Abstract: A present disclosure provides a garbage collector capable of freeing memory reachable only by a terminated thread immediately upon thread exit without scanning the heap or blocking other threads. A heap, including a plurality of spans, is walked, for example when the heap reaches a predetermined size. Reachable objects are marked in a mark bitmap. A mutator sweeps the mark bitmap without clearing mark bits, and newly allocated objects are not marked. When the thread terminates, sweep pointers in each of the spans owned by the terminated thread are reset. Moreover, a write barrier that makes a first unpublished object reachable from a second published object, also publishes the first object and all objects reachable from the first object.