Stay in your Lane: A NoC with Low-overhead Multi-packet Bypassing

NoCs are over-provisioned with large virtual channels to provide deadlock freedom and performance improvement. This use of virtual channels leads to considerable power and area overhead. In this paper, we introduce a novel flow control, called FastFlow, to enhance performance and avoid both protocol-and network-level deadlocks with an impressive reduction in number of virtual channels compared to the state-of-the-art NoCs. FastPass promotes a packet to traverse the network bufferlessly; the packet bypasses the routers to reach its destination. During the traversals, the packet is guaranteed to make forward progress every cycle. As a result, such a packet cannot be blocked by congestion nor deadlock. Promoting more packets to FastPass will provide higher throughput. To this end, FastPass allows multiple packets to be upgraded as FastPass packets simultaneously. To avoid any collision between these packets, FastPass provides multiple pre-defined non-overlapping lanes. Each lane is allowed to propagate only one FastPass packet. In a timedivision multiplexed way, each router gets a chance to upgrade its packets to the FastPass packets and then transfer them via the pre-defined non-overlapping lanes. FastPass not only provides high throughput but also resolves both protocoland network-level deadlocks. Compared to the state-of-theart, FastPass presents a 1.8× increase in throughput for synthetic traffic, 46% improvement in average packet latency for real applications, and 40% reduction in power and area consumption. * Must use multiple VNs to avoid protocol-level deadlock. ** No full path diversity within the escape VC. *** DRAIN can work with no VNs; however, it requires a large amount of buffer which is non-minimal [13]. **** Cannot support adaptive routing. ***** As the network size increases, the time for detecting/resolving deadlock increases.

• FastFlow: A new flow control scheme allowing

FastPass-Packets to make forward progress bufferlessly every cycle regardless of credit availability.

• Regular Pass: A credit-based flow control which is used when packets do not use the FastFlow control.

• Regular Packets: Packets using regular pass to make forward progress.