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OS Support for Building Distributed Applications:

Multithreaded Programming using Java

Threads

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Outline

Introduction Thread Applications Defining Threads Java Threads and States Architecture of Multithreaded servers Threads Synchronization Thread Concurrency Models Summary

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Introduction

We discuss how Middleware is supported by the Operating System (OS) facilities at the nodes of a distributed system.

The OS facilitates: Encapsulation and protection of resources

inside servers; It supports invocation of mechanisms

required to access those resources including concurrent access/processing.

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Middleware and Network Operating System (NOS)

Many DOS (Distributed OS) have been investigated, but there are none in general/wide use. But NOS are in wide use for various reasons both technical and non-technical.

Users have much invested in their application software; they will not adopt to new OS that will not run their applications.

The 2nd reason against the adoption of DOS is that users tend to prefer to have a degree of autonomy of their machines, even in a closely knit organisation.

A combination of middleware and NOSs provides an acceptable balance between the requirement of autonomy and network transparency.

NOS allows users to run their favorite word processor. Middleware enables users to take advantage of services

that become available in their distributed system.

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Operating system layers and Middleware

Unix and Windows are two examples of Network Operating Systems – have a networking capability built into them and so can be used to access remote resources using basic services such as rlogin, telnet.

Applications, services

Computer &

Platform

Middleware

OS: kernel,libraries & servers

network hardware

OS1

Computer & network hardware

Node 1 Node 2

Processes, threads,communication, ...

OS2Processes, threads,communication, ...

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Core OS components and functionality

Communication

manager

Thread manager Memory manager

Supervisor

Process manager

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A single threaded program

class ABC{….

public void main(..){…..}

}

begin

body

end

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A Multithreaded Program

Main Thread

Thread A Thread B Thread C

start startstart

Threads may switch or exchange data/results

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Single and Multithreaded Processes

Single-threaded Process

Single instruction stream Multiple instruction stream

Multiplethreaded ProcessThreads of

Execution

CommonAddress Space

threads are light-weight processes within a process

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Application

Application Application

Application

CPU

Better Response Times in Multiple Application Environments

Higher Throughput for Parallelizeable Applications

CPU

CPU

CPU CPU CPU

Threaded Libraries, Multi-threaded I/OThreaded Libraries, Multi-threaded I/O

Multi-Processing (clusters & grids) and Multi-Threaded

Computing

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Web/Internet Applications:Serving Many Users

Simultaneously

Internet Server

PC client

Local Area Network

PDA

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ServerThreads

Server ProcessClient 1 Process

Client 2 Process

Multithreaded Server: For Serving Multiple Clients Concurrently

Internet

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Printing ThreadPrinting Thread

Editing Thread

Editing Thread

Modern Applications need Threads (ex1):Editing and Printing documents in

background.

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Multithreaded/Parallel File Copy

reader()

{

- - - - - - - - - -

lock(buff[i]);

read(src,buff[i]);

unlock(buff[i]);

- - - - - - - - - -

}

reader()

{

- - - - - - - - - -

lock(buff[i]);

read(src,buff[i]);

unlock(buff[i]);

- - - - - - - - - -

}

writer()

{

- - - - - - - - - -

lock(buff[i]);

write(src,buff[i]);

unlock(buff[i]);

- - - - - - - - - -

}

writer()

{

- - - - - - - - - -

lock(buff[i]);

write(src,buff[i]);

unlock(buff[i]);

- - - - - - - - - -

}

buff[0]buff[0]

buff[1]buff[1]

Cooperative Parallel Synchronized Threads

Cooperative Parallel Synchronized Threads

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Code-GranularityCode ItemLarge grain(task level)Program

Medium grain(control level)Function (thread)

Fine grain(data level)Loop (Compiler)

Very fine grain(multiple issue)With hardware

Code-GranularityCode ItemLarge grain(task level)Program

Medium grain(control level)Function (thread)

Fine grain(data level)Loop (Compiler)

Very fine grain(multiple issue)With hardware

Task i-lTask i-l Task iTask i Task i+1Task i+1

func1 ( ){........}

func1 ( ){........}

func2 ( ){........}

func2 ( ){........}

func3 ( ){........}

func3 ( ){........}

a ( 0 ) =..b ( 0 ) =..

a ( 0 ) =..b ( 0 ) =..

a ( 1 )=..b ( 1 )=..

a ( 1 )=..b ( 1 )=..

a ( 2 )=..b ( 2 )=..

a ( 2 )=..b ( 2 )=..

++ xx LoadLoad

Sockets/PVM/MPI

Threads

Compilers

CPU

Levels of Parallelism

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Process Parallelism Process Parallelism Process Parallelism Process Parallelism

P1P1

P2P2

P3P3

time

time

No of executing process ≥ the number of CPUs

No of executing process ≥ the number of CPUs

CPU

CPU

CPU

Multithreading - Multiprocessors

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Multithreading on Uni-processor

Concurrency Vs Parallelism Process ConcurrencyProcess Concurrency Process ConcurrencyProcess Concurrency

Number of Simultaneous execution units > number of CPUs

Number of Simultaneous execution units > number of CPUs

P1P1

P2P2

P3P3

time

time

CPU

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What are Threads?

A piece of code that run in concurrent with other threads.

Each thread is a statically ordered sequence of instructions.

Threads are being extensively used to express concurrency on both single and multiprocessor machines.

Programming a task having multiple threads of control – Multithreading or Multithreaded Programming.

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Java Threads

Java has built in thread support for Multithreading

Synchronization Thread Scheduling Inter-Thread Communication:

currentThread start setPriority yield run getPriority sleep stop suspend resume

Java Garbage Collector is a low-priority thread.

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Threading Mechanisms... Create a class that extends the Thread

class Create a class that implements the

Runnable interface

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1st method: Extending Thread class

Threads are implemented as objects that contains a method called run() class MyThread extends Thread {

public void run() { // thread body of execution } } Create a thread: MyThread thr1 = new MyThread(); Start Execution of threads: thr1.start(); Create and Execute: new MyThread().start();

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An example

class MyThread extends Thread { // the thread public void run() { System.out.println(" this thread is running ... "); }} // end class MyThread

class ThreadEx1 { // a program that utilizes the thread public static void main(String [] args ) {

MyThread t = new MyThread();MyThread t2 = new MyThread();// due to extending the Thread class (above)// I can call start(), and this will call// run(). start() is a method in class Thread.t2.start();

t.start(); } // end main()} // end class ThreadEx1

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threads programming

More about threads programming Using interfaces Multiple threads in a program

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2nd method: Threads by implementing Runnable interfaceclass MyThread extends ABC implements Runnable{ ..... public void run() { // thread body of execution }} Creating Object: MyThread myObject = new MyThread(); Creating Thread Object: Thread thr1 = new Thread( myObject ); Start Execution: thr1.start();

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An example

class MyThread implements Runnable { public void run() { System.out.println(" this thread is running ... "); }} // end class MyThread

class ThreadEx2 { public static void main(String [] args ) { Thread t = new Thread(new MyThread()); // due to implementing the Runnable

interface // I can call start(), and this will call run(). t.start(); } // end main()} // end class ThreadEx2

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Life Cycle of Thread

new

runnable non-runnable

dead

wait()sleep()suspend()blocked

notify()sleptresume()unblocked

start()

stop()

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A Program with Three Java Threads

Write a program that creates 3 threads

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Three threads example

class A extends Thread { public void run() { for(int i=1;i<=5;i++) { System.out.println("\t From ThreadA: i= "+i); } System.out.println("Exit from A"); } }

class B extends Thread { public void run() { for(int j=1;j<=5;j++) { System.out.println("\t From ThreadB: j= "+j); } System.out.println("Exit from B"); } }

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Three threads example

class C extends Thread { public void run() { for(int k=1;k<=5;k++) { System.out.println("\t From ThreadC: k= "+k); }

System.out.println("Exit from C"); } }

class ThreadTest { public static void main(String args[]) { new A().start(); new B().start(); new C().start(); } }

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Run 1

[user@speedy] threads [1:76] java ThreadTest From ThreadA: i= 1 From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5Exit from A From ThreadC: k= 1 From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5Exit from C From ThreadB: j= 1 From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5Exit from B

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Run2

[user@speedy] threads [1:77] java ThreadTest From ThreadA: i= 1 From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5 From ThreadC: k= 1 From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5Exit from C From ThreadB: j= 1 From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5Exit from BExit from A

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ServerThreads

Message PassingFacility

Server ProcessClient Process

Client Process

User Mode

Kernel Mode

Multithreaded ServerMultithreaded Server

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Architecture for Multithread Servers

Multithreading enables servers to maximize their throughput, measured as the number of requests processed per second.

Threads may need to treat requests with varying priorities: A corporate server could prioritize request

processing according to class of customers. Architectures:

Worker pool Thread-per-request Thread-per-connection Thread-per-object

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Client and server with threads

In worker-pool architecture, the server creates a fixed pool of worker threads to process requests.

The module “receipt and queuing” receives requests from sockets/ports and places them on a shared request queue for retrieval by the workers.

Server

N threads

Input-output

Client

Thread 2 makes

T1

Thread 1

requests to server

generates results

Requests

Receipt &queuing

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Alternative server threading architectures

a. Thread-per-request b. Thread-per-connection c. Thread-per-object

remote

workers

I/O remoteremote I/O

per-connection threads per-object threads

objects objects objects

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Application 1: Multithreaded Dictionary Server – Demonstrate the use of Sockets

and Threads

MultithreadedDictionary Server

(try, dictionary.com)

A Client ProgramWhat is the Meaning of (“Love”)?

A Client ProgramWhat is the Meaning of (“Love”)?

A Client Program in “C++”A Client

Program in “C”

“Love”

“A deep, tender, ineffable feeling of affection and solicitude toward a

person ”

“care or concern, as for the well-being of another”

“Solicitude”

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Synchronisation & Priorities

Thread Synchronisation Thread Priorities

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Accessing Shared Resources

Applications Access to Shared Resources need to be coordinated. Printer (two person jobs cannot be printed at

the same time) Simultaneous operations on your bank

account. Can the following operations be done at the

same time on the same account? Deposit() Withdraw() Enquire()

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Online Bank: Serving Many Customers and Operations

Internet Bank Server

PC client

Local Area Network

PDABank

Database

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Shared Resources

If one thread tries to read the data and other thread tries to update the same data, it leads to inconsistent state.

This can be prevented by synchronising access to the data.

Use “Synchronized” method: public synchronized void update() {

… }

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the driver: 3 Threads sharing the same object

class InternetBankingSystem { public static void main(String [] args ) { Account accountObject = new Account (); Thread t1 = new Thread(new MyThread(accountObject)); Thread t2 = new Thread(new YourThread(accountObject)); Thread t3 = new Thread(new HerThread(accountObject));

t1.start(); t2.start(); t3.start(); // DO some other operation } // end main()}

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Shared account object between 3 threads

class MyThread implements Runnable { Account account; public MyThread (Account s) { account = s;} public void run() { account.deposit(); }} // end class MyThread

class YourThread implements Runnable { Account account; public YourThread (Account s) { account = s;} public void run() { account.withdraw(); } } // end class YourThread

class HerThread implements Runnable { Account account; public HerThread (Account s) { account = s; } public void run() {account.enquire(); }} // end class HerThread

account(shared object)

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Monitor (shared object access): serializes operation on shared

objectclass Account { // the 'monitor' int balance;

// if 'synchronized' is removed, the outcome is unpredictable public synchronized void deposit( ) { // METHOD BODY : balance += deposit_amount; }

public synchronized void withdraw( ) { // METHOD BODY: balance -= deposit_amount;

} public synchronized void enquire( ) {

// METHOD BODY: display balance. }

}

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Thread Priority

In Java, each thread is assigned priority, which affects the order in which it is scheduled for running. The threads so far had same default priority (NORM_PRIORITY) and they are served using FCFS policy. Java allows users to change priority:

ThreadName.setPriority(intNumber) MIN_PRIORITY = 1 NORM_PRIORITY=5 MAX_PRIORITY=10

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Thread Priority Example

class A extends Thread{ public void run() { System.out.println("Thread A started"); for(int i=1;i<=4;i++) { System.out.println("\t From ThreadA: i= "+i); } System.out.println("Exit from A"); }}class B extends Thread{ public void run() { System.out.println("Thread B started"); for(int j=1;j<=4;j++) { System.out.println("\t From ThreadB: j= "+j); } System.out.println("Exit from B"); }}

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Thread Priority Example

class C extends Thread{ public void run() { System.out.println("Thread C started"); for(int k=1;k<=4;k++) { System.out.println("\t From ThreadC: k= "+k); } System.out.println("Exit from C"); }}class ThreadPriority{ public static void main(String args[]) { A threadA=new A(); B threadB=new B(); C threadC=new C(); threadC.setPriority(Thread.MAX_PRIORITY); threadB.setPriority(threadA.getPriority()+1); threadA.setPriority(Thread.MIN_PRIORITY); System.out.println("Started Thread A"); threadA.start(); System.out.println("Started Thread B"); threadB.start(); System.out.println("Started Thread C"); threadC.start(); System.out.println("End of main thread"); }}

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Thread Programming models

Thread concurrency/operation models

The master/worker model The peer model A thread pipeline

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taskXtaskX

taskYtaskY

taskZtaskZ

main ( )main ( )

WorkersProgram

Files

Resources

Databases

Disks

SpecialDevices

Master

Input (Stream)

The master/worker model

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Example

main() /* the master */ { forever { get a request; switch( request ) case X: pthread_create(....,taskX); case Y: pthread_create(....,taskY); .... } } taskX() /* worker */ { perform the task, sync if accessing shared resources } taskY() /* worker */ { perform the task, sync if accessing shared resources } .... --Above runtime overhead of creating thread can be solved by thread pool * the master thread creates all worker thread at program initialization and each worker thread suspends itself immediately for a wakeup call from the master

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The peer model

taskXtaskX

taskYtaskY

WorkersProgram

Files

Resources

Databases

Disks

SpecialDevices

taskZtaskZ

InputInput

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Example

main() { pthread_create(....,thread1...taskX); pthread_create(....,thread2...taskY); .... signal all workers to start wait for all workers to finish do any cleanup } } taskX() /* worker */ { wait for start perform the task, sync if accessing shared resources } taskY() /* worker */ { wait for start perform the task, sync if accessing shared resources }

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A thread pipeline

Resources Files

Databases

Disks

Special Devices

Files

Databases

Disks

Special Devices

Files

Databases

Disks

Special Devices

Stage 1Stage 1 Stage 2Stage 2 Stage 3Stage 3

Program Filter Threads

Input (Stream)

A thread pipeline

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Examplemain()

{

pthread_create(....,stage1);

pthread_create(....,stage2);

....

wait for all pipeline threads to finish

do any cleanup

}

stage1() {

get next input for the program

do stage 1 processing of the input

pass result to next thread in pipeline

}

stage2(){

get input from previous thread in pipeline

do stage 2 processing of the input

pass result to next thread in pipeline

}

stageN()

{

get input from previous thread in pipeline

do stage N processing of the input

pass result to program output.

}

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Java thread constructor and management methods

Thread(ThreadGroup group, Runnable target, String name) Creates a new thread in the SUSPENDED state, which will belong to group and be identified as name; the thread will execute the run() method of target.

setPriority(int newPriority), getPriority()Set and return the thread’s priority.

run()A thread executes the run() method of its target object, if it has one, and otherwise its own run() method (Thread implements Runnable).

start()Change the state of the thread from SUSPENDED to RUNNABLE.

sleep(int millisecs)Cause the thread to enter the SUSPENDED state for the specified time.

yield()Enter the READY state and invoke the scheduler.

destroy()Destroy the thread.

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Java thread synchronization calls

thread.join(int millisecs)

Blocks the calling thread for up to the specified time until thread has terminated.

thread.interrupt()Interrupts thread: causes it to return from a blocking method call such as sleep().

object.wait(long millisecs, int nanosecs)Blocks the calling thread until a call made to notify() or notifyAll() on object wakes the thread, or the thread is interrupted, or the specified time has elapsed.

object.notify(), object.notifyAll()Wakes, respectively, one or all of any threads that have called wait() on object.

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RPC delay against parameter size

1000 2000

RPC delay

Requested datasize (bytes)

Packetsize

0

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A lightweight remote procedure call

1. Copy args

2. Trap to Kernel

4. Execute procedureand copy results

Client

User stub

Server

Kernel

stub

3. Upcall 5. Return (trap)

A A stack

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Times for serialized and concurrent invocations

Client Server

execute request

Send

Receiveunmarshal

marshal

Receiveunmarshal

process results

marshalSend

process args

marshalSend

process args

transmission

Receiveunmarshal

process results

execute request

Send

Receiveunmarshal

marshal

marshalSend

process args

marshalSend

process args

execute request

Send

Receiveunmarshal

marshal

execute request

Send

Receiveunmarshal

marshalReceive

unmarshalprocess results

Receiveunmarshal

process resultstime

Client Server

Serialised invocations Concurrent invocations

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Summary

Operating system provides various types of facilities to support middleware for distributed system:

encapsulation, protection, and concurrent access and management of node resources.

Multithreading enables servers to maximize their throughput, measured as the number of requests processed per second.

Threads support treating of requests with varying priorities. Various types of architectures can be used in current

processing: Worker pool Thread-per-request Thread-per-connection Thread-per-object

Threads need to be synchronized when accessing and manipulating shared resources.

New OS designs provide flexibility in terms of separating mechanisms from policies.