  Coupling regular topologies with optimized routing algorithms is key in
pushing the performance of interconnection networks of HPC systems. In this
paper we present Dmodc, a fast deterministic routing algorithm for Parallel
Generalized Fat-Trees (PGFTs) which minimizes congestion risk even under
massive topology degradation caused by equipment failure. It applies a
modulo-based computation of forwarding tables among switches closer to the
destination, using only knowledge of subtrees for pre-modulo division. Dmodc
allows complete rerouting of topologies with tens of thousands of nodes in less
than a second, which greatly helps centralized 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. We compare Dmodc against
routing algorithms available in the InfiniBand control software (OpenSM) first
for routing execution time to show feasibility at scale, and then for
congestion risk under degradation to demonstrate robustness. The latter
comparison is done using static analysis of routing tables under random
permutation (RP), shift permutation (SP) and all-to-all (A2A) traffic patterns.
Results for Dmodc show A2A and RP congestion risks similar under heavy
degradation as the most stable algorithms compared, and near-optimal SP
congestion risk up to 1% of random degradation.
