Global link arrangement for practical Dragonfly

The Dragonfly network organizes routers into groups, with connectivity within each group provided by local links and connectivity between groups provided by global links. The specification of Dragonfly leaves many options for arranging global links. In this work, we study global link arrangement for practical Dragonfly topologies where (1) there are multiple global links connecting each pair of groups, and (2) the global link bandwidth is similar to the local link bandwidth. We found that existing global link arrangement schemes such as the absolute, relative and circulant-based arrangements do not specify an important component in global connectivity for practical Dragonfly, which we call per-router arrangement. Per-router arrangement determines how the global links from each individual router are connected. We integrate per-router arrangement into existing schemes, develop a unified algorithm to compute a large class of global link arrangements for practical Dragonfly, and carry out an extensive simulation study to evaluate different global link arrangement schemes. Our results indicate that the existing understanding of the global link arrangement does not apply to practical Dragonfly: contradict to the existing understanding that global link arrangement does not make significant difference in performance when global links have similar bandwidth as local links, per-router arrangement significantly impacts the network performance of practical Dragonfly. We identify the schemes that yield high performance for practical Dragonfly.