Enabling vertical wormhole switching in 3D NoC-Bus hybrid systems

In Networks-on-Chip (NoC) systems Wormhole Switching (WS) enables lower packet transmission latency and requires less silicon real estate than the Packet Switching (PS). However, enabling vertical WS in conventional 3D NoC-Bus hybrid systems requires a large amount of TSVs, which have low yield in state of the art 3D stacking technology. In this paper, we alleviate this issue by introducing a Bus Virtual Channel (VC) Allocation (BVA) mechanism, which assigns to at most one cross layer packet a free input VC in its target router before injecting it into the bus. In this way, a routing path is reserved by the head flit, and the rest of the packet flits can be WS transmitted through the vertical buses. Given that VC allocation is performed only once per packet per hop BVA can be performed in such a way that it doesn´t become a system bottleneck. We evaluated our proposal with both synthetic and real application traffics and the experimental results indicate that when vertical WS is implemented, the bus critical path length is reduced by at least 31% and the average packet transmission latency is reduced by at least 22%, when compared with conventional pipelined bus or TDMA bus based systems. Moreover, the area cost and power consumption of the output buffer incident to the bus are reduced by 47% and 43%, respectively.