Chapter 5: Threads

1
Chapter 5 Threads

• Overview
• Multithreading Models Issues
• Read Chapter 5 pages 129-135

2
A Thread (Lightweight Process, LWP)

• A (traditional/ heavyweight) process has a single
  thread (of control).
• Modern operating systems allow a process to
  contain multiple threads of control.
• Each thread, is defined by its thread ID, program
  counter, register set stack (for temporary
  data).
• A thread shares with other threads of the same
  process its code section, data section, open
  files other operating system resources.

3
Multithreaded Applications

• Many applications are best executed as a single
  process with several threads of control
• Multithreading single program composed of a
  number of different concurrent activities

4
Example Multithreaded Applications

• Web browser
• A thread displays information
• A thread retrieves data from the network
• A Web Server
• A thread to listen for client requests
• A thread to service each request
• Word Processor
• Thread ONE displays graphics
• Thread TWO reads keystrokes
• Thread THREE performs spelling and grammar
  checking
• Thread FOUR .

5
Single and Multithreaded Processes
6
Benefits of Multithreaded Programming

• Responsiveness
• e.g. a multithreaded web browser allows user
  interaction in one thread and downloads an image
  in another thread
• Resource Sharingthreads share memory and other
  process resources

7
Benefits of Multithreaded Programming(continued)

• Economy
• Creation is 30 times slower for a process than a
  thread
• Context switching is 5 times slower for a process
  than a thread
• More Efficient Utilization of Multiprocessor
  Architectures
• threads run in parallel on different processors

8
Thread State

• Shared state
• Global variables
• Heap variables
• Private state
• Stack variables
• Registers
• When thread switch occurs, OS needs to save and
  restore private state

9
User Threads

• In user space and without kernel intervention,
  the library provides support for thread
• Creation
• Scheduling
• Management
• Thread management done by user-level threads
  library
• User threads are fast to create and manage

10
Example User Threads

• Solaris threads Sun Microsystems proprietary
  version of threads. Not portable.
• Pthreads
• An Application Programming Interface, (API),
  standard for thread creation and synchronization
  specified by the IEEE Portable Operating Systems
  Interface, POSIX in 1995.
• API specifies behavior of the thread library,
  implementation is up to development of the
  library.
• Common in UNIX operating systems.
• Pthreads are defined as a set of C language
  programming types and procedure calls,
  implemented with a pthread.h header/include file
  and a thread library.

11
Kernel Threads

• Kernel performs thread
• Creation
• Scheduling
• Management
• In a MP (multiprocessing) environment, the kernel
  schedules threads on different processors
• Example operating systems with kernel threads
• - Windows 95/98/NT/2000
• - Solaris
• - Linux

12
Multithreading Models

• Many-to-One
• One-to-One
• Many-to-Many

13
Many-to-One

• Many user-level threads mapped to single kernel
  thread.
• One user thread at a time can access the kernel
• Not useful for multiprocessing
• Used on systems that do not support kernel
  threads.

14
Many-to-One Model
15
One-to-One

• Each user-level thread maps to a kernel thread.
• Allows another thread to run when a thread makes
  a blocking system call (more concurrency than
  many to 1)
• Allows multiple threads to run in parallel on
  multiprocessors
• Drawback Creating a user thread requires
  creating a the corresponding kernel thread
• To reduce overhead, restrict the number of
  threads supported by the system
• Examples
• - Windows 95/98/NT/2000
• - OS/2

16
One-to-one Model
17
Many-to-Many Model

• Allows many user level threads to be multiplexed
  to a smaller or equal number of kernel threads.
• Allows the operating system to create a
  sufficient number of kernel threads.
• Solaris 2
• Windows NT/2000 with the ThreadFiber package

18
Many-to-Many Model