Hermes: An Efficient Building Block for RDMA Incast in Datacenters

In Remote Direct Memory Access (RDMA) data center networks, incast has a serious impact on non-incast flows due to its many-to-one and synchronous characteristics. With the increasing link rate, the response speed of RDMA Congestion Control (CC) alone fails to effectively cope with incast. However, existing RDMA incast solutions have limitations. They are unable to control the incast in time and require extensive device modifications. We propose Hermes, an efficient incast control scheme based on Top-of-Rack (ToR) switches. Hermes achieves accurate incast identification with the shortest control loop through the traffic monitor and uses the Virtual Output Queue (VOQ) to converge quickly to the proper rate of incast traffic. The core idea of Hermes is to minimize the accumulation of incast traffic on core switches and improve the performance of non-incast flows. Hermes only modifies ToR switches and is compatible with existing RDMA CCs. We evaluate Hermes through our Testbed and NS3 simulations. The results show that Hermes can reduce the average and 99th Flow Completion Time (FCT) slowdown of incast victim flows by up to 20% and 55% respectively, compared with the state-of-the-art solution.