ParallelGC splits large objarrays into partial subtasks, a leading chunk and the remaining tail task (PartialArrayScanTask), with the tail pushed on the mark stack.  When a tail task is encountered, it is similarly split.

As discussed in JDK-8253237, there are some problems with this mechanism, as it may not provide as much parallelism as one might hope.  That change introduced some new infrastructure (PartialArrayTaskStepper) to manage splitting in a way that provides more opportunities for parallelism without flooding the mark stack.  Only G1 currently uses this mechanism.  ParallelGC should be enhanced to also use it.