Fork Join pool understanding

Question

I have met a new class from java.util.concurrent package named ForkJoinPool As i understand it works using the following scenario:

Divide the big task into subtasks(fork) and when each task is completed, gather all subtask(join), then merge it.

Fork join pool has a constructor public ForkJoinPool(int parallelism).
From the official docs i have read the following:

By splitting itself up into subtasks, each subtask can be executed in parallel by different CPUs, or different threads on the same CPU.

The question is How to set the amount of threads to forkjoinpool.

Does parallelism in contructor mean the amount of threads or amount of processors. If it is the amount of processors, consider the following code:

Runtime.getRuntime().availableProcessors()
return 4

Does it mean that the following ForkJoinPool pool = new ForkJoinPool(8); has no effect because my PC has only 4 processors?

Code example

public static void main(String[] args) throws Exception {

        ForkJoinPool pool = new ForkJoinPool(8);
        final List<String> list = Arrays.asList("dasd","dasd")//for example 300 hundrends strings;
        pool.submit(()->{
            list.parallelStream().forEach(AvailableProc::test);
        }).get();

    }

    public static void test(final String code){
            Thread.sleep(1000);     
            System.out.println(code);
    }

Kirill Simonov Kirill Simonov · Accepted Answer · 2018-01-03T10:04:19

Does parallelism in contructor mean the amount of threads or amount of processors

The parallelism parameter tells ForkJoinPool how many worker threads to use. By default it equals to the number of processors available which is typically optimal. Of course you can set any number here, but setting it greater than the number of processors most likely will not benefit.

Does it mean that the following ForkJoinPool pool = new ForkJoinPool(8); has no effect because my PC has only 4 processors?

It depends on your actual task. For example, your code example will print 8 strings per second: you have 8 threads, and the operating system will schedule all of them on 4 processors using a context switching. Since most of the time the threads are just waiting, and the execution time is very short, you get eight strings per second.

Fork Join pool understanding

4 Answers