Summary

Exit the VM if users provide different NewSize and MaxNewSize when InitialHeapSize and MaxHeapSize are the same. Affects only Serial and Parallel GC.

Problem

MaxNewSize can be silently ignored in certain cases. For example:

java -XX:+UseSerialGC
  -XX:InitialHeapSize=256m -XX:MaxHeapSize=256M
  -XX:NewSize=8M -XX:MaxNewSize=80M
  '-Xlog:gc,gc+heap=trace' --version

sets initial and maximum young generation size to 8 MB, meaning that MaxNewSize=80M is silently ignored, which can be surprising. This is especially true when -XX:InitialHeapSize=256m (or an even larger value) is derived from the default and implicit InitialRAMPercentage.

Solution

Exit the VM with a warning to instruct users to adjust values for NewSize/MaxNewSize and/or InitialHeapSize/MaxHeapsize.

Specification

    // The maximum and initial heap sizes are the same, so the generation's
    // initial size must be the same as its maximum size.
    if (FLAG_IS_CMDLINE(NewSize) && FLAG_IS_CMDLINE(MaxNewSize) && NewSize != MaxNewSize) {
      vm_exit_during_initialization(
          "The MaxNewSize must be the same as NewSize because the MaxHeapSize and InitialHeapSize are the same.");
    }