Abstract
The need for computational cycles continues to exceed availability, driving commodity clusters to increasing scales. With upcoming clusters containing tens-ofthousands of cores, InfiniBand is a popular interconnect on these clusters, due to its low latency (1.5μsec) and high bandwidth (1.5 GB/sec). Since most scientific applications running on these clusters are written using the Message Passing Interface (MPI) as the parallel programming model, the MPI library plays a key role in the performance and scalability of the system. Nearly all MPIs implemented over InfiniBand currently use the Reliable Connection (RC) transport of InfiniBand to implement message passing. Using this transport exclusively, however, has been shown to potentially reach a memory footprint of over 200MB/task at 16K tasks for the MPI library. The Unreliable Datagram (UD) transport, however, offers higher scalability, but at the cost of medium and large message performance. In this paper we present a multi-transport MPI design, MVAPICH-Aptus, that uses both the RC and UD transports of InfiniBand to deliver scalability and performance higher than that of a single-transport MPI design. Evaluation of our hybrid design on 512 cores shows a 12% improvement over an RC-based design and 4% better than a UD-based design for the SMG2000 application benchmark. In addition, for the molecular dynamics application NAMD we show a 10% improvement over an RC-only design. To the best of our knowledge, this is the first such analysis and design of optimized MPI using both UD and RC.