XORAdap: A HoL-Blocking Aware Adaptive Routing Algorithm

Routing is a key parameter in the design of the interconnection network of large parallel computers. Depending on the number of routing options available for each packet, routing algorithms can be deterministic (one available path) or adaptive (several ones). Adaptive routing usually outperforms deterministic routing but it also may increase the Head-of-Line blocking effect. Usually, adaptive routing uses virtual channels to provide routing flexibility and to guarantee deadlock freedom. On the other hand, deterministic routing is simpler and therefore it has lower routing delay. In this paper, we take the challenge of developing new routing algorithms for direct topologies that exploit virtual channels in an efficient way combining the good properties of both routing algorithms types: flexibility and reduced HoL blocking. To do that, this paper proposes several hybrid (combination of adaptivity and determinism) simple mechanisms to perform an efficient distribution of packets among virtual channels based on their destination. The resulting routing mechanisms are able to adapt to the different traffic patterns to obtain the best performance while keeping the simplicity of routing.