A space-incremental garbage collector performs marking operations that are usually separated by several collection increments. It uses the marking results to compute collection-efficiency metrics for regions into which it treats the heap as divided. It bases its selection of regions for successive increments' collection sets on the metrics' values, whose computations also depend on the sizes of the regions' "remembered sets," i.e., on the lists of locations where references to objects in those regions have been observed. Although the remembered-set sizes therefore potentially change between collection increments, the collector re-computes metrics in most collection increments for only a subset of the regions. It selects the subset in accordance with a sorting of all regions that it performed at the end of the most recent completed marking operation.