从热点jvm源：

// 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，如果在jvm选项中没有指定，则用于计算默认区域大小。实际数字可能超过此值。