows multiple threads to run in parallel on multiprocessors
 The only drawback is that creating a user thread requires
creating the corresponding kernel thread
 Number of threads must be restricted to reduce the system
overhead
 Examples

Windows NT/XP/2000

Linux

Solaris 9 and later
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One-to-one Model
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Many-to-Many Model
 Allows many user level threads to be mapped to
many kernel threads
 Allows the operating system to create a sufficient
number of kernel threads
 Solaris prior to version 9
 Windows NT/2000 with the ThreadFiber package
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Many-to-Many Model
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Two-level Model
 Similar to M:M, except that it allows a user thread to
be bound to kernel thread
 Examples

IRIX

HP-UX

Tru64 UNIX

Solaris 8 and earlier
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Two-level Model
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Thread Libraries
 Thread library provides programmer with API for
creating and managing threads
 Two primary ways of implementing

Library entirely in user space

Kernel-level library supported by the OS
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Pthreads
 A POSIX standard (IEEE 1003.1c) API for thread
creation and synchronization
 A specification for thread behavior, not an
implementation
 API specifies behavior of the thread library,
implementation is up to development of the library
 May be provided either as user-level or kernel-level
 Common in UNIX operating systems (Solaris, Linux,
Mac OS X)
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Pthreads Example
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Pthreads Example (Cont.)
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Win32 API Multithreaded C Program
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Win32 API Multithreaded C Program (Cont.)
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Java Threads
 Java threads are managed by the JVM
 Typically implemented using the threads model
provided by underlying OS
 Java threads may be created by:

Extending Thread class

Implementing the Runnable interface (p154)
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Java Multithreaded Program
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Java Multithreaded Program (Cont.)
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Threading Issues ($)
 Semantics of fork() and exec() system calls
 Thread cancellation of target thread

Asynchronous or deferred
 Signal handling

Synchronous and asynchronous
 Thread pools
 Thread-specific data
 Create Facility needed for data private to thread
 Scheduler activations
 (see following slides)
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Semantics of fork() and exec() (p157)
 Does fork() duplicate only the calling thread or all
threads?

Some UNIX systems have chosen to have two versions of folk()

One that duplicates all threads and another that duplicates only
the thread invoked the fork() system call
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Thread Cancellation
 Terminating a thread before it has finished
 Two general approaches:

Asynchronous cancellation terminates the
target thread immediately.

Deferred cancellation allows the target thread
to periodically check if it should be cancelled.
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Signal Handling
 Signals are used in UNIX systems to notify a process that a
particular event has occurred.
 A signal handler is used to process signals
1.
Signal is generated by particular event
2.
Signal is delivered to a process
3.
Signal is handled
 Options:

Deliver the signal to the thread to which the signal applies

Deliver the signal to every thread in the process

Deliver the signal to certain threads in the process

Assign a specific thread to receive all signals for the process
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Thread Pools
 Create a number of threads in a pool where they
await work
 Advantages:

Usually slightly faster to service a request with an
existing thread than create a new thread

Allows the number of threads in the application(s)
to be bound to the size of the pool
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Thread Specific Data
 Allows each thread to have its own copy of data
 Useful when you do not have control over the thread
creation process (i.e., when using a thread pool)
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Scheduler Activations
 Both M:M and Two-level models require
communication to maintain the appropriate number
of kernel threads allocated to the application
 Scheduler activations provide upcalls - a
communication mechanism from the kernel to the
thread library
 This communication allows an application to
maintain the correct number kernel threads
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Lightweight Processes
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Operating System Examples
 Windows XP Threads
 Linux Thread
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Windows XP Threads
 Implements the one-to-one mapping, kernel-level
 Each thread contains

A thread id

Register set

Separate user and kernel stacks

Private data storage area
 The register set, stacks, and private storage area are known as the
context of the threads
 The primary data structures of a thread include:

ETHREAD (executive thread block)

KTHREAD (kernel thread block)

TEB (thread environment block)
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Windows XP Threads Data Structures
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Linux Threads
 Linux refers to them as tasks rather than threads
 Thread creation is done through clone()system call
 clone() allows a child task to share the address
space of the parent task (process)
 struct task_struct points to process data
structures (shared or unique)
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Linux Threads
 fork() and clone() system calls
 Doesn’t distinguish between process and thread
 Uses term task rather than thread
 clone() takes options to determine sharing on process
create
 struct task_struct points to process data
structures (shared or unique)
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Chapter 4 Quiz
 Due: 2 pm, 9/26/2011
 Pages: 166-167
 Exercises: 2, 3, 8, 9, 10, 11
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End of Chapter 4
Operating System Concepts Essentials – 8th Edition
Silberschatz, Galvin and Gagne ©2011