SoftReferences与Weakreferences/OutOfMemoryError

Question

处理软引用和弱引用时遇到问题.代码有一个标志,可以在软引用和弱引用之间切换所有逻辑.虽然弱引用似乎都工作正常,使用软引用我一直得到OutOfMemoryError.这种情况发生在MacOSX上的JDK7和JDK6以及Debian上的IcedTea6.然而,与JDK7 G1收集器的设置,我发现与软引用的工作,其他的一切我试过(串行/并行GC,-client/-server等)失败抛出异常.

代码有点大,但我试图在保留问题的同时尽量缩小代码.
我在其上面留下了一个大量的评论,更详细地描述了如何重现这个问题.

/*
 *
 * Leakling.java
 *
 *
 * Issue:
 *
 *
 *   This code throws OutOfMemoryError when using soft references, whereas weak references
 *   work ok. Moreover, with JDK7 G1 garbage collector soft references work as well. Other
 *   collectors seem to fail. Code was tested with MacOSX 10.8.2 JDKs 1.7.0_10-b18 and
 *   1.6.0_37-b06-434-11M3909, with Debian 6.0 IcedTea6 1.8.13.
 *   Various command line options including -Xmx, -client/-server, -XX:+UseParallelOldGC,
 *   -XX:+UseSerialGC were tested.
 *
 *
 * Examples:
 *
 *
 * 1. Default options, weak references, this works, counters go up and down,
 *    but everything keeps going just as expected:
 *
 *   java -Xmx50m Leakling \
 *       --loop-delay=10 --min-chunk-size=1000 --max-chunk-size=100000 --use-soft-references=false
 *
 *
 * 2. Default options, soft references, this eventually throws the exception:
 *
 *   java -Xmx50m Leakling \
 *       --loop-delay=10 --min-chunk-size=1000 --max-chunk-size=100000 --use-soft-references=true
 *
 *
 * 3. G1 collector (IcedTea6 doesn't support it), weak references, this works, but it did anyway:
 *
 *   java -XX:+UseG1GC -Xmx50m Leakling \
 *       --loop-delay=10 --min-chunk-size=1000 --max-chunk-size=100000 --use-soft-references=false
 *
 *
 * 4. G1 collector, soft references, this works with JDK7.
 *    JDK6 seems to just stop after having hit memory limit (with no message whatsoever).
 *
 *   java -XX:+UseG1GC -Xmx50m Leakling \
 *       --loop-delay=10 --min-chunk-size=1000 --max-chunk-size=100000 --use-soft-references=true
 *
 *
 * jarek, 02.01.2013
 *
 *
 */

import java.lang.ref.*;
import java.util.*;

public class Leakling {
  private static final String TAG = "memory-chunk-";


  class Chunk {
    final String name;
    final int size;
    final private byte[] mem;

    Chunk(String name, int minSize, int maxSize, Random randomizer) {
      int currSize = minSize;
      if (maxSize > minSize) {
        currSize += randomizer.nextInt(maxSize - minSize + 1);
      }
      this.size = currSize;
      this.mem = new byte[currSize];
      this.name = name;
      log(this + " has been created (" + currSize + " bytes)");
    }

    @Override
    public void finalize() throws Throwable {
      log(this + " is finalizing");
    }

    @Override
    public String toString() {
      return name + " of " + getReadableMemorySize(size);
    }
  }


  class WeakChunk extends WeakReference<Chunk> {
    final String name;

    public WeakChunk(Chunk chunk, ReferenceQueue<Chunk> queue) {
      super(chunk, queue);
      this.name = chunk.name;
    }

    @Override
    public String toString() {
      return "weak reference of " + name + " is " + ((get() == null) ? "null" : "alive");
    }
  }


  class SoftChunk extends SoftReference<Chunk> {
    final String name;

    public SoftChunk(Chunk chunk, ReferenceQueue<Chunk> queue) {
      super(chunk, queue);
      this.name = chunk.name;
    }

    @Override
    public String toString() {
      return "soft reference of " + name + " is " + ((get() == null) ? "null" : "alive");
    }
  }

  // Logging as implemented here gives extra timing info (secs.milis starting from the initialization).
  private final long start = System.currentTimeMillis();
  private final Formatter formatter = new Formatter(System.err);
  private final String formatString = "%1$d.%2$03d %3$s\n";

  // I found this be better synchronized...
  synchronized void log(Object o) {
    long curr = System.currentTimeMillis();
    long diff = curr - start;
    formatter.format(formatString, (int) (diff / 1000), (int) (diff % 1000), o.toString());
  }

  private final ArrayList<Chunk> allChunks = new ArrayList<Chunk>();
  private final ReferenceQueue<Chunk> softReferences = new ReferenceQueue<Chunk>();
  private final ReferenceQueue<Chunk> weakReferences = new ReferenceQueue<Chunk>();
  private final HashSet<Reference<Chunk>> allReferences = new HashSet<Reference<Chunk>>();
  private final Random randomizer = new Random();

  private int loopDelay = 200;
  private int minChunkSize = 100;
  private int maxChunkSize = 1000;
  private int chunkCounter = 0;
  private boolean useSoftReferences = false;
  private long minMemory = 10 * 1024 * 1024;  // Default range is 10-30MB
  private long maxMemory = 3 * minMemory;
  private long usedMemory = 0;

  private String getReadableMemorySize(long size) {
    if (size >= 1024 * 1024) {
      return (float) (Math.round((((float) size) / 1024f / 1024f) * 10f)) / 10f + "MB";
    }
    if (size >= 1024) {
      return (float) (Math.round((((float) size) / 1024f) * 10f)) / 10f + "kB";
    } else if (size > 0) {
      return size + "B";
    } else {
      return "0";
    }
  }

  private void allocMem() {
    Chunk chunk = new Chunk(TAG + chunkCounter++, minChunkSize, maxChunkSize, randomizer);
    allChunks.add(chunk);
    Reference ref = useSoftReferences
            ? (new SoftChunk(chunk, softReferences)) : (new WeakChunk(chunk, weakReferences));
    allReferences.add(ref);
    log(ref);
    usedMemory += chunk.size;
  }

  private void freeMem() {
    if (allChunks.size() < 1) {
      return;
    }
    int i = randomizer.nextInt(allChunks.size());
    Chunk chunk = allChunks.get(i);
    log("freeing " + chunk);
    usedMemory -= chunk.size;
    allChunks.remove(i);
  }

  private int statMem() throws Exception {
    for (Reference ref; (ref = softReferences.poll()) != null;) {
      log(ref);
      allReferences.remove(ref);
    }
    for (Reference ref; (ref = weakReferences.poll()) != null;) {
      log(ref);
      allReferences.remove(ref);
    }
    int weakRefs = 0;
    int softRefs = 0;
    for (Iterator<Reference<Chunk>> i = allReferences.iterator(); i.hasNext();) {
      Reference<Chunk> ref = i.next();
      if (ref.get() == null) {
        continue;
      }
      if (ref instanceof WeakChunk) {
        weakRefs++;
      }
      if (ref instanceof SoftChunk) {
        softRefs++;
      }
    }
    log(allChunks.size() + " chunks, "
            + softRefs + " soft refs, "
            + weakRefs + " weak refs, "
            + getReadableMemorySize(usedMemory) + " used, "
            + getReadableMemorySize(Runtime.getRuntime().freeMemory()) + " free, "
            + getReadableMemorySize(Runtime.getRuntime().totalMemory()) + " total, "
            + getReadableMemorySize(Runtime.getRuntime().maxMemory()) + " max");
    if (loopDelay > 1) {
      Thread.sleep(loopDelay);
    }
    return (int)((100 * usedMemory) / maxMemory); // Return % of maxMemory being used.
  }

  public Leakling(String[] args) throws Exception {
    for (String arg : args) {
      if (arg.startsWith("--min-memory=")) {
        minMemory = Long.parseLong(arg.substring("--min-memory=".length()));
      } else if (arg.startsWith("--max-memory=")) {
        maxMemory = Long.parseLong(arg.substring("--max-memory=".length()));
      } else if (arg.startsWith("--min-chunk-size=")) {
        minChunkSize = Integer.parseInt(arg.substring("--min-chunk-size=".length()));
      } else if (arg.startsWith("--max-chunk-size=")) {
        maxChunkSize = Integer.parseInt(arg.substring("--max-chunk-size=".length()));
      } else if (arg.startsWith("--loop-delay=")) {
        loopDelay = Integer.parseInt(arg.substring("--loop-delay=".length()));
      } else if (arg.startsWith("--use-soft-references=")) {
        useSoftReferences = Boolean.parseBoolean(arg.substring("--use-soft-references=".length()));
      } else {
        throw new Exception("Unknown command line option...");
      }
    }
  }

  public void run() throws Exception {
    log("Mem test started...");

    while(true) {
      log("going up...");
      do {// First loop allocates memory up to the given limit in a pseudo-random fashion.
          // Randomized rate of allocations/frees is about 4:1 as per the 10>=8 condition.
        if (randomizer.nextInt(10) >= 8) {
          freeMem();
        } else {
          allocMem();
        }
      } while (statMem() < 90); // Repeat until 90% of the given mem limit is hit...

      log("going down...");
      do {// Now do the reverse. Frees are four times more likely than allocations are.
        if (randomizer.nextInt(10) < 8) {
          freeMem();
        } else {
          allocMem();
        }
      } while (usedMemory > minMemory);
    }
  }

  public static void main(String[] args) throws Exception {
    (new Leakling(args)).run();
  }
}

Answer 1

首先,不要混合使用参考文献的终结器.两者都会影响对象从内存中删除的速度,并且使用适当的引用类型可以更好地完成终结器所能完成的一切.

其次,正如我所提到的,可能存在与使用引用相关的gc延迟.至少对于"常见"gc算法,弱/软引用的对象可能在完全回收之前需要额外的gc运行.弱引用和软引用之间的本质区别在于弱引用是积极的gc,而软引用通常是"尽可能长".这很可能是什么让你感到高兴.

当你使用弱引用对象运行时,东西会随着时间的推移被清理掉,让你避免使用OOME.

当您使用轻柔引用的对象运行时,将保持所有软引用的对象,直到您接近极限.然后,当内存变得紧,GC尝试开始释放软引用的对象,但它的时间太长(因为它可能需要多个GC通行证完全回收内存),你最终得到的OOME.

我只对G1 gc有肤浅的了解,所以我不知道为什么它在那个场景中"有效".

总而言之,软引用很不错,但由于回收延迟,它们并不总能像你想要的那样好.另外,这是一篇很棒的文章,还有一些额外的有用细节.

Answer 2

摆脱终结者。

对象终结发生在一个单独的线程中，并且在终结完成之前无法真正回收内存。在您的终结器中，您正在进行系统调用（输出），这将在该线程中引入等待。当您处于内存限制时，任何终结器等待都会导致 OOM，这很容易。

至于软与弱的区别：我们的引用将在次要集合期间被回收，而软引用不会（我没有检查过；可能是控制软引用生命周期的标志只会允许它存活多个次要集合）。您的终结器线程很可能可以跟上丢弃的弱引用对象。