G1 GC 是否有最大区域大小或最大区域数量?

2022-01-16 00:00:00 jvm garbage-collection java g1gc

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:

when i studied G1 GC, I found this article: http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html. In that article, there is something says as follow:

G1 GC 是一个区域化和分代的垃圾收集器，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) 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.

The G1 GC is a regionalized and generational garbage collector, which means that the Java object heap (heap) is divided into a number of equally sized regions. Upon startup, the Java Virtual Machine (JVM) sets the region size. The region sizes can vary from 1 MB to 32 MB depending on the heap size. The goal is to have no more than 2048 regions.

是不是说G1 GC可以处理的java heap memory的最大大小是2048 * 32M，如果超过了会怎样?

Does that mean the max size of java heap memory that G1 GC can deal with is 2048 * 32M, and if the size exceeds it, what will happen?

推荐答案

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，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.

    • MIN_REGION_SIZE (1MB) and MAX_REGION_SIZE (32MB) are hard limits;
    • TARGET_REGION_NUMBER is just a hint that is used to calculate default region size if it is not specified among JVM options. The actual number may exceed this value.

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