Efficient barrier using remote memory operations on VIA-based clusters

Most high performance scientific applications require efficient support for collective communication. Point-to-point message-passing communication in current generation clusters are based on the Send/Recv communication model. Collective communication operations built on top of such point-to-point message-passing operations might achieve suboptimal performance. VIA and the emerging InfiniBand architecture support remote DMA operations, which allow data to be moved between the nodes with low overhead; they also allow to create and provide a logical shared memory address space across the nodes. In this paper we focus on barrier, a frequently-used collective operations. We demonstrate how RDMA write operations can be used to support an inter-node barrier in a cluster with SMP nodes. Combining this with a scheme to exploit shared memory within a SMP node, we develop a fast barrier algorithm for a cluster of SMP nodes with a cLAN VIA interconnect. Compared to current barrier algorithms using the Send/Recv communication model, the new approach is shown to reduce barrier latency on a 64 processor (32 dual nodes) system by up to 66%. These results demonstrate that high performance and scalable barrier implementations can be delivered on current and next generation VIA/Infiniband-based clusters with RDMA support.