Work-Efficient Parallel Union-Find with Applications to Incremental Graph Connectivity

Natcha Simsiri, Kanat Tangwongsan, Srikanta Tirthapura, Kun-Lung Wu

Abstract: On an undirected graph, how can one quickly answer whether two vertices are connected while allowing more edges to be added incrementally? This is the well-studied incremental graph connectivity (IGC) problem, a fundamental problem that can be efficiently solved using solutions to the classical union-find problem. Motivated by the need to handle larger and rapidly-changing graphs, this paper presents the first shared-memory parallel algorithm for IGC and equivalently, Union-Find that is provably work-efficient (i.e., does no more work than the sequential optimal) and has polylogarithmic parallel depth. It performs path compression in parallel without a lock or speculative execution. We also present a simpler algorithm with slightly worse theoretical properties, but which is easier to implement, and has good practical performance.