G1垃圾回收器REGION SIZE说明

最新推荐文章于 2025-09-17 10:19:36 发布

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最新推荐文章于 2025-09-17 10:19:36 发布 · 5.8k 阅读

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#G1 #REGION SIZE

本文详细解析了JDK8中G1垃圾回收器的Region划分实现，包括Region大小的计算逻辑，以及如何确保Region大小符合特定范围，提供代码示例及验证方法。

说明：JDK7和JDK8的Region划分实现略有不同（差异非常小，且只有-Xmx和-Xms的值不一样才有区别），本篇文章讲解的是JDK8中Region的划分实现；如果要了解JDK7的Region划分实现，请参考JDK7 headpRegion.cpp

源码分析

G1 Region划分的实现源码在headpRegion.cpp中，摘取部分核心源码如下：

// 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.
#define 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.
#define 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).
#define TARGET_REGION_NUMBER          2048

size_t HeapRegion::max_region_size() {
  return (size_t)MAX_REGION_SIZE;
}

// 这个方法是计算region的核心实现
void HeapRegion::setup_heap_region_size(size_t initial_heap_size, size_t max_heap_size) {
  uintx region_size = G1HeapRegionSize;
  // 是否设置了G1HeapRegionSize参数，如果没有配置，那么按照下面的方法计算；如果设置了G1HeapRegionSize就按照设置的值计算
  if (FLAG_IS_DEFAULT(G1HeapRegionSize)) {
    // average_heap_size即平均堆的大小，(初始化堆的大小即Xms+最大堆的大小即Xmx)/2
    size_t average_heap_size = (initial_heap_size + max_heap_size) / 2;
    // average_heap