In G1 the G1CollectionSetChooser removes/prunes regions that meet the G1HeapWastePercent criteria detailed in JDK-8262185 in the Cleanup pause. These regions are removed from possible evacuation and their remembered sets cleared.

This implies that the work done in rebuilding these remembered sets is wasted. We can prune these regions before the rebuild during Remark and thus not construct these remembered sets which would eventually be cleared anyway.

Moving this pruning improves rebuild time in addition to remembered set memory usage.