Boomerang: Reducing Power Consumption of Response Packets in NoCs with Minimal Performance Impact

Most power reduction mechanisms for NoC channel buffers rely on on-demand wakeup to transition from a low-power state to the active state. Two drawbacks of on-demand wakeup limit its effectiveness: 1) performance impact caused by wakeup delays, and 2) energy and area cost of sleep circuitry itself. What makes the problem harder to solve is that solutions to either problem tend to exacerbate the other. For example, faster wakeup from a power-gated state requires greater charge/discharge current for the sleep transistors while using nimbler sleep transistors implies long wakeup delays. As a result, power downs have to be conservatively prescribed, missing many power-saving opportunities. We propose Boomerang, a novel power-saving method that overcomes the above drawbacks. Specifically, based on the observation that a response is always preceded by a request, we let the request trigger wakeup of the buffer that is to be used by its response in the (near) future, instead of using on-demand wakeups. Hiding the wakeup delay completely, Boomerang allows us to employ aggressive sleep policies and use low-cost power gating circuits on response buffers.