Efficient SMP-aware MPI-level broadcast over InfiniBand´s hardware multicast

Most of the high-end computing clusters found today feature multi-way SMP nodes interconnected by an ultra-low latency and high bandwidth network. InfiniBand is emerging as a high-speed network for such systems. InfiniBand provides a scalable and efficient hardware multicast primitive to efficiently implement many MPI collective operations. However, employing hardware multicast as the communication method may not perform well in all cases. This is true especially when more than one process is running per node. In this context, shared memory channel becomes the desired communication medium within the node as it delivers latencies which are of an order of magnitude lower than the inter-node message latencies. Thus, to deliver optimal collective performance, coupling hardware multicast with shared memory channel becomes necessary. In this paper we propose mechanisms to address this issue. On a 16-node 2-way SMP cluster, the Leader-based scheme proposed in this paper improves the performance of the MPI_Bcast operation by a factor of as much as 2.3 and 1.8 when compared to the point-to-point and original solution employing only hardware multicast. We have also evaluated our designs on NUMA based system and obtained a performance improvement of 1.7 using our designs on 2-node 4-way system. We also propose a dynamic attach policy as an enhancement to this scheme to mitigate the impact of process skew on the performance of the collective operation