Why does this parallel stream run 15x SLOWER? [duplicate]

问题

This question already has answers here:

Java 8's streams: why parallel stream is slower? (4 answers)

How do I write a correct micro-benchmark in Java? (11 answers)

Why is parallel stream slower? (3 answers)

Closed 2 years ago.

I was attempting to demonstrate to a Java novice streams and such. He said he had read that streams can be slower than for loops for primitive types. So, I set immediately to prove that wrong! What I got was a shock!

The for-loop and the serial stream run roughly the same, but the parallel stream is consistently and dramatically slower. Can someone explain why?

   @Test
    public void foo() throws Exception {

        Random r = new Random();

        IntSummaryStatistics stats = new IntSummaryStatistics();

        long start = System.currentTimeMillis();
        for(int i = 0; i < 100000000;i++) {
            stats.accept(r.nextInt() * 2);
        }
        System.out.println(System.currentTimeMillis() - start);

        start = System.currentTimeMillis();
        stats = r.ints(100000000).map(rn -> rn * 2).summaryStatistics();
        System.out.println(System.currentTimeMillis() - start);

        start = System.currentTimeMillis();
        stats = r.ints(100000000).parallel().map(rn -> rn * 2).summaryStatistics();
        System.out.println(System.currentTimeMillis() - start);


    }

resutls:

1067
1047
15184

来源：https://stackoverflow.com/questions/44294483/why-does-this-parallel-stream-run-15x-slower