Chapter 4: Threads

Chapter 4: Threads Overview Multithreading Models Threading Issues Pthreads Windows XP Threads Solaris 2 Threads Linux Threads Java Threads

Single and Multithreaded Processes

Threads A thread (or lightweight process) is a basic unit of CPU utilization; it consists of: program counter register set stack space A thread shares with its peer threads its: code section data section operating-system resources collectively known as a task. A traditional or heavyweight process is equal to a task with one thread

Benefits Responsiveness Resource Sharing Economy Utilization of MP Architectures

Threads (Cont.) In a multiple threaded task, while one server thread is blocked and waiting, a second thread in the same task can run. Cooperation of multiple threads in same job confers higher throughput and improved performance. Application that require sharing a common buffer (i.e., producer-consumer) benefit from thread utilization. Threads provide a mechanism that allows sequential processes to make blocking system calls while also achieving parallelism. Kernel-supported threads (Mach and OS/2) User-level threads; supported above the kernel, via a set of library calls at the user level (Project Andrew from CMU). Hybrid approach implements both user-level and kernel-supported threads (Solaris 2)

User Threads Thread management done by user-level threads library Examples POSIX Pthreads Win32 threads Java threads Solaris threads

Kernel Threads Supported by the Kernel Examples - Windows XP/2000 - Solaris - Linux - Tru64 UNIX - Mac OS X

Multithreading Models Many-to-One One-to-One Many-to-Many

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 NT/XP/2000 - Linux - Solaris 9 and later

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 prior to version 9 Windows NT/2000 with the ThreadFiber package

Many-to-Many Model

Two-level 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

Two-level Model

Multithreaded C program using the Pthreads API (1) #include <pthread.h> #include <stdio.h> int sum; /* this data is shared by the thread(s) */ void *runner( void * param ); /* the thread */ int main( int argc, char * argv[] ) { pthread_t tid; /* the thread identifier */ pthread_attr_t attr; /* set of thread attributes */ if( argc != 2 ) { fprintf( stderr, "usage: a.out <integer value>\n" ); return -1; } if( atoi(argv[1]) < 0 ) { fprintf( stderr, "%d must be >= 0\n", atoi( argv[1] ) ); /* get the default attributes */ pthread_attr_init( &attr, runner, argv[v] ); /* create the thread */ pthread_create( &tid, &attr, runner, argv[1] ); /* wait for the thread to exit */ pthread_join( tid, NULL ); printf( "sum = %d\n", sum );

Multithreaded C program using the Pthreads API (2) /* The thread will begin control in this function */ void *runner( void * param ) { int i, upper = atoi( param ); sum = 0; for( i = 1; i <= upper; i++ ) sum += i; pthread_exit( 0 ); }

Java program for the summation of a non-negative integer (1) class Sum { private int sum; public int getSum(){ return sum; } public void setSum( int sum ) { this.sum = sum; class Summation implements Runnable private int upper; private Sum sumValue; public Summation( int upper, Sum sumValue ) { this.upper = upper; this.sumValue = sumValue; public void run() { int sum = 0; for( int i; i <= upper; i++ ) sum += 1; sumValue.setSum( sum );

Java program for the summation of a non-negative integer (2) public class Driver { public static void main( String[] args ) { if( args.length > 0 ) { if( Integer.parseInt( args[0] ) < 0 ) System.err.println( args[0] + " must be >= 0." ); else { // create the object to be shared Sum sumObject = new Sum(); int upper = Integer.parseInt( args[0] ); Thread thrd = new Thread( new Summation( upper, sumObject ) ); thrd.start(); try { thrd.join(); System.out.println( "The sum of " + upper + " is " + sumObject.getSum() ); } catch ( InterruptedException ie ){} else System.err.println( "Usage: Summation <integer value>" );

Threading Issues Semantics of fork() and exec() system calls Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations

Semantics of fork() and exec() Does fork() duplicate only the calling thread or all threads?

Thread Cancellation Terminating a thread before it has finished Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled

Signal 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 Options: 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 threa to receive all signals for the process

Thread 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 Data Allows each thread to have its own copy of data Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

Scheduler 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

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 (Solaris, Linux, Mac OS X)

Threads Support in Solaris 2 Solaris 2 is a version of UNIX with support for threads at the kernel and user levels, symmetric multiprocessing, and real-time scheduling LWP – intermediate level between user-level threads and kernel-level threads. Resource needs of thread types: Kernel thread: small data structure and a stack; thread switching does not require changing memory access information – relatively fast. LWP: PCB with register data, accounting and memory information; switching between LWPs is relatively slow User-level thread: only need stack and program counter; no kernel involvement means fast switching. Kernel only sees the LWPs that support user-level threads.

Solaris 2 Threads

Solaris Process

Windows 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 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 are managed by the JVM Java threads may be created by: Extending Thread class Implementing the Runnable interface

Java Thread States

Extentding the Thread Class Class Worker1 extends Thread { public void run() { System.out.println(“I am a Worker Thread”); }

Creating the Thread Public class First { public static void main(String args[]) { Worker runner = new Worker1(); runner.start(); System.out.println(“I am the main thread”); }

The Runnable Interface Public interface Runnable { public abstract void run(); }

Implementing the Runnable Interface Class Worker2 implements Runnable { public void run() { System.out.println(“I am a Worker Thread”); }

Creating the Thread Public class Second { public static void main(String args[]) { Runnable runner = new Worker2(); Thread thrd = new Thread(runner); thrd.start(); System.out.println(“I am the main thread”); }

Java Thread Management suspend() – suspends execution of the currently running thread sleep() – puts the currently running thread to sleep for a specified amount of time resume() – resumes execution of a suspended thread stop() – stops execution of a thread

/** * ClockApplet.java (그림5.9) * This applet displays the time of day. * Note that this program uses deprecated methods, * an alternative version will be shown in chapter 8. */ import java.applet.*; import java.awt.*; public class ClockApplet extends Applet implements Runnable { public void run() { while (true) { try { Thread.sleep(1000); } catch (InterruptedException e) {}

repaint(); } public void start() { if (clockThread == null) { clockThread = new Thread(this); clockThread.start(); else clockThread.resume();

// this method is called when we leave the page the applet is on public void stop() { if (clockThread != null) clockThread.suspend(); } // this method is called when the applet is removed from the cache public void destroy() { if (clockThread != null) { clockThread.stop(); clockThread = null;

public void paint(Graphics g) { g.drawString(new java.util.Date().toString(), 10, 30); } private Thread clockThread;

/* Server.java (그림 5.11) * This creates the buffer and the producer and consumer threads. */ public class Server { public Server() // first create the message buffer MessageQueue mailBox = new MessageQueue(); // now create the producer and consumer threads Producer producerThread = new Producer(mailBox); Consumer consumerThread = new Consumer(mailBox);

producerThread.start(); consumerThread.start(); } public static void main(String args[]) { Server server = new Server(); public static final int NAP_TIME = 5;

/* * Producer.java (그림 5.12) * * This is the producer thread for the bounded buffer problem. */ import java.util.*; class Producer extends Thread { public Producer(MessageQueue m) mbox = m; }

public void run() { Date message; while (true) int sleetime = (int) (Server.NAP_TIME * Math.random()); System.out.println("Producer sleeping for " + sleeptime + " seconds"); try { Thread.sleep(sleeptime * 1000); } catch (InterruptedException e) {}

message = new Data(); // produce an item & enter it into the buffer System.out.println("Producer produced " + message); mbox.send(message); } private MessageQueue mbox;

/** * Consumer.java (그림5.13) * * This is the consumer thread for the bounded buffer problem */ import java.util.*; class Consumer extends Thread { public Consumer(MessageQueue m) mbox = m; }

public void run() { Date message; while (true) int sleeptime = (int) (Server.NAP_TIME * Math.random()); System.out.println("Consumer sleeping for " + sleeptime + " seconds"); try { sleep(sleeptime * 1000); } catch(InterruptedException e) {} // consume an item from the buffer System.out.println("Consumer wants to consume.");

message = (Date)mbox.receive(); if (message != null) System.out.println("Consumer consumed " + message); } private MessageQueue mbox;

End of Chapter 4