1. Chapter 4: Threads

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

3. Single and Multithreaded Processes

4. 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

5. Benefits Responsiveness Resource Sharing Economy
Utilization of MP Architectures

6. 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)

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

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

9. Multithreading Models
Many-to-One
One-to-One
Many-to-Many

10. Many-to-One Many user-level threads mapped to single kernel thread.
Used on systems that do not support kernel threads.

11. Many-to-One Model

12. One-to-One Each user-level thread maps to kernel thread. Examples
- Windows NT/XP/2000
- Linux
- Solaris 9 and later

13. One-to-one Model

14. 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

15. Many-to-Many Model

16. 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

17. Two-level Model

18. 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 );

19. 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 ); }

20. 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 );

21. 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>" );

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

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

24. 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

25. 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

26. 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

27. 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)

28. 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

29. Pthreads
A POSIX standard (IEEE c) 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)

30. 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.

31. Solaris 2 Threads

32. Solaris Process

33. 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)

34. 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)

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

36. Java Thread States

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

38. 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”); }

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

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

41. 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”); }

42. 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

43. /**
* 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) {}

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

45. // 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;

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

47. /* 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);

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

49. /*
* 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; }

50. 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) {}

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

52. /**
* 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; }

53. 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.");

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

55. End of Chapter 4