Chapter 4: Multithreaded Programminghscc.cs.nthu.edu.tw/~sheujp/lecture_note/os08_CH04.pdf ·...

Chapter 4: Multithreaded Chapter 4: Multithreaded ProgrammingProgramming

4.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts

Chapter 4: Multithreaded Chapter 4: Multithreaded ProgrammingProgramming

Overview

Multithreading Models

Thread Libraries

Threading Issues

Operating-System Examples


OverviewOverview

A thread is a basic unit of CPU utilization; it comprises a thread ID, a program counter, a register set, and a stack.

A traditional, or heavyweight process has a single thread of control.

It shares with other threads belonging to the same process its code section, data section, and other OS resources, such as open files and signals.

In busy WWW server: The server creates a separate thread that would listen for clients requests, when a request was made, creates a thread to service the request.


Single and Multithreaded ProcessesSingle and Multithreaded Processes


Benefits (1)Benefits (1)

Responsiveness: Allow a program to continue running even if part of it is blocked or is performing a lengthy operation.

Resource sharing: several different threads of activity all within the same address space.

Economy: Allocating memory and resources for process creation is costly. In Solaris, creating a process is about 30 times slower than is creating a thread, and context switching is about five times slower. A register set switch is still required, but no no memorymemory--management related work is neededmanagement related work is needed.


Benefits (2)Benefits (2)

Utilization of multiprocessor architecture: Several thread may be running in parallel on different processors.

Of course, multithreading a process may introduce concurrency control problem that requires the use of critical sections or locks.


User ThreadsUser Threads

Thread management done by user-level threads library.

Fast: All thread creating and scheduling are done in user space without the need for kernel intervention.

Any user-level thread performing a blocking system call will cause the entire process to block, even if there are other threads available to run within the applications, if the kernel is single-thread.

Examples

Pthreads

Win32 threads


Kernel ThreadsKernel Threads

Supported and managed directly by the OS

Examples

Windows XP/2000

Solaris

Linux

Tru64 UNIX

Mac OS X


Multithreading Models (1)Multithreading Models (1)

(Multi-Thread vs. Multi-process)

Multiple processes

Each is independent and has it own program counter, stack register, and address space. This is useful for unrelated jobs.

Multiple processes can perform the same task as well. (e.g., provide data to remote machines in a network file system). Each executes the same code but has it own memory and file resources.

Multiple-thread process

It is more efficient to have one process containing multiple threads serve the same task.

Most Systems Support for both user and kernel threads


Multithreading Models (2)Multithreading Models (2) Uses fewer resources, including memory, open files and CPU scheduling.

Threads are not independent to each other. This structure does not provide protection.

Only a single user can own an individual task with multiple threads. The threads should be designed to assist one another.

Threads can create child threads. If one thread is blocked, another thread can run.

Threads provide a mechanism that allows sequential processes to make blocking system calls while also achieving parallelism.


Multithreading ModelsMultithreading Models

Many-to-One

One-to-One

Many-to-Many


ManyMany--toto--OneOne

Many user-level threads mapped to single kernel thread.

Used on systems that do not support kernel threads.

Thread management is done in user space, so it is efficient.

The entire process will block if a thread makes a blocking system call.

Only one thread can access the kernel at a time, multiple threads are unable to run in parallel on multiprocessors.


ManyMany--toto--One ModelOne Model


OneOne--toto--OneOne

Each user-level thread maps to kernel thread.

More concurrency

Overhead: Creating a thread requires creating the corresponding kernel thread.

Examples

- Windows XP/NT/2000

- Linux

- Solaris 9 and later


OneOne--toto--one Modelone Model


ManyMany--toto--Many ModelMany 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

Multiplexes many user-level threads to a smaller or equal number of kernel threads

The corresponding kernel threads can run in parallel on a multiprocessor.

When a thread performs a blocking call, the kernel can schedule another thread for execution

Solaris prior to version 9

Windows NT/2000 with the ThreadFiber package


ManyMany--toto--Many ModelMany Model


TwoTwo--level Modellevel 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


TwoTwo--level Modellevel Model


Threading IssuesThreading Issues

Semantics of fork() and exec() system calls. Duplicate all the threads or not?

Thread cancellation: Asynchronous or deferred

Signal handling: Where then should a signal be delivered?

Thread pools: Create a number of threads at process startup.

Thread specific data: Each thread might need its own copy of certain data.

Scheduler activations


Semantics of fork() and exec()Semantics of fork() and exec()

Does fork() duplicate only the calling thread or all threads?

Two versions of fork(): duplicates all threads and another duplicates only the thread that invoked the fork() system call

If exec() is called immediately after forking, then duplicating all threads is unnecessary.


Thread CancellationThread Cancellation

Terminating a thread before it has completed.

Two general approaches:

Asynchronous cancellation terminates the target thread immediately

Deferred cancellation The target thread to periodically check whether it should be terminate, allowing it an opportunity to terminate itself in an orderly fashion (canceled safely).

Cancellation points


Signal HandlingSignal Handling

Signals are used in UNIX systems to notify a process that a particular event has occurred

A signal handler is used to process signals

Signal is generated by particular event

Signal is delivered to a process

Signal is handled

Delivering Signals:

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


Thread PoolsThread 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


Thread Specific DataThread Specific Data

Allows each thread to have its own copy of data

Each transaction assigned a unique number in the transaction-processing system

Useful when you do not have control over the thread creation process (i.e., when using a thread pool)


Scheduler ActivationsScheduler Activations

Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application

Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library

This communication allows an application to maintain the correct number kernel threads


OperatingOperating--system Examplesystem Example

Explore how threads are implemented in Windows XP and Linux systems.


Windows XP ThreadsWindows XP Threads

Implements the one-to-one mapping

Each thread contains

A thread id

Register set

Separate user and kernel stacks

Private data storage area

The register set, stacks, and private storage area are known as the context of the threads

The primary data structures of a thread include:

ETHREAD (executive thread block)

KTHREAD (kernel thread block)

TEB (thread environment block)


Linux ThreadsLinux 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)


Home worksHome works

1, 3, 4, 5, 6

Project- Matrix Multiplication (multi-thread vs. multi-process)

End of Chapter 4End of Chapter 4

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