Method and system for managing a heap. A map is employed to indicate which areas of the heap are inaccessible to a program. Such areas constitute garbage which is potentially returnable to a data structure which identifies free storage available for reallocation to the program. By choosing map entries corresponding to portions of storage whose size is of the same order of magnitude as a predetermined minimum unit size for the data structure, a small map can be used. Such a map can be scanned more quickly than a larger map of higher granularity to identify only relatively larger areas of garbage for reallocation.