Context Switching vs Parallel Execution

user_14240830 0
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I am trying to learn multi-threads, and parallel execution in Java. I wrote example code like this:

public class MemoryManagement1 {

public static int counter1 = 0;
public static int counter2 = 0;

public static final Object lock1= new Object();
public static final Object lock2= new Object();



public static  void increment1() {
    synchronized(lock1) {
        counter1 ++;
    }
    
}

public static  void increment2() {
    synchronized(lock2) {
        counter2 ++;
    }
}

public static void processes() {

    Thread thread1 = new Thread(new Runnable() {

        @Override
        public void run() {
            for (int i = 0; i < 4; i++) {
                increment1();
            }

        }

    });

    Thread thread2 = new Thread(new Runnable() {

        @Override
        public void run() {
            for (int i = 0; i < 4; i++) {
                increment2();
            }

        }

    });

    thread1.start();
    thread2.start();


    try {
        thread1.join();
        thread2.join();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    System.out.println("Counter value is :" + counter1);
    System.out.println("Counter value is :" + counter2);


}

public static void main(String[] args) {

    processes();

 }

}

The code is running properly, but how can I know that the code is running according to time-slicing or whether it is running with parallel execution. I have a CPU with 4 cores. As I understand it, the program should be run with parallel execution, but I am not sure.

parallel-processing multithreading java performance
2 Answers

The code is running properly, but how can I know that the code is running according to time-slicing or whether it is running with parallel execution.

A complete answer to this question would have to cover several factors, but I will be concise and focus mainly on the two most relevant points (IMO) to this question. For simplicity, let us assume that whenever possible each thread (created by the application) will be assigned to a different core.

First, it depends on the number of cores of the hardware that the application is being executed on, and how many threads (created by the application) are running simultaneously. For instance, if the hardware only has a single core or if the application creates more threads than the number of cores available, then some of those threads will inevitably not be executing truly in parallel (i.e., will be mapped to the same core).

Second, it depends if the threads executing their work synchronize with each other or not. In your code, two threads are created, synchronizing using a different object, and since your machine has 4 cores, in theory, each thread is running in parallel to each other.

It gets more complex than that because you can have parts of your code that are executed in parallel, and other parts that are executed sequentially by the threads involved. For instance, if the increment1 and increment2 methods were synchronizing on the same object, then those methods would not be executed in parallel.

user_13668710

Your program is indeed running in parallel execution. In this particular example however you don't need locks in your code, it would run perfectly well without them.

user_23185060
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