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Chapter 4: Threads
Operating System Concepts – 8th Edition,
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
 To discuss the APIs for the Pthreads, Win32, and Java thread libraries
 To examine issues related to multithreaded programming
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Threads Concept
 So far we assumed a process was an executing program with a single
thread of control.
 Motivating example - A web server accepts client requests for Web pages,
images, sound, and so forth (see example in next page)
 A thread is a basic unit of CPU utilization (why CPU? What if it differed just in a
specific device utilization?);

Thread ID

Program counter

Register set

Stack
it comprises:
 It shares with other threads belonging to the same process, its

Code section

Data section and

Other OS resources such as open files
 If a process has multiple threads of control, it can perform more than one
task at a time.
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Multithreaded Server Architecture
Operating System Concepts – 8th Edition
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Single and Multithreaded Processes
Example – word processor: (a) displaying graphics,
(b) responding to keystroke, (c) spell checker
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Benefits
 Responsiveness – allow a program to continue running even if
part of it is blocked or performing a lengthy operation (e.g., webbrowser loading an image)
 Resource Sharing – threads share the memory and resources of
the process (allows an application to have several threads of activity
within the same address space)
 Economy – it is more economical to create and context-switch
threads (because they share resources of the process to which they
belong)

In Solaris a process is thirty times slower to create and five times
slower to context-switch
 Scalability (utilizing multiprocessor architectures) – same
process can make use of several CPUs by using threads
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Multicore Programming
 Multicore systems putting pressure on programmers, challenges include:

Dividing activities –find areas that can be divided into separate,
concurrent tasks and thus can run in parallel on individual cores.

Balance – ensure tasks perform equal work of equal value

Data splitting – ensure that the data be divided to run on separate
cores just like the application is divided into separate tasks.

Data dependency – ensure that the execution of the tasks is
synchronized to accommodate data dependency.

Testing and debugging – more difficult than for single-threaded
applications since there are many different execution paths when a
program is running in parallel on multiple cores.
Operating System Concepts – 8th Edition
4.7
Silberschatz, Galvin and Gagne ©2009
Concurrent Execution on a Single/Multi-core System
Operating System Concepts – 8th Edition
4.8
Silberschatz, Galvin and Gagne ©2009
User and Kernel Threads
 Kernel thread (sometimes called a Lightweight Process - LWP) is created
and scheduled by the kernel:

More expensive to create than user threads
 User thread is created by a threading library and scheduling is managed by
the threading library itself (Which runs in user mode).

All user threads belong to process that created them.

Kernel is unaware of user-level threads so all thread
 The major difference is when using multiprocessor systems:

User threads completely managed by the threading library can't be ran
in parallel on the different CPUs (will run fine on uniprocessor systems)

Since kernel threads use the kernel scheduler, different kernel threads
can run on different CPUs.
 Programs can have user-space threads when threading with timers, signals,
or other methods to interrupt their own execution, performing a sort of adhoc time-slicing.
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
User Threads
 Thread management done by user-level threads library
 Three primary thread libraries:

POSIX Pthreads

Win32 threads

Java threads
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Kernel Threads
 Supported by the Kernel
 Examples

Windows XP/2000

Solaris

Linux

Mac OS X
Operating System Concepts – 8th Edition
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Multithreading Models
 Many-to-One
 One-to-One
 Many-to-Many
Operating System Concepts – 8th Edition
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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 is
efficient.
 Drawback - the entire process will block if a thread makes a blocking
system call.
 Examples:

Solaris Green Threads

GNU Portable Threads
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
One-to-One
 Each user-level thread maps to kernel thread
 More concurrency compared to many-to-one model since another thread
will be able to run when a thread makes a blocking system call.
 Allows multiple threads to run in parallel on multiprocessors.

Drawback:

Creating a user thread requires creating a corresponding kernel thread.

Performance will degrade and hence, most systems has a restriction on the number of
threads allowed.
 Examples

Windows NT/XP/2000

Linux

Solaris 9 and later
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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
Operating System Concepts – 8th Edition
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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
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
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
Operating System Concepts – 8th Edition
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Pthreads
 May be provided either as user-level or kernel-level
 A POSIX standard (IEEE 1003.1c) API for thread creation
and synchronization
 API specifies behavior of the thread library, implementation
is up to development of the library (i.e. the OS designers)
 Common in UNIX operating systems (Solaris, Linux, Mac
OS X)
Operating System Concepts – 8th Edition
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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 (and override its run() method)

Implementing the Runnable interface
 Calling start()

Allocates memory and initializes a new thread in the JVM

Calls the run() method, making the thread eligible to run on
the JVM
Operating System Concepts – 8th Edition
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Threading Issues
 Semantics of fork() and exec() system calls
 Thread cancellation of target thread

Asynchronous or deferred
 Signal handling
 Thread pools
 Thread-specific data
 Scheduler activations
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Semantics of fork() and exec()
 Does fork() duplicate only the calling thread or all threads?
 If one thread in a program calls fork(), does the new process duplicate all
threads, or is the new process single-threaded? Unix has both versions.

When exec() system call is executed, the program specified in its
parameter will replace the entire process, including all the threads. Hence,
duplicating only the calling thread is appropriate.
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Thread Cancellation
 Terminating a thread before it has finished


Example: multiple threads concurrently searching
through a database and one thread returns the result
Stopping a webpage from loading any further
 Two general approaches:

Asynchronous cancellation terminates the target
thread immediately – may not free its resources

Deferred cancellation allows the target thread to
periodically check if it should be cancelled – checking a
flag
Operating System Concepts – 8th Edition
4.22
Silberschatz, Galvin and Gagne ©2009
Signal Handling

Signals are used in UNIX systems to notify a process that a
particular event has occurred

Example: illegal memory access, division by zero

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
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
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
 Unlimited threads could exhaust system resources, such as CPU time
or memory
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Scheduler Activations
 Both M:M and Two-level models require communication to maintain
the appropriate number of kernel threads allocated to the application
 Such coordination allows the number of kernel threads to be
dynamically adjusted to help ensure the best performance
 Communication between the user-level thread library and the kernel
is known as Scheduler activations.
 Scheduler activations provide upcalls - a communication mechanism
from the kernel to the thread library
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009
Operating System Concepts – 8th Edition
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Silberschatz, Galvin and Gagne ©2009