JAVA: How to scale threads according to cpu cores?

Question

Hi everyone, I'm new to this site and glad to be here :).

I'm not a good java programmer, it's just my hobby, but I'm eager to know more than average stuff.

I want to solve a mathematical problem with multiple threads in java. my math problem can be separated into work units, that I want to have solved in several threads.

but I don't want to have a fixed amount of threads working on it, but instead a coresponding amount of threads to the amount of cpu cores. and my problem is, that I couldn't find an easy tutorial in the internet for this. all I found are examples with fixed threads.

So could you help me with a link to a good tuturial or could give me an easy and good example? That would be really nice :)

thank you in advance,

Andreas

Answer 1

On the Runtime class, there is a method called availableProcessors(). You can use that to figure out how many CPUs you have. Since your program is CPU bound, you would probably want to have (at most) one thread per available CPU.

Answer 2

You can determine the number of processes available to the Java Virtual Machine by using the static Runtime method, availableProcessors. Once you have determined the number of processors available, create that number of threads and split up your work accordingly.

Update: To further clarify, a Thread is just an Object in Java, so you can create it just like you would create any other object. So, let's say that you call the above method and find that it returns 2 processors. Awesome. Now, you can create a loop that generates a new Thread, and splits the work off for that thread, and fires off the thread. Here's some psuedocode to demonstrate what I mean:

int processors = Runtime.getRuntime().availableProcessors();
for(int i=0; i < processors; i++) {
  Thread yourThread = new AThreadYouCreated();
  // You may need to pass in parameters depending on what work you are doing and how you setup your thread.
  yourThread.start();
}

For more information on creating your own thread, head to this tutorial. Also, you may want to look at Thread Pooling for the creation of the threads.

Answer 3

You probably want to look at the java.util.concurrent framework for this stuff too. Something like:

ExecutorService e = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
// Do work using something like either
e.execute(new Runnable() {
        public void run() {
            // do one task
        }
    });

or

    Future<String> future = pool.submit(new Callable<String>() {
        public String call() throws Exception {
            return null;
        }
    });
    future.get();  // Will block till result available

This is a lot nicer than coping with your own thread pools etc.

Answer 4

Doug Lea (author of the concurrent package) has this paper which may be relevant: http://gee.cs.oswego.edu/dl/papers/fj.pdf

Answer 5

Hi again,

thanks to Jason and the rest of you, I did this

but I don't like my way in joining them. is there a more elegant way in doing this?

Andreas

public class Main {
static class JoinerThread extends Thread
  {
    public int id;
    public int result;
    @Override
public void run()
{
    for(int i=0; i<999999999; i++){
        Math.pow(i,2);
    }

    result = id;
}
  }
  public static void main( String[] args ) throws InterruptedException
  {
int processors = 3;
JoinerThread threads [] = new JoinerThread[processors];
for(int i=0;i<processors;i++){
    threads[i] = new JoinerThread();
    threads[i].id = i;
    threads[i].start();
}
for(int i=0;i<processors;i++){
    threads[i].join();
    System.out.println("gnaa");
}

for(int i=0;i<processors; i++){
    System.out.println( threads[i].result );
}
  }
}