Adaptive Flow Control for Robust Performance and Energy

As chip multiprocessors scale the number of on chip cores, the superior scalability of multihop networks compared to buses and crossbars makes multihop networks the choice interconnection strategy. However, a significant part of the networks' energy is consumed in the buffers used to handle link contention via back pressured routing. Recent work proposes to apply well-known backpressure less routing techniques, which eliminate buffers, and hence buffer power(static and dynamic), at the cost of some misrouting/dropping upon link contention (misrouted/dropped flits are eventually recovered/retransmitted). At low loads, misrouting (dropping)is rare and hence backpressure less routing performs well. Unfortunately, backpressure less routers incur significant misrouting/dropping under high loads and saturate at lower throughputs than back pressured networks, resulting in poorer performance and energy. We make the key observation that because load varies significantly across applications, back pressureless and back pressured networks are not robust in performance-energy across the spectrum of high and low loads. That is, at high loads backpressure less networks suffer considerable performance and energy disadvantage compared to back pressured networks, and the energy disadvantage reverse sat low loads. To address this robustness issue, we propose a novel adaptive flow control (AFC) router which dynamically adapts between back pressured and backpressure less flow control. AFC employs three novel mechanisms, namely local contention thresholds, gossip-induced mode-switch, and lazy VCallocation. The first mechanism maximizes performance (and minimizes energy) in the common case, and the second mechanism ensures correctness in corner cases. The third mechanism exploits flit-by-flit routing in AFC's back pressured mode to simplify VC allocation and reduces the buffer requirements by a factor of two in AFC's back pressured mode. Simulations using commercial workloads and SPLASH-2 confirm AFC'srobustness by showing that AFC achieves performance and energy that are closer to that of the better of backpressure and backpressure less networks.