A method for collecting garbage in a computing environment, the method including tracing root objects to their reachable objects in a population of objects, marking the traced objects, unmarking a marked card including any of the objects, tracing any marked object on the unmarked card to an unmarked referent object of the marked object, marking the unmarked referent object, and tracing the marked referent object to its reachable objects, concurrently with the operation of a mutator upon the population of objects within the computing environment, and, while no mutator operates upon the population of objects within the computing environment, marking the traced objects, tracing unmarked root object referents to their reachable objects, marking any of the objects, and designating any unmarked object in the population of objects as available for reallocation.