Java 8 流串行与并行性能

It's not clear to my why executing the program in serial is faster than executing it in parallel. I've given both programs -Xms2g -Xmx2g on an 8gb box thats relatively quiet. Can someone clarify whats going on?

import java.util.stream.LongStream;
import java.util.stream.LongStream.Builder;

public class Problem47 {

    public static void main(String[] args) {

        final long startTime = System.currentTimeMillis();
        System.out.println(LongStream.iterate(1, n -> n + 1).parallel().limit(1000000).filter(n -> fourConsecutives(n)).findFirst());
        final long endTime = System.currentTimeMillis();
        System.out.println(" PARALLEL took " +(endTime - startTime) + " ms");

        final long startTime2 = System.currentTimeMillis();
        System.out.println(LongStream.iterate(1, n -> n + 1).limit(1000000).filter(n -> fourConsecutives(n)).findFirst());
        final long endTime2 = System.currentTimeMillis();
        System.out.println(" SERIAL took " +(endTime2 - startTime2) + " ms");
    }

    static boolean fourConsecutives(final long n) {
        return distinctPrimeFactors(n).count() == 4 &&
                distinctPrimeFactors(n + 1).count() == 4 &&
                distinctPrimeFactors(n + 2).count() == 4 &&
                distinctPrimeFactors(n + 3).count() == 4;
    }

    static LongStream distinctPrimeFactors(long number) {
        final Builder builder = LongStream.builder();
        final long limit = number / 2;
        long n = number;
        for (long i = 2; i <= limit; i++) {
            while (n % i == 0) {
                builder.accept(i);
                n /= i;
            }
        }
        return builder.build().distinct();
    }

}

推荐答案

虽然 Brian Goetz 对您的设置是正确的，例如你应该使用 .range(1, 1000000) 而不是 .iterate(1, n -> n + 1).limit(1000000) 并且你的基准测试方法很简单，我想强调一下重点:

ExecutorService es=Executors.newFixedThreadPool(
                       Runtime.getRuntime().availableProcessors());
AtomicLong found=new AtomicLong(Long.MAX_VALUE);
LongStream.range(1, 1000000).filter(n -> found.get()==Long.MAX_VALUE)
    .forEach(n -> es.submit(()->{
        if(found.get()>n && fourConsecutives(n)) for(;;) {
            long x=found.get();
            if(x<n || found.compareAndSet(x, n)) break;
        }
    }));
es.shutdown();
try { es.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS); }
catch (InterruptedException ex) {throw new AssertionError(ex); }
long result=found.get();
System.out.println(result==Long.MAX_VALUE? "not found": result);