There are remnants from Solaris/SPARC left in the NUMA code in Parallel GC (MutableNUMASpace) that handles cases where lgrp_id is set to -1 and if the locality groups internal data structure is not sized properly for the hardware. This should never be the case, as HotSpot does not support CPU/NUMA hotplugging.

Solaris/SPARC had support for hotplugging hardware, which meant that the internal datastructures had to be updated to reflect such a change. The lgrp_id is set during thread creation and changed during allocation, it should never be "reset" to -1. The last code that ever set lgrp_id to -1 was removed in JDK-8301149.