Question

Suppose there is an array of n elements. Use n threads in CUDA to reduce (sum) these elements to one element. Do “Reduction” using single thread block in CUDA.

Will your program work with multiple thread blocks. What changes are required in case of multiple thread blocks?

Answer 1

Java's multithreading support is centered around the java.lang.Thread class. The Thread class provides the capability to create objects of class Thread, each with its own separate flow of control. The Thread class encapsulates the data and methods associated with separate threads of execution and allows multithreading to be integrated within the object-oriented framework.

Java provides two approaches to creating threads. In the first approach, you create a subclass of class Thread and override the run() method to provide an entry point into the thread's execution. When you create an instance of your Thread subclass, you invoke its start() method to cause the thread to execute as an independent sequence of instructions. The start() method is inherited from the Thread class. It initializes the Thread object using your operating system's multithreading capabilities and invokes the run() method. You learn how to create threads using this approach in the next section.

The approach to creating threads identified in the previous paragraph is very simple and straightforward. However, it has the drawback of requiring your Thread objects to be under the Thread class in the class hierarchy. In some cases, as you'll see when you study applets in Part VI, "Programming the Web with Applets and Scripts," this requirement can be somewhat limiting.

Java's other approach to creating threads does not limit the location of your Thread objects within the class hierarchy. In this approach, your class implements the java.lang.Runnable interface. The Runnable interface consists of a single method, the run() method, which must be overridden by your class. The run() method provides an entry point into your thread's execution. In order to run an object of your class as an independent thread, you pass it as an argument to a constructor of class Thread. You learn how to create threads using this approach later in this chapter in the section titled "Implementing Runnable."

Creating Subclasses of Thread

In this section, you create your first multithreaded program by creating a subclass of Thread and then creating, initializing, and starting two Thread objects from your class. The threads will execute concurrently and display Java is hot, aromatic, and invigorating. to the console window.

The source code of the ThreadTest1 program.

class ThreadTest1

{

    public static void main(String args[])

    {

        MyThread thread1 = new MyThread("thread1: ");

        MyThread thread2 = new MyThread("thread2: ");

        thread1.start();

        thread2.start();

        boolean thread1IsAlive = true;

        boolean thread2IsAlive = true;

        do {

           if (thread1IsAlive && !thread1.isAlive()) {

               thread1IsAlive = false;

System.out.println("Thread 1 is dead.");

           }

           if (thread2IsAlive && !thread2.isAlive()) {

               thread2IsAlive = false;

               System.out.println("Thread 2 is dead.");

           }

        } while(thread1IsAlive || thread2IsAlive);

    }

}

class MyThread extends Thread

{

static String message[] =

{ "Java", "is", "hot,", "aromatic,", "and", "invigorating."};

    public MyThread(String id)

    {

        super(id);

    }

    public void run()

    {

        String name = getName();

        for (int i=0;i<message.length;++i) {

           randomWait();

           System.out.println(name + message[i]);

        }

    }

    void randomWait()

    {

        try {

           sleep((long)(3000*Math.random()));

        } catch (InterruptedException x) {

           System.out.println("Interrupted!");

        }

    }

}