On the usefulness of type and liveness accuracy for garbage collection and leak detection

Abstract

The effectiveness of garbage collectors and leak detectors in identifying dead objects depends on theaccuracyof their reachability traversal. Accuracy has two orthogonal dimensions: (i) whether the reachability traversal can distinguish between pointers and nonpointers (type accuracy), and (ii) whether the reachability traversal can identify memory locations that will be dereferenced in the future (liveness accuracy). This article presents an experimental study of the importance of type and liveness accuracy for reachability traversals. We show that liveness accuracy reduces the reachable heap size by up to 62% for our benchmark programs. However, the simpler liveness schemes (e.g., intraprocedural analysis of local variables) are largely ineffective for our benchmark runs: one must analyze global variables using interprocedural analysis to obtain significant benefits. Type accuracy has an insignificant impact on a garbage collector's ability to find unreachable objects in our benchmark runs. We report results for programs written in C, C++, and Eiffel.