Live Webcast 15th Annual Charm++ Workshop

Collective communication operations often involve massive data movement over the entire network. A bad implementation of these operations can affect the scalability of an application to a large number of processors. In this paper we study the collective multicast operation. The extreme case of collective multicast is all-to-all multicast MPI_Allgather. Here each processor multicasts a message to all the other processors. We present optimizations and performance studies for all-to-all multicast. These optimizations need to be different for small and large messages. For small messages, the major issue is minimization of software overhead. This can be achieved by message combining. For large messages it is network contention that can be reduced by intelligent message sequencing. Modern NIC's have a communication co-processor that performs message management through zero copy remote DMA operations. We present an asynchronous non blocking collective multicast framework that allows the processor do other computation while the collective operation is in progress. We will also present performance comparisons of the various algorithms implemented by our framework with many relevant applications and benchmarks.