JVM内存参数的学习之三

背景

研究启动性能时, 顺便看到了jmap -heap 1 的部分信息
看到:
MinHeapFreeRatio、MaxHeapFreeRatio
自己突然以为是 Percentage的参数, 恍惚了好久. 
才发现自己对内存的学习不够, 所以想多学习一下. 

参数解释

MinHeapFreeRatio: 
空闲堆空间的最小百分比，计算公式为：HeapFreeRatio =(CurrentFreeHeapSize/CurrentTotalHeapSize) * 100，
值的区间为0到100，默认值为 40。如果HeapFreeRatio < MinHeapFreeRatio，则需要进行堆扩容，扩容的时机应该在每次垃圾回收之后。

MaxHeapFreeRatio : 
空闲堆空间的最大百分比，计算公式为：HeapFreeRatio =(CurrentFreeHeapSize/CurrentTotalHeapSize) * 100，
值的区间为0到100，默认值为 70。如果HeapFreeRatio > MaxHeapFreeRatio，则需要进行堆缩容，缩容的时机应该在每次垃圾回收之后。

来源: 
https://blog.csdn.net/claram/article/details/104635114

参数默认值

如果启动参数为: 
"-XX:MaxRAMPercentage=80.0","-XX:MinRAMPercentage=50.0"
参数值应该是默认的: 
Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 25891438592 (24692.0MB)
   NewSize                  = 169148416 (161.3125MB)
   MaxNewSize               = 8630435840 (8230.625MB)
   OldSize                  = 338362368 (322.6875MB)

如果关闭自适应的话:
"-XX:-UseAdaptiveSizePolicy","-XX:MaxRAMPercentage=80.0","-XX:MinRAMPercentage=50.0"
发现这个参数值并不会发生变化
   Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 25891438592 (24692.0MB)
   NewSize                  = 169148416 (161.3125MB)
   MaxNewSize               = 8630435840 (8230.625MB)
   OldSize                  = 338362368 (322.6875MB)

增加初始化的设置时
"-XX:InitialRAMPercentage=75.0","-XX:MaxRAMPercentage=75.0","-XX:MinRAMPercentage=75.0"
Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 22548578304 (21504.0MB)
   NewSize                  = 7516192768 (7168.0MB)
   MaxNewSize               = 7516192768 (7168.0MB)
   OldSize                  = 15032385536 (14336.0MB)

感觉增加初始化 InitialRAMPercentage 非常管用,会自动分配堆区设置, 并且能够将old区域 固定下来. 


如果不设置 InitialRAMPercentage时
"-XX:MaxRAMPercentage=75.0","-XX:MinRAMPercentage=75.0"
Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 22548578304 (21504.0MB)
   NewSize                  = 234881024 (224.0MB)
   MaxNewSize               = 11274289152 (10752.0MB)
   OldSize                  = 234881024 (224.0MB)
但是这个时候内存的分配不是严格的 1:2配比的new和old   old区域一开始很小.  感觉这样并不是很优秀. 

可以增加几个内存参数对象的变量设置. 
"-XX:InitialRAMPercentage=75.0","-XX:MaxRAMPercentage=75.0","-XX:MinRAMPercentage=75.0","-XX:SurvivorRatio=3","-XX:NewRatio=1"
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 22548578304 (21504.0MB)
   NewSize                  = 11274289152 (10752.0MB)
   MaxNewSize               = 11274289152 (10752.0MB)
   OldSize                  = 11274289152 (10752.0MB)