Feed-Forward Routing for the Wormhole Switching Network-on-Chip of the Kalray MPPA2 Processor.

The Kalray MPPA2-256 Bostan processor network-on-chip (NoC) implements wormhole switching without virtual channels and with source routing. As shown in earlier work, this NoC can be configured for guaranteed services by solving a set of linear inequalities based on deterministic network calculus (DNC). However, DNC assumes that a unique route is assigned to each flow and requires that the set of flows be feed-forward. We show that ensuring the feed-forward flows property is equivalent to deterministic deadlock-free routing on a wormhole switching NoC. While routing each flow between its given endpoints, minimal path diversity occurs and the choice of a particular unique path for each flow has an impact on the NoC bandwidth exploitation. We cast this choice as a max-min fairness with unsplittable paths problem, whose instances are solved by enumeration and by an efficient heuristic.