Straightforward solutions to reduce HoL blocking in different Dragonfly fully-connected interconnection patterns

The performance of interconnection networks is a challenging issue for High-Performance Computing (HPC) systems, which becomes even more important when the number of interconnected endnodes grows. In that sense, Dragonfly interconnection patterns are a very popular option to configure the network topology, especially for large systems, as they are able to achieve a high scalability relying on high-radix switches. This kind of hierarchical topologies has two levels of interconnection (i.e., connections within the element of a group and connections among groups) and each one can be interconnected using different patterns. However, regardless of the Dragonfly interconnection pattern, the Head-of-Line (HoL) blocking effect derived from congestion situations may jeopardize the Dragonfly performance. This paper analyzes the dynamics of congestion in different Dragonfly fully-connected interconnection patterns. Also, we describe a queuing scheme called Hierarchical Two-Level Queuing (H2LQ), designed specially to reduce HoL blocking in any fully-connected Dragonfly network that uses minimal-path routing. Finally, we present experiment results which show that this scheme significantly boost Dragonfly performance, regardless the interconnection pattern, especially when congestion arises, while requiring fewer network resources than other techniques oriented to deal with the effects of congestion.