Faster asynchronous MST and low diameter tree construction with sublinear communication

Abstract

Building a spanning tree, minimum spanning tree (MST), and BFS tree in a distributed network are fundamental problems which are still not fully understood in terms of time and communication cost. x The first work to succeed in computing a spanning tree with communication sublinear in the number of edges in an asynchronous CONGEST network appeared in DISC 2018. That algorithm which constructs an MST is sequential in the worst case; its running time is proportional to the total number of messages sent. Our paper matches its message complexity but brings the running time down to linear in $n$. Our techniques can also be used to provide an asynchronous algorithm with sublinear communication to construct a tree in which the distance from a source to each node is within an additive term of $\sqrt{n}$ of its actual distance. We can convert any asynchronous MST algorithm with time $T(n, m)$ and message complexity of $M(n, m)$ to an algorithm with time $O(n^{1 - 2\epsilon} + T(n, n^{3/2 + \epsilon}))$ and message complexity of $\tilde{O}(n^{3/2 + \epsilon} + M(n, n^{3/2+\epsilon}))$, for $\epsilon \in [0, 1/4]$. Picking $\epsilon = 0$ and using Awerbuch's algorithm \cite{awerbuch1987optimal}, this results in an MST algorithm with time $O(n)$ and message complexity $\tilde{O}(n^{3/2})$. However, if there were an asynchronous MST algorithm that takes time sublinear in $n$ and requires messages linear in $m$, by picking $\epsilon > 0$ we could achieve sublinear time (in $n$) and sublinear communication (in $m$), simultaneously. To the best of our knowledge, there is no such algorithm. All the algorithms presented here are Monte Carlo and succeed with high probability, in the KT1 CONGEST asynchronous model.