CMSInitiatingOccupancyFraction计算释疑

JVM CMS垃圾收集器相关一个重要参数CMSInitiatingOccupancyFraction，默认值具体是多少，众说纷纭，这里分析和验证一下。

-XX:CMSInitiatingOccupancyFraction: 当老年代使用达到该比例时会触发FullGC，百分比格式。

1. 计算CMSInitiatingOccupancyFraction

具体还是要看openJDK的代码(基于JDK8)：

// hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp// The field "_initiating_occupancy" represents the occupancy percentage
// at which we trigger a new collection cycle.  Unless explicitly specified
// via CMSInitiatingOccupancyFraction (argument "io" below), it
// is calculated by:
//
//   Let "f" be MinHeapFreeRatio in
//
//    _intiating_occupancy = 100-f +
//                           f * (CMSTriggerRatio/100)
//   where CMSTriggerRatio is the argument "tr" below.
//
// That is, if we assume the heap is at its desired maximum occupancy at the
// end of a collection, we let CMSTriggerRatio of the (purported) free
// space be allocated before initiating a new collection cycle.
//
void ConcurrentMarkSweepGeneration::init_initiating_occupancy(intx io, uintx tr) {assert(io <= 100 && tr <= 100, "Check the arguments");if (io >= 0) {_initiating_occupancy = (double)io / 100.0;} else {_initiating_occupancy = ((100 - MinHeapFreeRatio) +(double)(tr * MinHeapFreeRatio) / 100.0)/ 100.0;}
}

其中：
io参数是：输入的CMSInitiatingOccupancyFraction值
tr参数是：CMSTriggerRatio的值，JDK8默认是80

代码逻辑比较简单：

若指定的CMSInitiatingOccupancyFraction为正值，例如80，则老年代到达80%就会触发FullGC。
CMSInitiatingOccupancyFraction默认为-1，走下面的分支。
基于JDK8，结合其余2个参数MinHeapFreeRatio默认值40、CMSTriggerRatio默认值80，计算出的默认百分比是92。
若MinHeapFreeRatio默认值0、CMSTriggerRatio默认值80，则计算出的默认百分比是100。

2. MinHeapFreeRatio的默认值到底是多少？0还是40？

这里有个疑问点：MinHeapFreeRatio的默认值到底是多少？0还是40？

通过-XX:+PrintFlagsFinal参数打印出来的MinHeapFreeRatio默认值是0：

uintx MinHeapFreeRatio                          = 0                                   {manageable}

再借助jinfo -flag MinHeapFreeRatio PID打印出来的MinHeapFreeRatio默认值都是0。

C:\Users\root>jinfo -flag MinHeapFreeRatio 18896
-XX:MinHeapFreeRatio=0

但是从源码上看MinHeapFreeRatio默认值从JDK6开始默认值就一直是40了。

// hotspot/src/share/vm/runtime/globals.hpp
manageable(uintx, MinHeapFreeRatio, 40,                                   \"The minimum percentage of heap free after GC to avoid expansion."\" For most GCs this applies to the old generation. In G1 and"     \" ParallelGC it applies to the whole heap.")                      \\
manageable(uintx, MaxHeapFreeRatio, 70,                                   \"The maximum percentage of heap free after GC to avoid shrinking."\" For most GCs this applies to the old generation. In G1 and"     \" ParallelGC it applies to the whole heap.")

说明MinHeapFreeRatio在哪里进行修改了！

查了一下openJDK相关的代码，发现有这个UseAdaptiveSizePolicy配置，打开时，会默认将MinHeapFreeRatio设置为0，MaxHeapFreeRatio设置为100。
而UseAdaptiveSizePolicy默认就是开启的！这也就能解释为什么代码中默认值是40，而实际运行PrintFlagsFinal时则是0。

bool UseAdaptiveSizePolicy                     = true                                {product}

// hotspot/src/share/vm/runtime/arguments.cpp
if (UseAdaptiveSizePolicy) {// We don't want to limit adaptive heap sizing's freedom to adjust the heap// unless the user actually sets these flags.if (FLAG_IS_DEFAULT(MinHeapFreeRatio)) {FLAG_SET_DEFAULT(MinHeapFreeRatio, 0);_min_heap_free_ratio = MinHeapFreeRatio;}if (FLAG_IS_DEFAULT(MaxHeapFreeRatio)) {FLAG_SET_DEFAULT(MaxHeapFreeRatio, 100);_max_heap_free_ratio = MaxHeapFreeRatio;}
}

关闭UseAdaptiveSizePolicy，然后再查看MinHeapFreeRatio：

java -XX:+PrintFlagsFinal -XX:-UseAdaptiveSizePolicy -version | grep MinHeapFreeRatio

结果符合预期了：

uintx MinHeapFreeRatio                          = 40                                  {manageable}

3. UseAdaptiveSizePolicy和CMS

从网上查阅到的信息是，对于JDK8，当启用CMS时，无论UseAdaptiveSizePolicy设置成什么值，都会关闭UseAdaptiveSizePolicy，自然而然MinHeapFreeRatio就不会被调整为0了。

java -XX:+PrintFlagsFinal -XX:+UseConcMarkSweepGC -version | grep -E "MinHeapFreeRatio | UseAdaptiveSizePolicy"
java -XX:+PrintFlagsFinal -XX:+UseConcMarkSweepGC -XX:+UseAdaptiveSizePolicy -version | grep -E "MinHeapFreeRatio | UseAdaptiveSizePolicy"

结果符合预期：UseAdaptiveSizePolicy关闭，且MinHeapFreeRatio变为默认值40。

uintx MinHeapFreeRatio                          = 40                                  {manageable}bool UseAdaptiveSizePolicy                     = false                               {product}

Java HotSpot(TM) 64-Bit Server VM warning: disabling UseAdaptiveSizePolicy; it is incompatible with UseConcMarkSweepGC.
uintx MinHeapFreeRatio                          = 40                                  {manageable}bool UseAdaptiveSizePolicy                    := false                               {product}

4. 结论

对于JDK6/JDK7/JDK8，CMS的参数：CMSInitiatingOccupancyFraction，肯定需要启用CMS（-XX:+UseConcMarkSweepGC），此时UseAdaptiveSizePolicy会强制关闭，所以MinHeapFreeRatio的默认值就是40，CMSInitiatingOccupancyFraction默认值就是92，百分比类型。不存在100%这样的情况。