Is it possible to set the priority of the threads in Stream.parallel()?

Question

If I want to run a Stream in parallel in a background task is it possible to run it in lower priority? And if so how?

Coucher

Answer 1

Yes it is possible.

The procedure is as follows:

1. Create a ForkJoinWorkerThreadFactory that creates threads with an appropriate priority.

2. Create a ForkJoinPool using the above thread factory.

3. Instantiate the parallel stream.

4. Run the stream by submitting it to the ForkJoinPool

Something like this:

public class MyThread extends ForkJoinWorkerThread {
    public MyThread(ForkJoinPool pool, int priority) {
        super(pool);
        setPriority(priority);
    }
}

final int poolSize = ...
final int priority = ...

List<Long> aList = LongStream.rangeClosed(firstNum, lastNum).boxed()
  .collect(Collectors.toList());

ForkJoinWorkerThreadFactory factory = new ForkJoinWorkerThreadFactory() {
    public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
         return new MyThread(pool, priority);
    }
};
/*
ForkJoinWorkerThreadFactory factory = pool -> new MyThread(
  pool,
  priority
);
*/

ForkJoinPool customThreadPool = new ForkJoinPool(
    poolSize, factory, null, false);
long actualTotal = customThreadPool.submit(
    () -> aList.parallelStream().reduce(0L, Long::sum)).get();

(Example code adapted from http://www.baeldung.com/java-8-parallel-streams-custom-threadpool)

Overstuff

I said "something like" :-) It needed a pool argument. – Overstuff

It still doesn't compile. I think you have to subclass it. – Birth

This should do it: ForkJoinWorkerThreadFactory factory = pool -> new ForkJoinWorkerThread(pool) {{ setPriority(priority); }}; – Birth

Did it a slightly different way ... but it compiles and runs. The only gotcha is that submit throws InterruptedException which needs to be handled. – Overstuff

The second gotcha might be that (unless you're on Java 10) the number of fork join tasks created will be in proportion to the common pool parallelism – Disgusting

ForkJoinPool.ForkJoinWorkerThreadFactory is (structurally) a functional interface, so factory can be defined as pool -> new MyThread(pool, priority). – Evangelize

Should be noted that this behavior is an undocumented implementation details, as is the entire use of the Fork/Join pool. There is no guaranty that this works in other implementations. – Bathetic

Answer 2

I think a better way to do this is like described here:

public class CustomForkJoinWorkerThreadFactory implements ForkJoinWorkerThreadFactory {

    private final int threadPriority;

    public CustomForkJoinWorkerThreadFactory(int threadPriority) {
        this.threadPriority = threadPriority;
    }

    @Override           
    public ForkJoinWorkerThread newThread(ForkJoinPool pool)
    {
        final ForkJoinWorkerThread worker = ForkJoinPool.defaultForkJoinWorkerThreadFactory.newThread(pool);
        worker.setPriority(threadPriority);
        return worker;
    }
}

It allows you to still use a "default" ForkJoinWorkerThread, but you can set priority / name / etc. Use like this:

new ForkJoinPool(poolSize, new CustomForkJoinWorkerThreadFactory(Thread.MIN_PRIORITY), null, false);

Wessex

Answer 3

Inline version of the previous answer

new ForkJoinPool(parallelism, (pool) -> {
    ForkJoinWorkerThread thread = ForkJoinPool.defaultForkJoinWorkerThreadFactory.newThread(pool);
    thread.setPriority(NORM_PRIORITY + 1);
    return thread;
}, null, false);

Dialogize