Efficient multicast on irregular switch-based cut-through networks with up-down routing

The irregular switch-based network of workstations is fast becoming a cost-effective platform for high performance computing. This paper presents efficient multicasting with reduced link contention on irregular switch-based cut-through interconnection using the popular up*/down* (UD) routing and unicast message passing. First, it is proven that, for an arbitrary irregular network with UD routing, it is not possible to create an ordered list of nodes to implement an arbitrary multicast in a link contention-free manner with a minimal number of communication steps. Next, three different multicast algorithms are proposed with their respective node orderings to reduce link contention: switch-based ordering (SO), switch-based hierarchical ordering (SHO), and chain concatenation ordering (CCO). A variation of the binomial tree-based communication pattern, with unicast message passing, is used on the above orderings to implement multicast. Then, the problem of node contention is described in the case when multiple multicasts occur concurrently in a system. Using source-based information, the CCO algorithm is modified to propose a source-partitioned chain concatenation ordering (SPCCO) algorithm. It is also shown how the SPCCO algorithm reduces the effect of node contention at the cost of link contention. Using detailed simulation experiments, the proposed multicast algorithms are compared with each other as well as with the naive random ordering (RO) algorithm for a range of system sizes, switch sizes, message lengths, input buffer sizes, degrees of connectivity, destination set sizes, and communication start-up times. For the case of single multicast, the CCO algorithm is shown to be the best to implement multicast with reduced link contention and minimum latency, For the case of multiple multicasts, the SPCCO algorithm is shown to be the best when the start-up overhead dominates the propagation overhead and the CCO algorithm is shown to be the best otherwise. The results also highlight the importance of reducing link contention when designing efficient multicast, even for systems with large input buffers in the switches. Thus, these results demonstrate significant potential to be applied to current and future generation NOW systems with irregular interconnection