The par compaction summary phase makes two passes over each Space in the GC heap. The first pass updates the data as if each Space were to be compacted completely (i.e., leaving no dead space) into itself. This is used to determine how much data is live in the Space and allows a dense prefix to be computed where applicable (the old gen and the perm gen). The second pass then computes the final destinations. Live data from the young gen Spaces is normally copied to the old gen, assuming it will fit. For young gen spaces that fit into the old gen, certain fields of the summary data must be cleared. More data than necessary is being cleared.