cs presentation template · ("1:1") and user-level ("n:1") threading. many to...

Department of Computer Science

DCS

COMSATS Institute of Information Technology

Threads

Rab Nawaz JadoonAssistant Professor

COMSATS Lahore

Pakistan

Operating System Concepts

Department of Computer Science 2

Threads

Definitions

In the previous discussion, a process was an

executing program with a single thread of control.

Most modern OS now provide features enabling a process to contain multiple threads of control.

A thread is a basic unit of CPU utilization.

It is a light weight process.

If a process has multiple thread of control, it can

perform more than one task at a time.

Seamless execution of two or more sections of a program at the same time. Threads allows us to do

this.


Process, threads

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What is the relationship of a thread, a process and a program?


Threads

Writing multithreaded programs can be tricky,

Although human mind can function concurrently,

people find it difficult to jump between parallel thoughts.

Why it is difficult to handled multithreaded

applications or programs, just see the example,

Open three books to page 1, and try reading the books concurrently,

Read a few words from book 1, then from 2 and then from 3rd one.

Execute the same work repeatedly.

After this experiment, you will appreciate some of the key challenges of the multithreading.

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Threads

i.e. switching between books, reading briefly,

remembering your place in each book etc are

hectic at the end.

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Threads

Thread possesses a subset of the resources

contained in a process, such as,

Processor registers,

Stack and other,

TSD (thread specific data)

Such as signal masks (data that describes which signals a thread will not receive)

Many OS supports threads, Win32 threads used by MS-32 bit Windows.

C-threads these are created from a thread library in the MAC Microkernel.

POSIX threads Pthread standards

Java threads Thread Class / runnable interface

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Threads

The primary goad of POSIX is to allow

Multithreaded programs to be portable across

multiple OS platforms.

POSIX has been implemented in a variety of

OS, Including,

Solaris

Linux

Windows XP etc.

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Threads

Single threaded & multithreaded processes

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Question???

Why is it difficult to write portable multithreaded

applications?

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ANSWER!There is no standard threading library that is implemented on all platforms.


Threads

Many s/w packages that run on modern desktop

PCs are mulithreaded.

An application typically is implemented as a separate

process with several threads of control.

For example a web browser might have one thread displaying images while other retrieves data from the network.

Next example is of word processor may have

displaying graphics, another thread for responding the keystrokes from the user and a 3rd thread for

performing spelling and grammar checking in the background.

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Threads

Benefits

Responsiveness

Multithreading interactive application may allow a program to continue running even if a part of it is blocked or is performing a lengthy operation.

For example, a multithreaded web browser could still allow user interaction in one thread while an image was being loaded in another thread.

Resource sharing

By default, threads share the memory and the resources of the process to which they belong.

The benefits of sharing code and data is that it allows an application to have several different threads of activity within the same address space.

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Threads

Economy

Allocating memory and resources for process

creation is costly.

It is more economical to create and context switch threads.

In Solaris, creating a process is about thirty times

slower than is a creating a thread, and context switching is about five time slower.

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Threads

Utilization of multiprocessor architecture

The benefit of multithreading can be greatly

increased in a multiprocessor architecture, where threads may be running parallel on different

processor.

A single threaded process can only run on one CPU, no matter how many are available.

Multithreading on a multi CPU machine increases concurrency.

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Threads

Different threads in a process are not

independent as different processes.

All threads have exactly the same address space

which means that they share the same global variables.

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Thread states

Like a traditional process, a thread can be in

any one of several states,

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States Description

Ready The thread is ready to run and waiting for the CPU.

Running The thread is executing on the CPU.

Waiting The thread is waiting for some event to happen.

Sleeping The thread has been told to sleep for a time

Blocked The thread is waiting for I/O to finish

dead The thread is terminated


Thread life cycle (JAVA)

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Born


Thread stack

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Thread states

If a process is swapped out, all of its threads

are swapped out, b/c they all share the same

address space of the process.

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Thread Operations

Threads and processes have many operations in

common such as,

Create

Exit

Suspend

Resume

Sleep

Wake

Etc.

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Threading Models

Almost all OS’s support one of three primary

threading models,

User level threads (ULTs)

Kernel level threads (KLTs)

Combination of ULTs and KLTs

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User Level Threads

User Level Threads

These performs operation in user space.

These are created by run time libraries that cannot

access kernel primitives directly.

These are transparent to the OS.

Single execution context i.e. OS dispatches the multithreaded process as a unit, as opposed to

dispatching each individual thread.

For this reason ULTs are also called many to one thread mapping, bc OS maps all threads in a

multithreaded process to a single execution context.

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User level thread

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User level threads

In ULTs, OS is unaware that process contains

multiple threads.

In ULTs the multithreaded process continues until its quantum expires.

ULTs are more portable b/c they do not rely on

a particular OS’s threading API.

Application developers can tune the threading

library’s scheduling algorithm to meet the needs of specific applications.

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ULTs

ULTs presents an API to applications that is independent of the operating system APIs.

Deficiencies

Kernel views a multithreaded process as a single thread of control.

For instance, ULTs do not scale well to multiprocessor systems, b/c the kernel can not simultaneously dispatch a process thread to multiple

processors.

So ULTs can result in suboptimal performance in multiprocessor systems.

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Kernel Level threads

The KLTs are used to address the limitations in

ULTs.

This is done by mapping each thread to its own

execution context.

As a result KLTs are often described as one to one thread mapping.

Such mappings require the OS to provide each user thread with a kernel thread that the OS can dispatch.

When a user requests a KLT using system calls defined by OS’s API, the OS create a KLT that executes the user thread’s instructions.

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Kernel level thread

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Benefits of KTLs

The kernel can dispatch a process’s threads to

several processors at one, which can improve

performance for application designed for

concurrent execution.

Kernel can manage each thread individually,

meaning that OS can dispatch a process ‘s

ready threads even if one of its thread is

blocked.

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Difficiencies

KLTs are not always the optimal solution for

multithreaded applications.

KLTs implementation is less efficient than ULTs implementation b/c scheduling and

synchronization operations invoke the kernel

which increases overhead.

Software that employs KLTs, are less portable

than the S/W that employs ULTs – b/c

The application programmer that uses KLTs must modify the program to use the thread API for each

OS on which it runs.

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Hybrid Threading

This is a compromise between kernel-level

("1:1") and user-level ("N:1") threading.

Many to many association mechanism.

In general, "M:N" threading systems are more

complex to implement than either kernel or userthreads, because changes to both kernel and user-space code are required.

In the M:N implementation, the threading library is

responsible for scheduling user threads on theavailable schedulable entities; this makes context

switching of threads very fast, as it avoids systemcalls.

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Hybrid Threading Model

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T1 T2 T3 T4 T1 T2 T3 T1 T2

User

Sp

ace

Kern

el

Sp

ace

Process P1 Process P2 Process P3

Thread

Execution Context


Threads Implementation(JAVA)

public class ThreadTester {

public static void main(String args[])

{

//create and name each thread.

PrintThread thread1 = new PrintThread();



system.err.println("starting thread");

thread1.start();

thread2.start();

thread3.start();

system.err.println("Threads Started, main ended");

}//end of main

}//end class ThreadTester

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Threads in JAVA

// Class PrintThread controls thread execution.

Class PrintThread extends Thread {

private int sleepTime;

//assign name to threads by calling super class constructor.

public PrintThread ( Stirng name )

{

super(name);

//pick random sleep time between 0 and 5 second

sleepTime = ( int ) ( math.random ( ) * 5001);

}

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Threads in JAVA

pubic void run()

{

try {

system.err.println(getName() + "going to sleep for " + sleep time + "milliseconds");

thread.sleep(sleeptime);

} //end try

catch(InterruptedException e)

{

e.printStackTrace();

} //end catch

system.err.println(getName() + "done sleeping");

}//end method run

}// end class PrintThread.

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Output

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Starting threadsTreads started, main ends

thread1 going to sleep for 1217 millisecondsthread2 going to sleep for 3989 millisecondsthread3 going to sleep for 662 milliseconds

thread3 done sleepingthread1 done sleepingthread2 done sleeping

Department of Computer Science 35

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