Efficient communication is a requirement for application scalability on High Performance Computing systems. In this paper we argue for incorporating proactive congestion avoidance mechanisms into the design of communication layers on manycore systems. This is in contrast with the status quo which employs a reactive approach, e.g. congestion control mechanisms are activated only when resources have been exhausted. We present a core stateless optimization approach based on open loop end-point

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... ttling, implemented for two UPC runtimes (Cray and Berkeley UPC) and validated on InfiniBand and the Cray Gemini networks. Microbenchmark results indicate that throttling the number of messages in flight per core can provide up to 4X performance improvements, while throttling the number of active cores per node can provide additional 40% and 6X performance improvement for UPC and MPI respectively. We evaluate inline (each task makes independent decisions) and proxy (server) congestion avoidance designs. Our runtime provides both performance and performance portability. We improve allto-all collective performance by up to 4X and provide better performance than vendor provided MPI and UPC implementations. We also demonstrate performance improvements of up to 60% in application settings. Overall, our results indicate that modern systems accommodate only a surprisingly small number of messages in flight per node. As Exascale projections indicate that future systems are likely to contain hundreds to thousands of cores per node, we believe that their networks will be underprovisioned. In this situation, proactive congestion avoidance might become mandatory for performance improvement and portability.