Multithreading and recursion together

Question

I have recursive code that processes a tree structure in a depth first manner. The code basically looks like this:

function(TreeNode curr) 
{
    if (curr.children != null && !curr.children.isEmpty()) 
    {
        for (TreeNode n : curr.children) 
    {
            //do some stuff
            function(n);
        }
    } 
    else 
    {
        //do some other processing
    }
}

I want to use threads to make this complete faster. Most of the time is spent traversing so I don't want to just create a thread to handle "the other processing" because it doesn't take that long. I think I want to fork threads at "do some stuff" but how would that work?

Answer 1

It's a good case for Fork/Join framework which is to be included into Java 7. As a standalone library for use with Java 6 it can be downloaded here.

Something like this:

public class TreeTask extends RecursiveAction {
    private final TreeNode node;
    private final int level;

    public TreeTask(TreeNode node, int level) {
        this.node = node;
        this.level = leve;
    }

    public void compute() {
        // It makes sense to switch to single-threaded execution after some threshold
        if (level > THRESHOLD) function(node);

        if (node.children != null && !node.children.isEmpty()) {
            List<TreeTask> subtasks = new ArrayList<TreeTask>(node.children.size());
            for (TreeNode n : node.children) {
                // do some stuff
                subtasks.add(new TreeTask(n, level + 1));
            }
            invokeAll(subtasks); // Invoke and wait for completion
        } else {
            //do some other processing
        }
    }
}

...
ForkJoinPool p = new ForkJoinPool(N_THREADS);
p.invoke(root, 0);

The key point of fork/join framework is work stealing - while waiting for completion of subtasks thread executes other tasks. It allows you to write algorithm in straightforward way, while avoiding problems with thread exhausting as a naive apporach with ExecutorService would have.