chapter 5: threads

Silberschatz, Galvin and Gagne 20025.1Operating System Concepts

Chapter 5: Threads

Overview Multithreading Models Threading Issues Pthreads Solaris 2 Threads Windows 2000 Threads Linux Threads Java Threads


Thread vs. Process

A thread – lightweight process (LWP) is a basic unit of CPU utilization.

It comprises a thread ID, a program counter, a register set, and a stack.

A traditional (heavyweight) process has a single thread of control.

If the process has multiple threads of control, it can do more than one task at a time.


Single and Multithreaded Processes


Motivation

An application typically is implemented as a separate process with several threads of control.

For example, a web browser might have one thread display images or text while another thread retrieves data from the network.

It is more efficient for a process that contains multiple threads to serve the same purpose.

This approach would multithread the web-server process.


Benefits

Responsiveness: Multithreading an interactive application may allow 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: Threads share the memory and the resources of the process to which they belong.

Economy: Allocating memory and resources for process creation is costly.

Utilization of MP (multiprocessor) Architectures: Each thread may be running in parallel on a different processor.


User Threads

Thread management done by user-level threads library User-level threads are fast to create and manage. If the kernel is single-threaded, then any user-level thread

performing a blocking system call will cause the entire process to block.

Examples

- POSIX Pthreads

- Mach C-threads

- Solaris UI-threads


Kernel Threads

Supported by the Kernel: The kernel performs thread creation, scheduling, and management in kernel space.

Examples

- Windows 95/98/NT/2000

- Solaris

- Tru64 UNIX

- BeOS

- OpenBSD

- FreeBSD

- Linux


Multithreading Models

Many-to-One: The many-to-one model maps many user-level threads to one kernel thread.

One-to-One: The one-to-one model maps each user thread to a kernel thread.

Many-to-Many: The many-to-many multiplexes many user-level threads to a smaller or equal number of kernel threads.


Many-to-One

Many user-level threads mapped to single kernel thread.

Used on systems that do not support kernel threads.


Many-to-One Model


One-to-One

Each user-level thread maps to kernel thread.

Examples

- Windows 95/98/NT/2000

- OS/2


One-to-one Model


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 2 Windows NT/2000 with the ThreadFiber package


Many-to-Many Model


Multithreading Models - Conclusion

Whereas the many-to-one model allows the developer to create as many user threads as she wishes, true concurrency is not gained because the kernel can schedule only one thread at a time.

The one-to-one model allows for greater concurrency, but the developer has to be careful not to create too many threads within an application.

The many-to-many model suffers from neither of these shortcomings: Developer can create as many user threads as necessary, and the corresponding kernel threads can run in parallel on a multiprocessor. Also, when a thread performs a blocking system call, the kernel can schedule another thread for execution.


Threading Issues

Semantics of fork() and exec() system calls: Two versions of fork are one that duplicates all threads and another that duplicates only the thread that invoked the fork system call.

Thread cancellation is the task of terminating a thread before it has completed.

• Two different scenarios are asynchronous cancellation and deferred cancellation.

• A thread checks if it should be cancelled at a point referred as cancellation point.

Signal handling Thread pools Thread specific data is a copy of data owned by a

thread.


Signal Handling

A signal is used in UNIX systems to notify a process that a particular event has occurred.

All signals follow the same pattern:1. A signal is generated by the occurrence of a particular

event.

2. A generated signal is delivered to a process.

3. Once delivered, the signal must be handled. A signal can be synchronous or asynchronous:

• Synchronous signals are delivered to the same process.• Asynchronous signal is generated by an external event.


Signal Handling

Every signal may be handled by one of two possible handlers: • A default signal handler• A user-defined signal handler

The options exist for delivering signals to a multithreaded program:1. Deliver the signal to the thread to which the signal applies.

2. Deliver the signal to every thread in the process.

3. Deliver the signal to certain threads in the process.

4. Assign a specific thread to receive all signals for the process.

Windows 2000 uses asynchronous procedure calls (APCs) to emulate signals.


Thread Pools

A multithread server has potential problems: • The time required includes thread creating and discarding.• Unlimited threads could exhaust system resources.

One solution to this issue is to use thread pools. A thread pool has a number of threads being created at

process startup, where they sit and wait for work. The benefits of thread pools are:

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

• A thread pool limits the number of threads at any one point.


Thread-Specific Data

A multithread server has potential problems: • The time required includes thread creating and discarding.• Unlimited threads could exhaust system resources.

One solution to this issue is to use thread pools. A thread pool has a number of threads being created at

process startup, where they sit and wait for work. The benefits of thread pools are:

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

• A thread pool limits the number of threads at any one point.


Pthreads

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.

Common in UNIX operating systems. Refer to the program shown in Figure 5.5.


Solaris 2 Threads

Solaris 2 is a version of UNIX with support for threads at the kernel and user levels, SMP, and real-time scheduling.

Solaris 2 implements the Pthread API and UI threads. Between user- and kernel-level threads are ligthweight

processes (LWPs). Threads in a process multiplex to connect an LWP. An

LWP corresponds a kernel thread. A (un)bound user-level thread is (not) permanently

attached to an LWP.


Solaris 2 Threads


Solaris Process


Windows 2000 Threads

Implements the one-to-one mapping. Each thread contains

- a thread id

- register set

- separate user and kernel stacks

- private data storage area


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)


Java Threads

Java threads may be created by:

Extending Thread class Implementing the Runnable interface

Calling the start method for the new object does two things:

1. It allocates memory and initializes a new thread in the JVM.

2. It calls the run method, making the thread eligible to be run by JVM.

Java threads are managed by the JVM.


Java Thread States

chapter 5: threads

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