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Chapter 5: Threads
• Overview
• Multithreading Models & Issues
Read Chapter 5 pages 129-135
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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.
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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
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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: ….
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Single and Multithreaded Processes
5
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 Sharing
threads share memory and other process
resources
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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
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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
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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
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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.
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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
11
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.
• One user thread at a time can access the kernel
– Not useful for multiprocessing
• Used on systems that do not support kernel
threads.
13
Many-to-One Model
14
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
15
One-to-one Model
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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
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Many-to-Many Model
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