Java虚拟机中 -XX:+PrintFlagsFinal与XX:+PrintCommandLineFlags 中MaxHeapSize的值不同的原因

size_t CollectorPolicy::compute_heap_alignment() {
  // The card marking array and the offset arrays for old generations are
  // committed in os pages as well. Make sure they are entirely full (to
  // avoid partial page problems), e.g. if 512 bytes heap corresponds to 1
  // byte entry and the os page size is 4096, the maximum heap size should
  // be 512*4096 = 2MB aligned.

  // There is only the GenRemSet in Hotspot and only the GenRemSet::CardTable
  // is supported.
  // Requirements of any new remembered set implementations must be added here.
  size_t alignment = GenRemSet::max_alignment_constraint(GenRemSet::CardTable);

  // Parallel GC does its own alignment of the generations to avoid requiring a
  // large page (256M on some platforms) for the permanent generation.  The
  // other collectors should also be updated to do their own alignment and then
  // this use of lcm() should be removed.
  if (UseLargePages && !UseParallelGC) {
      // in presence of large pages we have to make sure that our
      // alignment is large page aware
      alignment = lcm(os::large_page_size(), alignment);
  }

  return alignment;
}

原因堆被划了一个个card page。

512个card page 便是一个Card Table。

这里假设：一个Card Page的是一个card table 的entry 与一个对应的操作系统内存页是4KB

一个Card Table 总体堆内存最大的size便是 512个（Card Page)*4k(系统内存页）=2MB的内存