Java : HEAP Memory 参数设置

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1. https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html

   -Xmxsize
   Specifies the maximum size (in bytes) of the memory allocation pool in bytes.
   

   -XX:NewRatio=old/young   By default, this option is set to 2.
   

   -XX:SurvivorRatio=eden/survivor   By default, this option is set to 8.
   

2. 你比如说我设置-Xmx3212M -XX:NewRatio=3 -XX:SurvivorRatio=9
3. https://github.com/alibaba/arthas/blob/555958c6083dd69fdde25b732c831af99215c53c/core/src/main/java/com/taobao/arthas/core/command/monitor200/DashboardCommand.java#L183

   MemoryMXBean memoryMXBean = ManagementFactory.getMemoryMXBean();
   List<MemoryPoolMXBean> memoryPoolMXBeans = ManagementFactory.getMemoryPoolMXBeans();
   
   MemoryUsage heapMemoryUsage = memoryMXBean.getHeapMemoryUsage();
   System.out.println(heapMemoryUsage);
   for (MemoryPoolMXBean memoryPoolMXBean : memoryPoolMXBeans) {
       if(MemoryType.HEAP.equals(memoryPoolMXBean.getType())) {
           System.out.println(memoryPoolMXBean.getName()+"\t"+memoryPoolMXBean.getUsage());
       }
   }
   

4. console

   init = 134217728(131072K) used = 1698944(1659K) committed = 131596288(128512K) max = 3291480064(3214336K)
   PS Eden Space   init = 28311552(27648K) used = 1698944(1659K) committed = 28311552(27648K) max = 836763648(817152K)
   PS Survivor Space   init = 2621440(2560K) used = 0(0K) committed = 2621440(2560K) max = 2621440(2560K)
   PS Old Gen  init = 100663296(98304K) used = 0(0K) committed = 100663296(98304K) max = 2526019584(2466816K)
   

5. Q1: console 和我设的参数是怎么个对应法？
   • init used committed max 它们之间关系？
   • 3291480064=-Xmx3212M ？就是有点小偏差的？
   • -XX:NewRatio=3，指哪列哦？ committed ？ init ？ 3=100663296/(28311552+2621440*2) ？
   • -XX:SurvivorRatio=9，指哪列？怎么算？
6. Q2: console Survivor 的几个值都是 2621440 ？

For example, setting -XX:NewRatio=3 means that the ratio between the young and tenured generation is 1:3. In other words, the combined size of the eden and survivor spaces will be one-fourth of the total heap size.

For example, -XX:SurvivorRatio=6 sets the ratio between eden and a survivor space to 1:6. In other words, each survivor space will be one-sixth the size of eden, and thus one-eighth the size of the young generation (not one-seventh, because there are two survivor spaces).

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https://blog.csdn.net/fanwu72/article/details/8936746
```结论：init 约等于 xms 的值，max 约等于 xmx 的值。used 是已经被使用的内存大小，committed 是当前可使用的内存大小（包括已使用的），committed >= used。committed 不足时 jvm 向系统申请，若超过 max 则发生 OutOfMemoryError 错误。``` committed 初始运行时候约等于 init，然后随着使用 /gc 增大 /减小(与 init 无关)

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猜测: XX:NewRatio XX:SurvivorRatio 可以使用 committed 列来算，可能之前选的值不太好，试了下```-XX:NewRatio=3 -XX:SurvivorRatio=6```是符合的

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@c4f36e5766583218 #2 好像不对。重新猜测:
* XX:NewRatio XX:SurvivorRatio 可能之前选的值不太好，试```-XX:NewRatio=3 -XX:SurvivorRatio=6```
* PS Survivor Space 是指其中之一，共 2 个
* 使用 init 值来计算
* HeapMemory.init = (Eden.init + Survivor.init * 2) + Old.init
* Old.init / (Eden.init + Survivor.init * 2) = 3
* Eden.init / Survivor.init = 6