Incremental copying garbage collection for WAM-based Prolog systems

Abstract

The design and implementation of an incremental copying heap garbage collector for WAM-based Prolog systems is presented. Its heap layout consists of a number of equalsized blocks. Other changes to the standard WAM allow these blocks to be garbage collected independently. The independent collection of heap blocks forms the basis of an incremental collecting algorithm which employs copying without marking (contrary to the more frequently used mark&copy or mark&slide algorithms in the context of Prolog). Compared to standard semi-space copying collectors, this approach to heap garbage collection lowers in many cases the memory usage and reduces pause times. The algorithm also allows for a wide variety of garbage collection policies including generational ones. The algorithm is implemented and evaluated in the context of hProlog.

The YAP Prolog system

“…A second drawback is that it requires explicit code for efficiency, making it hard to take advantage of 64 bit machines. Notice that YAP does not need tag bits for garbage collection: instead, we use a separate memory area to store the garbage collector state (Vandeginste and Demoen 2007).…”