Java 8的流：为什么并行流较慢？(Java 8's streams: why parallel stream is slower?)

 我在玩Java 8的流，不能理解我获得的性能结果。 
 
 我有2个核心CPU（Intel i73520M），Windows 8 x64和64位Java 8更新5.我正在做简单的映射流/并行流Strings，发现并行版本有点慢。 
 
 当我运行这个代码： 
 
String[] array = new String[1000000];
Arrays.fill(array, "AbabagalamagA");

Stream<String> stream = Arrays.stream(array);

long time1 = System.nanoTime();

List<String> list = stream.map((x) -> x.toLowerCase()).collect(Collectors.toList());

long time2 = System.nanoTime();

System.out.println((time2 - time1) / 1000000f);
 
 ...我得到了600左右的结果。这个版本使用并行流： 
 
String[] array = new String[1000000];
Arrays.fill(array, "AbabagalamagA");

Stream<String> stream = Arrays.stream(array).parallel();

long time1 = System.nanoTime();

List<String> list = stream.map((x) -> x.toLowerCase()).collect(Collectors.toList());

long time2 = System.nanoTime();


System.out.println((time2 - time1) / 1000000f);
 
 给我一些约900个。 
 
 考虑到我有2个CPU内核，并行版本不应该更快， 有人可以给我一个提示，为什么并行版本较慢？ 

I am playing with Java 8's streams and cannot understand the performance results I am getting. I have 2 core CPU (Intel i73520M), Windows 8 x64, and 64-bit Java 8 update 5. I am doing simple map over stream/parallel stream of Strings and found that parallel version is somewhat slower.
 
Function<Stream<String>, Long> timeOperation = (Stream<String> stream) -> {
  long time1 = System.nanoTime();
  final List<String> list = 
     stream
       .map(String::toLowerCase)
       .collect(Collectors.toList());
  long time2 = System.nanoTime();
  return time2 - time1;
};

Consumer<Stream<String>> printTime = stream ->
  System.out.println(timeOperation.apply(stream) / 1000000f);

String[] array = new String[1000000];
Arrays.fill(array, "AbabagalamagA");

printTime.accept(Arrays.stream(array));            // prints around 600
printTime.accept(Arrays.stream(array).parallel()); // prints around 900
 
Shouldn't the parallel version be faster, considering the fact that I have 2 CPU cores? Could someone give me a hint why parallel version is slower?

原文：https://stackoverflow.com/questions/23170832