Dart: Divide and Specialize for Fast Response to Congestion in RDMA-Based Datacenter Networks

Though Remote Direct Memory Access (RDMA) promises to reduce datacenter network latencies significantly compared to TCP (e.g., 10<inline-formula> <tex-math notation="LaTeX">$\times$ </tex-math></inline-formula>), end-to-end congestion control in the presence of incasts is a challenge. Targeting the full generality of the congestion problem, previous schemes rely on slow, iterative convergence to the appropriate sending rates (e.g., TIMELY takes 50 RTTs). Several papers have shown that even in oversubscribed datacenter networks most congestion occurs at the receiver. Accordingly, we propose a divide-and-specialize approach, called <italic>Dart</italic>, which isolates the common case of receiver congestion and further subdivides the remaining in-network congestion into the simpler spatially-localized and the harder spatially-dispersed cases. For receiver congestion, we propose <italic>direct apportioning of sending rates (DASR)</italic> in which a receiver for <inline-formula> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> senders directs each sender to cut its rate by a factor of <inline-formula> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula>, converging in only one RTT. For the spatially-localized case, Dart provides fast (under one RTT) response by adding novel switch hardware for <italic>in-order flow deflection (IOFD)</italic> because RDMA disallows packet reordering on which previous load balancing schemes rely. For the uncommon spatially-dispersed case, Dart falls back to DCQCN. Small-scale testbed measurements and at-scale simulations, respectively, show that Dart achieves 60% (2.5<inline-formula> <tex-math notation="LaTeX">$\times$ </tex-math></inline-formula>) and 79% (4.8<inline-formula> <tex-math notation="LaTeX">$\times$ </tex-math></inline-formula>) lower <inline-formula> <tex-math notation="LaTeX">$99^{th}$ </tex-math></inline-formula>-percentile latency, and similar and 58% higher throughput than InfiniBand, and TIMELY and DCQCN.