Reader#lines()由于其spliterator中的不可配置的批量大小策略而严重并行化

Question

当流源是a时,我无法实现流处理的良好并行化Reader.在四核CPU上运行下面的代码我首先观察到3个核心,然后突然下降到两个核心,然后是一个核心.整体CPU利用率徘徊在50%左右.

请注意示例的以下特征:

• 只有6,000行;
• 每条线需要大约20毫秒来处理;
• 整个过程大约需要一分钟.

这意味着所有压力都在CPU上,I/O很小.这个例子是一个用于自动并行化的坐鸭.

import static java.util.concurrent.TimeUnit.NANOSECONDS;
import static java.util.concurrent.TimeUnit.SECONDS;

... class imports elided ...    

public class Main
{
  static final AtomicLong totalTime = new AtomicLong();

  public static void main(String[] args) throws IOException {
    final long start = System.nanoTime();
    final Path inputPath = createInput();
    System.out.println("Start processing");

    try (PrintWriter w = new PrintWriter(Files.newBufferedWriter(Paths.get("output.txt")))) {
      Files.lines(inputPath).parallel().map(Main::processLine)
        .forEach(w::println);
    }

    final double cpuTime = totalTime.get(),
                 realTime = System.nanoTime()-start;
    final int cores = Runtime.getRuntime().availableProcessors();
    System.out.println("          Cores: " + cores);
    System.out.format("       CPU time: %.2f s\n", cpuTime/SECONDS.toNanos(1));
    System.out.format("      Real time: %.2f s\n", realTime/SECONDS.toNanos(1));
    System.out.format("CPU utilization: %.2f%%", 100.0*cpuTime/realTime/cores);
  }

  private static String processLine(String line) {
    final long localStart = System.nanoTime();
    double ret = 0;
    for (int i = 0; i < line.length(); i++)
      for (int j = 0; j < line.length(); j++)
        ret += Math.pow(line.charAt(i), line.charAt(j)/32.0);
    final long took = System.nanoTime()-localStart;
    totalTime.getAndAdd(took);
    return NANOSECONDS.toMillis(took) + " " + ret;
  }

  private static Path createInput() throws IOException {
    final Path inputPath = Paths.get("input.txt");
    try (PrintWriter w = new PrintWriter(Files.newBufferedWriter(inputPath))) {
      for (int i = 0; i < 6_000; i++) {
        final String text = String.valueOf(System.nanoTime());
        for (int j = 0; j < 25; j++) w.print(text);
        w.println();
      }
    }
    return inputPath;
  }
}

我的典型输出:

          Cores: 4
       CPU time: 110.23 s
      Real time: 53.60 s
CPU utilization: 51.41%

为了比较,如果我使用稍微修改的变体,我首先收集到列表然后处理:

Files.lines(inputPath).collect(toList()).parallelStream().map(Main::processLine)
  .forEach(w::println);

我得到这个典型的输出:

          Cores: 4
       CPU time: 138.43 s
      Real time: 35.00 s
CPU utilization: 98.87%

什么可以解释这种影响,我如何解决它以获得充分利用？

请注意,我最初在servlet输入流的读者上观察到这一点,因此它不是特定于a FileReader.

Answer 1

下面是答案,在源代码中阐明Spliterators.IteratorSpliterator,通过所使用的一个BufferedReader#lines():

    @Override
    public Spliterator<T> trySplit() {
        /*
         * Split into arrays of arithmetically increasing batch
         * sizes.  This will only improve parallel performance if
         * per-element Consumer actions are more costly than
         * transferring them into an array.  The use of an
         * arithmetic progression in split sizes provides overhead
         * vs parallelism bounds that do not particularly favor or
         * penalize cases of lightweight vs heavyweight element
         * operations, across combinations of #elements vs #cores,
         * whether or not either are known.  We generate
         * O(sqrt(#elements)) splits, allowing O(sqrt(#cores))
         * potential speedup.
         */
        Iterator<? extends T> i;
        long s;
        if ((i = it) == null) {
            i = it = collection.iterator();
            s = est = (long) collection.size();
        }
        else
            s = est;
        if (s > 1 && i.hasNext()) {
            int n = batch + BATCH_UNIT;
            if (n > s)
                n = (int) s;
            if (n > MAX_BATCH)
                n = MAX_BATCH;
            Object[] a = new Object[n];
            int j = 0;
            do { a[j] = i.next(); } while (++j < n && i.hasNext());
            batch = j;
            if (est != Long.MAX_VALUE)
                est -= j;
            return new ArraySpliterator<>(a, 0, j, characteristics);
        }
        return null;
    }

同样值得注意的是常数:

static final int BATCH_UNIT = 1 << 10;  // batch array size increment
static final int MAX_BATCH = 1 << 25;  // max batch array size;

因此,在我的示例中,我使用了6,000个元素,因为批量大小步长为1024,我只得到三个批次.这正好解释了我的观察结果,最初使用了三个核心,当小批量完成时,使用的是两个核心,然后是一个核心.与此同时,我尝试了一个包含60,000个元素的修改示例,然后我获得了几乎100%的CPU利用率.

为了解决我的问题,我开发了下面的代码,它允许我将任何现有的流转换为Spliterator#trySplit将其分成指定大小的批次的流.从我的问题中将它用于用例的最简单方法是这样的:

toFixedBatchStream(Files.newBufferedReader(inputPath).lines(), 20)

在较低级别,下面的类是spliterator包装器,它更改包装的spliterator的trySplit行为并保持其他方面不变.

import static java.util.Spliterators.spliterator;
import static java.util.stream.StreamSupport.stream;

import java.util.Comparator;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.stream.Stream;

public class FixedBatchSpliteratorWrapper<T> implements Spliterator<T> {
  private final Spliterator<T> spliterator;
  private final int batchSize;
  private final int characteristics;
  private long est;

  public FixedBatchSpliteratorWrapper(Spliterator<T> toWrap, long est, int batchSize) {
    final int c = toWrap.characteristics();
    this.characteristics = (c & SIZED) != 0 ? c | SUBSIZED : c;
    this.spliterator = toWrap;
    this.est = est;
    this.batchSize = batchSize;
  }
  public FixedBatchSpliteratorWrapper(Spliterator<T> toWrap, int batchSize) {
    this(toWrap, toWrap.estimateSize(), batchSize);
  }

  public static <T> Stream<T> toFixedBatchStream(Stream<T> in, int batchSize) {
    return stream(new FixedBatchSpliteratorWrapper<>(in.spliterator(), batchSize), true);
  }

  @Override public Spliterator<T> trySplit() {
    final HoldingConsumer<T> holder = new HoldingConsumer<>();
    if (!spliterator.tryAdvance(holder)) return null;
    final Object[] a = new Object[batchSize];
    int j = 0;
    do a[j] = holder.value; while (++j < batchSize && tryAdvance(holder));
    if (est != Long.MAX_VALUE) est -= j;
    return spliterator(a, 0, j, characteristics());
  }
  @Override public boolean tryAdvance(Consumer<? super T> action) {
    return spliterator.tryAdvance(action);
  }
  @Override public void forEachRemaining(Consumer<? super T> action) {
    spliterator.forEachRemaining(action);
  }
  @Override public Comparator<? super T> getComparator() {
    if (hasCharacteristics(SORTED)) return null;
    throw new IllegalStateException();
  }
  @Override public long estimateSize() { return est; }
  @Override public int characteristics() { return characteristics; }

  static final class HoldingConsumer<T> implements Consumer<T> {
    Object value;
    @Override public void accept(T value) { this.value = value; }
  }
}

Answer 2

这个问题在一定程度上在 Java-9 早期访问版本中得到解决。将Files.lines被改写，现在在拆分它实际上跳进内存映射文件的中间。这是我的机器上的结果（它有 4 个超线程内核 = 8 个硬件线程）：

Java 8u60：

Start processing
          Cores: 8
       CPU time: 73,50 s
      Real time: 36,54 s
CPU utilization: 25,15%

Java 9b82：

Start processing
          Cores: 8
       CPU time: 79,64 s
      Real time: 10,48 s
CPU utilization: 94,95%

如您所见，实时性和 CPU 利用率都得到了极大的提高。

不过，这种优化有一些限制。目前它仅适用于几种编码（即UTF-8、ISO_8859_1 和 US_ASCII），因为对于任意编码，您并不确切知道换行符是如何编码的。它仅限于不超过 2Gb 大小的文件（由于MappedByteBufferJava 中的限制），当然不适用于某些非常规文件（如字符设备、无法进行内存映射的命名管道）。在这种情况下，旧的实现被用作后备。