G1 GC的回收区域有最大的大小限制还是最大数量限制？

Question

G1 GC的回收区域有最大的大小限制还是最大数量限制？

javagarbage-collectionjvmg1gc

6

当我学习G1 GC时，我发现了这篇文章：http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html。在这篇文章中，有一段话如下所述:

G1 GC是一个分区和分代垃圾收集器，这意味着Java对象堆(heap)被分成许多大小相等的区域。在启动时，Java虚拟机(JVM)设置区域大小。根据堆的大小，区域大小可以从1 MB到32 MB不等。目标是不超过2048个区域。

这是否意味着G1 GC可以处理的java堆内存的最大大小为2048 * 32M？如果超过了这个大小会发生什么呢？

- ho yoje

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- apangin · Accepted Answer

以下内容来自HotSpot JVM源代码:

  // Minimum region size; we won't go lower than that.
  // We might want to decrease this in the future, to deal with small
  // heaps a bit more efficiently.
  static const size_t MIN_REGION_SIZE = 1024 * 1024;

  // Maximum region size; we don't go higher than that. There's a good
  // reason for having an upper bound. We don't want regions to get too
  // large, otherwise cleanup's effectiveness would decrease as there
  // will be fewer opportunities to find totally empty regions after
  // marking.
  static const size_t MAX_REGION_SIZE = 32 * 1024 * 1024;

  // The automatic region size calculation will try to have around this
  // many regions in the heap (based on the min heap size).
  static const size_t TARGET_REGION_NUMBER = 2048;

MIN_REGION_SIZE (1MB)和MAX_REGION_SIZE (32MB)是硬限制；
TARGET_REGION_NUMBER只是一个提示，用于计算默认的区域大小（如果在JVM选项中未指定）。实际数量可能超过此值。