Multi-homed Fat-Tree Routing with InfiniBand

For clusters where the topology consists of a fat-tree or more than one fat-tree combined into one subnet, there are several properties that the routing algorithms should support, beyond what exists today. One of the missing properties is that current fat-tree routing algorithm does not guarantee that each port on a multi-homed node is routed through redundant spines, even if these ports are connected to redundant leaves. As a consequence, in case of a spine failure, there is a small window where the node is unreachable until the subnet manager has rerouted to another spine. In this paper, we discuss the need for independent routes for multi-homed nodes in fat-trees by providing real-life examples when a single point of failure leads to complete outage of a multi-port node. We present and implement methods that may be used to alleviate this problem and perform simulations that demonstrate improvements in performance, scalability, availability and predictability of InfiniBand fat-tree topologies. We show that our methods not only increase the performance by up to 52.6%, but also, and more importantly, that there is no downtime associated with spine switch failure.