-XX:CMSInitiatingOccupancyFracti

使用例子：
-XX:CMSInitiatingOccupancyFraction=70
CMS垃圾收集器，当老年代达到70%时，触发CMS垃圾回收。

查看CMSInitiatingOccupancyFraction的初始值为-1

intx CMSInitiatingOccupancyFraction            = -1                                  {product}

那么-1代表着什么呢？

查看jvm源码可知

product(intx, CMSInitiatingOccupancyFraction, -1,                         \
          "Percentage CMS generation occupancy to start a CMS collection "  \
          "cycle. A negative value means that CMSTriggerRatio is used")   

注释里也说了，如果CMSInitiatingOccupancyFraction是个负值，那么CMSTriggerRatio将被用到

那么具体是如何用到的呢？

_cmsGen ->init_initiating_occupancy(CMSInitiatingOccupancyFraction, CMSTriggerRatio);

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;
  }
}

如果CMSInitiatingOccupancyFraction在0~100之间，那么由CMSInitiatingOccupancyFraction决定。

否则由按 ((100 - MinHeapFreeRatio) + (double)( CMSTriggerRatio * MinHeapFreeRatio) / 100.0) / 100.0 决定。

那么MinHeapFreeRatio，CMSTriggerRatio的初始值是多少？

uintx MinHeapFreeRatio                          = 40                           {manageable}
uintx CMSTriggerRatio                           = 80                                  {product}

即最终当老年代达到 ((100 - 40) + (double) 80 * 40 / 100 ) / 100 = 92 %时，会触发CMS回收。