Java8的流：为什么并行流比较慢？

Question

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

考虑到我有2个CPU核心的事实，并行版本不是应该更快吗？有人能给我一个提示为什么并行版本比较慢吗？

Answer 1

事实上，这里有几个问题是并行的。

第一，并行解决一个问题总是需要执行更多的实际工作，而不是按顺序执行。在多个线程之间拆分工作以及连接或合并结果时会涉及开销。像将短字符串转换为小写的问题非常小，以至于它们有被并行拆分开销淹没的危险。

第二个问题是，对Java程序进行基准测试是非常微妙的，非常容易得到令人困惑的结果。两个常见的问题是JIT编译和死代码消除。短基准通常在JIT编译之前或期间完成，因此它们不是在测量峰值吞吐量，实际上它们可能是在测量JIT本身。编译发生的时间多少是不确定的，因此它也可能导致结果发生很大的变化。

package com.stackoverflow.questions;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.concurrent.TimeUnit;

import org.openjdk.jmh.annotations.*;

public class SO23170832 {
    @State(Scope.Benchmark)
    public static class BenchmarkState {
        static String[] array;
        static {
            array = new String[1000000];
            Arrays.fill(array, "AbabagalamagA");
        }
    }

    @GenerateMicroBenchmark
    @OutputTimeUnit(TimeUnit.SECONDS)
    public List<String> sequential(BenchmarkState state) {
        return
            Arrays.stream(state.array)
                  .map(x -> x.toLowerCase())
                  .collect(Collectors.toList());
    }

    @GenerateMicroBenchmark
    @OutputTimeUnit(TimeUnit.SECONDS)
    public List<String> parallel(BenchmarkState state) {
        return
            Arrays.stream(state.array)
                  .parallel()
                  .map(x -> x.toLowerCase())
                  .collect(Collectors.toList());
    }
}

java -jar dist/microbenchmarks.jar ".*SO23170832.*" -wi 5 -i 5 -f 1

Benchmark                       Mode   Samples         Mean   Mean error    Units
c.s.q.SO23170832.parallel      thrpt         5        4.600        5.995    ops/s
c.s.q.SO23170832.sequential    thrpt         5        1.500        1.727    ops/s

java -verbose:gc -jar dist/microbenchmarks.jar ".*SO23170832.*" -wi 5 -i 5 -f 1

[GC (Allocation Failure)  512K->432K(130560K), 0.0024130 secs]
[GC (Allocation Failure)  944K->520K(131072K), 0.0015740 secs]
[GC (Allocation Failure)  1544K->777K(131072K), 0.0032490 secs]
[GC (Allocation Failure)  1801K->1027K(132096K), 0.0023940 secs]
# Run progress: 0.00% complete, ETA 00:00:20
# VM invoker: /Users/src/jdk/jdk8-b132.jdk/Contents/Home/jre/bin/java
# VM options: -verbose:gc
# Fork: 1 of 1
[GC (Allocation Failure)  512K->424K(130560K), 0.0015460 secs]
[GC (Allocation Failure)  933K->552K(131072K), 0.0014050 secs]
[GC (Allocation Failure)  1576K->850K(131072K), 0.0023050 secs]
[GC (Allocation Failure)  3075K->1561K(132096K), 0.0045140 secs]
[GC (Allocation Failure)  1874K->1059K(132096K), 0.0062330 secs]
# Warmup: 5 iterations, 1 s each
# Measurement: 5 iterations, 1 s each
# Threads: 1 thread, will synchronize iterations
# Benchmark mode: Throughput, ops/time
# Benchmark: com.stackoverflow.questions.SO23170832.parallel
# Warmup Iteration   1: [GC (Allocation Failure)  7014K->5445K(132096K), 0.0184680 secs]
[GC (Allocation Failure)  7493K->6346K(135168K), 0.0068380 secs]
[GC (Allocation Failure)  10442K->8663K(135168K), 0.0155600 secs]
[GC (Allocation Failure)  12759K->11051K(139776K), 0.0148190 secs]
[GC (Allocation Failure)  18219K->15067K(140800K), 0.0241780 secs]
[GC (Allocation Failure)  22167K->19214K(145920K), 0.0208510 secs]
[GC (Allocation Failure)  29454K->25065K(147456K), 0.0333080 secs]
[GC (Allocation Failure)  35305K->30729K(153600K), 0.0376610 secs]
[GC (Allocation Failure)  46089K->39406K(154624K), 0.0406060 secs]
[GC (Allocation Failure)  54766K->48299K(164352K), 0.0550140 secs]
[GC (Allocation Failure)  71851K->62725K(165376K), 0.0612780 secs]
[GC (Allocation Failure)  86277K->74864K(184320K), 0.0649210 secs]
[GC (Allocation Failure)  111216K->94203K(185856K), 0.0875710 secs]
[GC (Allocation Failure)  130555K->114932K(199680K), 0.1030540 secs]
[GC (Allocation Failure)  162548K->141952K(203264K), 0.1315720 secs]
[Full GC (Ergonomics)  141952K->59696K(159232K), 0.5150890 secs]
[GC (Allocation Failure)  105613K->85547K(184832K), 0.0738530 secs]
1.183 ops/s