Efficient Topologies for Large-Scale Cluster Networks
Abstract: Increasing integrated-circuit pin bandwidth has motivated a
corresponding increase in the degree or radix of interconnection networks and their
routers. This paper describes the flattened butterfly, a cost-efficient topology for
high-radix networks. On benign (load-balanced) traffic, the flattened butterfly
approaches the cost/performance of a butterfly network and has roughly half the cost of
a comparable performance Clos network. The advantage over the Clos is achieved by
eliminating redundant hops when they are not needed for load balance. On adversarial
traffic, the flattened butterfly matches the cost/performance of a folded-Clos network
and provides an order of magnitude better performance than a conventional butterfly. In
this case, global adaptive routing is used to switch the flattened butterfly from
minimal to non-minimal routing — using redundant hops only when they are needed.
Different routing algorithms are evaluated on the flattened butterfly and compared
against alternative topologies. We also provide a detailed cost model for an
interconnection network and compare the cost of the flattened butterfly to alternative
topologies to show the cost advantages of the flattened butterfly.