The Network Adapter: The Missing Link between MPI Applications and Network Performance

Network design aspects that influence cost and performance can be classified according to their distance from the applications, into issues concerning topology, switch technology, link technology, network adapter, and communication library. The network adapter has a privileged position to take decisions with more global information than any other component in the network. It receives feedback from the switches and requests from the communication libraries and applications. Also, compared to a network switch, an adapter has access to significantly more memory (host memory and on-chip memory) and memory bandwidth (which typically exceeds network bandwidth). The potential of the adapter to improve global network performance has not yet been fully exploited. In this work we show a series of noticeable performance improvements (of at least 10% to 15%) for medium-sized message exchanges in typical HPC communication patterns by optimizing message segmentation and packet injection policies, that can be implemented in an adapter's firmware inexpensively. We also show that implementing equivalent solutions in the switch (as opposed to the adapter) leads to only marginal performance improvements as the ones obtained by controlling the segmentation and injection policy at the adapter, while involving significantly more cost. In addition, enhancing the adapter will lead to less hardware complexity in the switches, thus reducing cost and energy consumption.