Bubble-Swap Flow Control

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Deadlock-free adaptive routing is extensively adopted in both on-chip and off-chip interconnection networks to improve communication bandwidth and reduce latency. Introducing virtual channels (VCs), also known as virtual lanes (VLs), is the mainstream technique to handle deadlocks incurred by adaptive routing, and provides VC preemption for higher priority traffic. However, existing deadlock-free flow control schemes either underutilize memory resources due to inefficient buffer management to simplify hardware implementation, or rely on complicated global coordination and synchronization with very high hardware complexity. Most hardware-friendly schemes use more VCs and memory resources to enable ease of implementation of deadlock-free flow control. In contrast, sophisticated schemes achieve deadlock freedom with minimum VC cost, even eliminating additional buffer requirement through the complicated control mechanisms. In this work, we rethink the root cause of the deadlock problem from a different perspective by considering it as a lack of credit, which makes us find an efficient solution to the deadlock problem. With minor modification of credit accumulation and return, our proposed bubble-swap flow control (BSFC) ensures atomic buffer swap between two adjacent routers only based on local credit status while making full use of the buffer space. BSFC achieves a better tradeoff between implementation complexity and memory overhead and can be easily integrated in the industrial router with no modification on buffer allocation or port arbitration. The simulation results demonstrate BSFC outperforms existing bubble-based deadlock-free methods by average 64% higher throughput. We further propose a credit reservation strategy to eliminate the escape virtual channel (VC) cost for fully adaptive routing implementation. The synthesizing results demonstrate that BSFC along with credit reservation (BSFC-CR) can reduce the area and power consumption by respectively 29% and 26% in contrast to the traditional critical bubble scheme (CBS).