A single field in the heap region structure prohibits more than one concurrent job at a time.

From the technical debt page:

Separate iterator claim values from the data structures iterated over

    potentially think of making the interface of iterators the same, at least within a single collector's code base
    these two things are conceptually unrelated
    claim values are temporary, the data structures iterated are long-living
    lots of bloat regarding resetting/managing the iterator claim values detracting from the point of these data structures
    extension and changing the iteration/claiming mechanism does not change the data structure all the time
    there is a lot of code repetition with minor differences in the implementations
    should allow easy and generic application of padding etc. too
    should allow generic introduction of measuring properties like contention