# Extending the Performance of Hybrid NoCs beyond the Limitations of Network Heterogeneity
## 4 Evaluation
### Packet Delay of Synthetic Traffic  
In this section, we present the packet latency results under various synthetic traffic patterns. We use four traffic patterns: shuffle, hot-spot, transpose and bit-reversal from Noxim [54]. Table 1 shows the basic setup of these simulations. Figure 9: Bypassing rate under random traffic. Figure 10 shows the average packet latency of an \(8\times 8\) mesh network under four traffic patterns. In all four traffic patterns, at low injection rate (\(PIR=0.005\)), SlideAcross and DSR have the lowest latency due to the lowest zero-load latency. In hot-spot, transpose and bit-reversal traffic, bypassing in SlideAcross works effectively, reducing packet delay by 6.2%, 9.8% and 13.1%, respectively, at \(PIR~{}=~{}0.025\), compared to SWIFT. In shuffle traffic, SWIFT has lower latency than SlideAcross when PIR is lower than 0.04. This is because the communication distance is usually short in shuffle traffic, and hence, the bypass channels in SlideAcross are not fully utilized. The adaptiveness of the network determines packet delay at high loads and the network's saturation point. The saturation points of DSR and SWIFT are the same in all traffic patterns due to the same deterministic DoR routing algorithm employed. The baseline adaptive router, although, is slower at low PIR, but can achieve lower latency and higher throughput at high loads. SlideAcross achieves the largest network saturation point in the four synthetic traffic patterns evaluated. Although the same amount of buffer size and the same adaptive routing algorithm are employed, the saturation point of SlideAcross is 13.2%, 9.2% and 22.2% higher in shuffle, transpose and bit-reversal traffic than baseline. This demonstrates that intra-dimension bypassing does not only reduce low-load packet latency, but also improves the network's maximal throughput. Figure 11 shows the average packet latency of the \(12\times 12\) mesh network. SlideAcross benefits from longer communication distance and has lower latency than SWIFT in all four traffic patterns under low loads. SlideAcross has an average reduction of 15.6% of the latency compared to SWIFT at \(PIR=0.005\) due to the higher passing rate. Still adaptive routers (baseline and SlideAcross) achieve a higher network saturation point than DSR and SWIFT. SlideAcross also significantly improves the throughput of network in shuffle, transpose and bit-reversal traffic compared to SWIFT. In summary, Figures 9-11 reveal that the at low PIR, where the network is less congested, the bypass links are highly utilized to improve the packet latency of SlideAcross compared with SWIFT and DSR. Consequently, SlideAcross can sustain a higher saturation point as packets can move more freely in the NoC compared with existing architectures. It should be noted that if the NoC is configured with a larger packet size to buffer depth ratio, the saturation point will reduce in all cases due to high congestion probabilities. However, as shown in Figures 10 and 11, SlideAcross has much lower average packet latency and can sustain more packets with a higher network saturation point compared to SWIFT and DSR. Specifically, under wormhole switching, flits in both SlideAcross and the baseline exploit the available buffer spaces to adaptively route packets in the network to their destinations with reduced latency. However, due to the effect of the bypass paths at low loads (although the bypass rate will be reduced), SlideAcross can still sustain load with a higher injection load compared with the baseline router. \begin{table}
\begin{tabular}{c c} \hline \hline
**Mesh Size** & \(\mathbf{8\times 8}\), \(\mathbf{12\times 12}\) \\ \hline Number of virtual-channels per port & 4 \\ Escape channel & 1 \\ Buffer depth & 6 flits \\ Packet length & 2–7 flits \\ PIR (packets/cycle/core) & 0.005 to 0.1 \\ Warm up cycles & 2000 \\ Simulation cycles & 50,000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation configuration for synthetic traffic.