How can we bound the number of threads used in a ForJoinPool?

Question

I have 8-cores CPU and I'm going parallelise one recursive task into a few subtask. The thing is I wouldn't like that the amount of threads would be more than the amount of the CPU-cores. So, I have a class

public class ValueSumCounter extends RecursiveTask<Long>{
    byte[] bytesToProcess;
    public ValueSumCounter(byte[] b) {
        this.bytesToProcess = b;
    }

    @Override
    protected Long compute() {
        List<ValueSumCounter> tasksToCompute;
        //Obtaining list of tasks (more than 100)
    }        
    for(ValueSumCounter task : tasksToCompute) {
        task.join();
    }
}

and now, I wannta create the ForkJoinPool as follows:

public static void main(String[] args) {
    byte[] bytes;
    //Obtaining bytes of an image and set their to bytes
    new ForkJoinPool().invoke(new ValueSumCounter(bytes));
}

And I don't know how much threads will be created during this invocation. How can I bound the amount of threads explicitly less than the number of cores (8 in my case)?

Answer 1

You can create the ForkJoinPool with an int as Constructor parameter ( http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ForkJoinPool.html#ForkJoinPool-int- ), so you can controll how many worker threads the ForkJoinPool will use. In your case you want to have 8, because you have 8 cores and can call it like:

new ForkJoinPool(Runtime.getRuntime().availableProcessors());

Edit: As "isnot2bad" noted you actually don't need to specify the number of worker threads, because the no-argument constructor already uses your desired number: http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ForkJoinPool.html#ForkJoinPool--

I overlooked this, thanks for the addition.