G1 GC 是否有最大区域大小或最大区域数量？

Question

我在学习 G1 GC 时，发现了这篇文章：http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html。在那篇文章中，有一段话是这样说的：

G1 GC 是一个区域化的分代垃圾收集器，这意味着 Java 对象堆（heap）被划分为多个大小相等的区域。启动时，Java 虚拟机 (JVM) 会设置区域大小。区域大小可以从 1 MB 到 32 MB 不等，具体取决于堆大小。目标是不超过 2048 个区域。

是不是说G1 GC可以处理的java堆内存最大大小是2048 * 32M，如果超过了会怎么样？

Answer 1

来自HotSpot JVM sources：

  // 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，如果未在 JVM 选项中指定，则用于计算默认区域大小。实际数量可能会超过此值。