During a young collection, Parallel GC searches for dirty cards in the card table to locate old-to-young pointers. After finding dirty cards, Parallel GC uses the internal bookkeeping data structures to locate object starts for heap-parsing to be able to walk the heap within these dirty cards object-by-object.

This change modifies the internal bookkeeping data structure to the one used by Serial and G1. As a result, the object start lookup time is improved and one can observe about a 20% reduction of Young-GC pause in some benchmarks using large object arrays.