Parallel Spectral Graph Sparsification via Low Diameter Decompositions
We present a new solver-free parallel spectral sparsification algorithm for weighted graphs that relies only on parallel low-diameter decompositions and independent sampling. This yields the first algorithmic improvement over prior, solver-free parallel sparsification approaches since Koutis (2014) and, for the first time for a practical algorithm, eliminates any dependence on the target approximation accuracy $\epsilon$ in the algorithm's work and depth. Our algorithm works by sub-sampling edges according to their robust connectivity, as introduced by Kapralov and Panigrahy (2012). We show how to estimate the robust connectivities of $G$ in an extremely simple manner: we create multiple random sub graphs $G_p$, where each edge in $G$ is sub-sampled independently with probability $p_e = \min \{w_e \cdot p, 1\}$. Then, we run a Low Diameter Decomposition in each of the graphs. If $u$ and $v$ often share a cluster in the LDDs, then this provides us with an upper bound on the robust connectivity of the edge $e = (u,v)$. Carefully invoking this procedure for $O(\log n)$ different values of the probabilities $p$ then allows us to obtain sufficiently good estimates for sub-sampling. We additionally complement the theory with an experimental evaluation demonstrating strong performance across relevant graphs and sparsity regimes.