Concurrent Programming

COMP124 Lectures 2021-04-20

Mathematical Example

Consider finding a single root using the quadratic formula:

\[x=\frac{-b+\sqrt{b^2-4ac}}{2a}\]

This can be split into several operations.

Concurrent Operations

These operations don’t rely on any other part of the expression.

1. $t_1=-b$
2. $t_2=b\times b$
3. $t_3=4\times a$
4. $t_4=2\times a$

Serial Operations

These parts have prerequisites.

1. $t_5=t_3\times c$
2. $t_5=t_2-t_5$
3. $t_5=\sqrt{t_5}$
4. $t_5=t_1+t_5$
5. $x=\frac{t_5}{t_4}$

Java Threads

When a Java program starts up, a single thread is always created for the program

JVM also has own threads for garbage collection, screen updates, event handling…

• New threads may be created by the programmer by extending the Thread class.
• Threads may be managed directly by kernel, or implemented at user level by a library.
  • Former is less efficient.
  • In latter, if one thread blocks, all threads will block.

Java Thread Class

public class Thread extends Object implements Runnable

• A Thread implements a run() method that defines what processing will be carried out during the Thread’s lifetime.
• Threads may be started within main() and run simultaneously, sharing variables, etc.

Example

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

This doesn’t have a definite order. Worker1 or main may get run first.

Method

The class Worker1 is derived from the Thread class and the work of the new thread is specified in the run() method.

1. In main() we create a new Worker1 object.
2. Calling the start() method:
   • Allocates memory and initialises the new thread.
   • Causes the run() method to be called.
3. Original thread and new thread now run in parallel.

Java Thread States

graph LR
a[ ] -->|new| New
New -->|"start()"| Runnable
Runnable -->|"I/O, sleep()..."| Blocked
Blocked --> Runnable
Runnable -->|termination| Dead

All threads capable of execution are in the runnable state.

This includes the currently executing thread.