# Extending the Performance of Hybrid NoCs beyond the Limitations of Network Heterogeneity
## 4 Evaluation
### Impact of the Proposed Router on Inhomogeneous 3D NoCs  
To evaluate the performance of the proposed bypassing technique in 3D NoCs, an extended version of \(Worm\_sim\) (here, \(Worm\_sim\) is employed to facilitate correlation with existing work on inhomogeneous 3D NoCs.), a cycle-accurate NoC simulator is used [3]. In the simulation, a fixed packet size of five flits is used. We employ a complex multimedia traffic (MMS) [55], Auto-indust and Telecom (from the E3S benchmark suite) [56] as the workload for our evaluation. The branch-and-bound [3] mapping algorithm is used to map the applications. The setup is run for a warm-up period of 2000 cycles and a simulation length of 200,000 simulation cycles. Hence, by introducing different delay models of 2D and 3D routers in the system, we have compared the average packet latency.  
For inhomogeneous 3D NoCs with bypass techniques (also known as SlideAcross), we replace the conventional 2D routers by the SlideAcross routers. Hence, packets destined for other layers could be routed to the 3D router either via the bypass links or by the proposed deadlock-free adaptive routing to get access to the destination layer. Moreover, in the destination layer, packets can either exploit the bypass links or adaptive routing depending on the traffic conditions of the network, to the destination node. For a fair comparison, the performance-efficient buffer-nearest vertical hub (Buff NVH) [39] routing algorithm is employed for routing in existing inhomogeneous 3D NoCs. The Buff NVH routing algorithm always forwards packets towards the 3D node whose path provides the maximum output channel buffer space on the current core and has the closest Cartesian \(x\),\(y\) to the current core, as well as the minimum Manhattan distance to the destination. We compare the proposed inhomogeneous 3D NoC with existing inhomogeneous 3D NoC architectures: Periphery, Chess, \(m\) column and half/lo. In Periphery, 3D routers are distributed along the peripherals of each layer, whereas in Chess, 3D and 2D routers are evenly distributed in each layer of the NoC by placing the 2D routers at least one hop away from a 3D router. In \(m\) column, 2D routers are placed along (n-m) number of columns in each layer with 2D routers placed on the same column number of every layer; where, \(n\) is the total number of columns in each layer and m can range between 1 and n\(-\)1. In half/lo, 2D routers placed as close to 3D routers as possible in each layer [2].  
Figure 13 shows the average packet latency of various inhomogeneous architectures under different realistic benchmarks. By bypassing the links between 2D and 3D layers, SlideAcross has reduced average hop-count with less traffic loads within the layers and exploits the performance benefits of short vertical wires for inter-layer traversal. Hence, inhomogeneous 3D NoCs with SlidAcross have much lower packet latencies compared to existing inhomogeneous architectures.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Title** & **Dynamic (mW)** & **Leakage (mW)** \\ \hline Data Flit & 1.025 & \(4.847\times 10^{-2}\) \\ Data + Lookahead Flit & 1.249 & \(5.907\times 10^{-2}\) \\ Overhead & 21.85\% & 21.87\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Lookahead signal overhead.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Title** & **Area (um\({}^{2}\))** & **Power (mW)** \\ \hline Adaptive Router & 56,896.4 & 95.6 \\ Slide Across & 59,059.8 & 96.9 \\ Bypass Overhead & 3.66\% & 1.40\% \\ \hline \hline \end{tabular}
\end{table}
Table 3: Area and power estimation.  
This is due to the extra delays introduced by the multi-hops between 2D routers and 3D routers, which reduce the performance of the NoC by increasing delays in the network, which consequently causes contention in conventional inhomogeneous 3D NoCs.