  Coupling regular topologies with optimised routing algorithms is key in
pushing the performance of interconnection networks of supercomputers.In this
paper we present Dmodc, a fast deterministic routing algorithm for Parallel
Generalised Fat-Trees (PGFTs) which minimises congestion risk even under
massive network degradation caused by equipment failure.Dmodc computes
forwarding tables with a closed-form arithmetic formula by relying on a fast
preprocessing phase.This allows complete re-routing of networks with tens of
thousands of nodes in less than a second.In turn, this greatly helps
centralised fabric management react to faults with high-quality routing tables
and no impact to running applications in current and future very large-scale
HPC clusters.
