Uniprocessor Garbage Collection Techniques 1 Automatic Storage Reclamation

We survey basic garbage collection algorithms, and variations such as incremental and generational collection. The basic algorithms include reference counting, mark-sweep, mark-compact, copying, and treadmill collection. Incremental techniques can keep garbage collection pause times short, by interleaving small amounts of collection work with program execution. Generational schemes improve efficiency and locality by garbage collecting a smaller area more often, while exploiting typical lifetime characteristics to avoid undue overhead from long-lived objects. 1 Automatic Storage Reclamation Garbage collection is the automatic reclamation of computer storage [Knu69, Coh81, App91]. While in many systems programmers must explicitly reclaim heap memory at some point in the program, by using a “free” or “dispose” statement, garbage collected systems free the programmer from this burden. The garbage collector’s function is to find data objects that are no longer in use and make their space available for reuse by the the running program. An object is considered garbage (and subject to reclamation) if it is not reachable by the running program via any path of pointer traversals. Live (potentially reachable) objects are preserved by the collector, ensuring that the program can never traverse a “dangling pointer” into a deallocated object. This paper is intended to be an introductory survey of garbage collectors for uniprocessors, especially those developed in the last decade. For a more thorough treatment of older techniques, see [Knu69, Coh81].