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Prof. D.S.R. Murthy OS-5 Threads 1

Threads

OverviewMultithreading Models

Threading Issues

Pthreads

Solaris 2 Threads

Windows 2000 Threads

Linux Threads

Java Threads

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Overview

Thread

Light Weight Process (LWP).

Basic unit of CPU utilisation.

Comprises ofThread ID

Program Counter

Register Set

Stack

TypesUser Threads

Kernel Threads

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Overview

Single and Multithreaded Processes

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Overview

Benefits of Multithreading

ResponsivenessAllows program to continue running even if part of it is blocked or

performing lengthy operation.

Resource SharingShares memory and resources of the process.

EconomyMore economical to create and context switch threads.

Utilization of Multiprocessor ArchitecturesEach thread may be running in parallel on a different processor.

Increases concurrency.

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Overview

User Threads

Thread management

done by user-level threads library.

Fast to create and manage.

Examples

- POSIX Pthreads

- Mach C-threads- Solaris 2 UI-threads

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Overview

KernelThreads

Supported by the Kernel.

Slow to create and manage.

Examples

- Windows 95/98/NT/2000

- Solaris 2

- Tru64 UNIX (formerly Digital UNIX)

- BeOS

- Linux

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MultithreadingModels

Many-to-One ModelOne-to-One Model

Many-to-Many Model

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Multithreading Models

Many-to-One Model

Many user-level threads mappedto single kernel thread.

Used on systems that do not support kernel threads.

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One-to-One Model

Each user-level thread maps to kernel thread.Provides more concurrency.

Allows multiple threads to run in parallelon multiprocessors.

Drawback

Creating user thread requires creating the corresponding kernel thread.

Examples- Windows 95/98/NT/2000

- OS/2

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Many-to-Many Model

Allows many userlevel threads to be mapped to many kernel threads.Allows the operating system to create a sufficient no. of kernel threads.

Examples

Windows NT/2000 with the ThreadFiberpackage

Solaris 2

IRIX HP-UX

Tru64 UNIX

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Threading Issues

fork and exec System Calls

Thread Cancellation

Signal Handling

Thread Pools

Thread Specific Data

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Threading Issues

fork and exec System Calls

Versions of fork System call

1. Duplicates all threads.

2. Duplicates only the threadthat invokedthe fork system call.

exec System callreplaces entire process.

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Threading Issues

Thread Cancellation

Task ofterminating thread(target thread) before it has completed.

Asynchronous cancellationOne thread immediately terminates the target thread.

Deferred cancellationTarget thread periodically checks if it should terminate,

allowing the target thread an opportunity to terminate itself

in an orderly fashion.

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Threading Issues

SignalHandling

Signal

Notifies process a particular event occurred.

Received either synchronously or asynchronously.

1. Signal generated by the occurrence of a particular event.

2. Generated signal delivered to a process.

3. Once delivered, the signal must be handled.

4. Default signal handler or user-defined signal handler handles every signal.

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Threading Issues

Thread Pools

Create no. of threads at process startup

and place them into a pool.

Benefits It is usually faster to service a request with an existing thread

than waiting to create a thread.

A thread pool limits the no. of threads

that exist at any one point.

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Threading Issues

ThreadSpecific Data

Each thread

needs its own copy of certain data.

Examples

- Win32

- Pthreads

- Java

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Pthreads

POSIX standard (IEEE 1003.1c)

Application Program Interface (API)

for thread creation and synchronization.

specifies behavior ofthe threadlibrary;

implementation is up to development of the library.

Common in

UNIX operating systems.

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Solaris 2 Threads

Version of UNIX

with supportforthreads at the kerneland userlevels.

Implements

Pthread API.

many-to-many model.

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Solaris 2 Threads

Solaris Process

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Windows 2000 Threads

Implements

Win32 API.

one-to-one mapping.

Each thread contains

- Thread ID

- Register set

- Separate user and kernel stacks

- Private data storage area

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Linux Threads

Refers as tasks rather than threads.

Thread creation

done through clone system call.

Clone system call

allows a child task to sharethe address space of the parent task (process).

Does not support multithreading.

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Java Threads

Created byy Extending Thread class.

y Implementing the Runnable interface.

Managed byJavaVirtual Machine (JVM).

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Java Threads

Java ThreadStates

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