HotSpot JVM中的“ulimited”大小由适合64位整数变量的最大值（即264-1）表示。
MaxMetaSpaceSize然后被进一步对齐（向下舍入）到最小元空间分配单元

// Ideally, we would be able to set the default value of MaxMetaspaceSize in
  // globals.hpp to the aligned value, but this is not possible, since the
  // alignment depends on other flags being parsed.
  MaxMetaspaceSize = align_size_down_bounded(MaxMetaspaceSize, _reserve_alignment);

在Windows上等于64 K（216）。