DETERMINISTIC MASSIVELY PARALLEL CONNECTIVITY

We consider the problem of designing fundamental graph algorithms on the model of massively parallel computation (MPC). The input to the problem is an undirected graph G with n vertices and m edges and with D being the maximum diameter of any connected component in G. We consider the MPC with low local space, allowing each machine to store only Theta(n(delta)) words for an arbitrary constant delta> 0 and with linear global space (which is the number of machines times the local space available), that is, with optimal utilization. In a recent breakthrough, Andoni et al. [Parallel graph connectivity in log diameter rounds, 2018] and Behnezhad, Hajiaghayi, and Harris [Exponentially faster massively parallel maximal matching, 2019] designed parallel randomized algorithms that in O(logD+log log n) rounds on an MPC with low local space determine all connected components of a graph, improving on the classic bound of O(log n) derived from earlier works on PRAM algorithms. In this paper, we show that asymptotically identical bounds can be also achieved for deterministic algorithms: We present a deterministic MPC low local space algorithm that in O(log D+ log log n) rounds determines connected components of the input graph. Our result matches the complexity of state-of-the-art randomized algorithms for this task. We complement our upper bounds by extending a recent lower bound for the connectivity on an MPC conditioned on the 1-vs 2-cycles conjecture (which requires D >= log(1+Omega(1)) n) by showing a related conditional hardness of Omega (log D) MPC rounds for the entire spectrum of D, covering a particularly interesting range when D <= O(log n).