Diffusion tree restructuring for indirect reference counting

A new variant algorithm for distributed acyclic garbage detection is presented for use in hybrid garbage collectors. The existing fault-tolerance of Piquer's Indirect Reference Counting (IRC) is qualitatively improved by this new approach. The key insight that underpins this work is the observation that the parent of a node in the IRC diffusion tree need not remain constant. The new variant exploits standard mechanisms for implementing diffusion trees and remote references, using four simple low-cost techniques to dynamically restructure the trees to reduce their depth. This variant reduces third-party dependencies, which make standard IRC vulnerable to process failure, while retaining tolerance of message reordering and without incurring substantial overheads. The paper carefully motivates the algorithm, presents the full technical basis for its development, provides a clear explanation of implementation details and includes an initial discussion of performance issues.