Thresholds for Extreme Orientability

Multiple-choice load balancing has been a topic of intense study since the seminal paper of Azar, Broder, Karlin, and Upfal. Questions in this area can be phrased in terms of orientations of a graph, or more generally a k-uniform random hypergraph. A (d,b)-orientation is an assignment of each edge to d of its vertices, such that no vertex has more than b edges assigned to it. Conditions for the existence of such orientations have been completely documented except for the “extreme” case of (k?1,1)-orientations. We consider this remaining case, and establish:

• The density threshold below which an orientation exists with high probability, and above which it does not exist with high probability.
• An algorithm for finding an orientation that runs in linear time with high probability, with explicit polynomial bounds on the failure probability.

Previously, no closed-form expression for the threshold was known. The only known algorithms for constructing (k?1,1)-orientations worked for k≤3, and were only shown to have expected linear running time.