Restrict CPU usage by java8 parallel stream/High CPU usage by Java 8 parallel stream

I was testing the performance of java 8 parallel stream api by creating a simple loop and adding elements in an array.

I am getting a huge performance boost over non parallel one.

But when I am checking my task manager I am seeing an uncontrolled CPU usage, it eats up all my CPU during that period. Check the CPU usage

here is my sample code,

    public static void withIfElse(boolean sorted) {

    int[] arr = new int[32768];
    Random rand = new Random();
    for(int i = 0;i < arr.length;i++)
        arr[i] = rand.nextInt(256);

    long nst = System.currentTimeMillis();
    long sum = 0;
    for(int j=0;j<100000;j++) {
        sum +=>m>=128).reduce((m,n)-> m+n).getAsInt();

    System.out.printf("Time taken for boolean %b is %d ms. \n",sorted,(System.currentTimeMillis()-nst));


I am able to achieve almost 2x to 8x performance boost.

But ,

Cpu usage is uncontrolled. Is there any way to force java to only 2 cores ?

I have also tried setting

System.setProperty("java.util.concurrent.ForkJoinPool.common.parallelism", "1");

But still CPU usage is high.

Also is it recommended to use parallel stream?, because suppose in my application I have 10 user threads, or a web application where each request is a thread, in that case if I start a parallel stream on multiple threads which eventually eat all my cpu, there will be so many context switching.

2 answers

  • answered 2017-06-17 18:42 Joe C

    Parallel streams run, by default, in the system-wide ForkJoinPool, which gives one thread for each core available on your machine.

    If you want to change this behaviour, you can run your parallel stream in a ForkJoinPool of your choosing:

        ForkJoinPool fjp = new ForkJoinPool(2);
        ForkJoinTask<Long> sumFuture = fjp.submit(() -> {
            long sum = 0;
            for(int j=0;j<100000;j++) {
                sum +=>m>=128).reduce((m,n)-> m+n).getAsInt();
            return sum;

  • answered 2017-06-17 18:42 Eugene

    My 0.02$: huge performance boost over non parallel one. huge is probably a bit too much here. And there might be good reasons for this - first, the way you test. It is so critical that there has been a question with over 500 up-votes especially for this: micro-benchmark. You might think you have good tests written, but jvm could have different plans for you.

    For moderate data, indeed a sequential stream might be faster than a parallel one. It's a lot more work that has to be done by a parallel stream. But without actual measurements, it's close to impossible to say; but here are recommendations from the best in the field, like this.

    When you say that the you want to limit java to some cores - the plain answer is that you can't; from java code that is. It has to be done from the OS, if it doable at all. What you have enable there via java.util.concurrent.ForkJoinPool.common.parallelism is limiting the threads, not the cores.

    And a single thread can be split over multiple cpus - it depends on the Operating system itself.