# Router Activation Heuristics for Energy-Saving ECMP and Valiant Routing in Data Center Networks
## 6 Simulation Experiments and Performance Evaluation
### Valiant Routing and Pivot Selection Heuristic  
The ECMP routing with 40% device activation threshold was used as a reference point defining 100% energy consumption level to test the efficiency of Valiant routing and the heuristic for pivot selection (Figure 7). Using data centers' load statistics, the load threshold was chosen and included overhead to handle predictable traffic changes. The Valiant routing scenarios evaluated in experiments presented in this section use two heuristics:  
* Node activation heuristic with 40% device activation threshold;
* Pivot selection heuristic with average port utilization threshold varying from 0% (i.e., standard Valiant pivot selection) to 100%.  
Valiant routing without pivot selection heuristics significantly increased the power consumption (Figure 7). This is due to the need to activate some previously deactivated nodes as pivots. Using heuristics made it possible to reduce the need to activate new switches during pivot selection. In the case of the fat-tree topology, it allowed reducing the cost of device activation to a level close to the ECMP routing. However, data transfer energy costs remained at a higher level, as Valiant routing uses longer paths than in the case of ECMP routing.  
Beneficially, the heuristics did not significantly influence the standard deviations of active devices' port load (see Figure 8). The port load standard deviation decreased for a butterfly topology, while for the fat-tree topology it increased. Such a result suggests that restricting the set of available pivots did not result in visible traffic load imbalance.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Topology** & **Device Delay (ns)** & **Power on Cost (\(\mu\)J)** & **Transmission Cost (\(\mu\)J)** \\ \hline \multirow{3}{*}{Fat-tree} & 0 & 140,000,000 & 2,164,548 \\  & 7000 & 99,921,125 & 2,166,556 \\  & 70,000 & 101,036,774 & 2,167,525 \\ \hline \multirow{3}{*}{ Butterfly} & 0 & 224,000,000 & 2,794,195 \\  & 7000 & 155,758,495 & 2,794,737 \\ \cline{1-1}  & 70,000 & 155,618,216 & 1,794,708 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Results of node deactivation experiments.  
Figure 7: Energy cost of Valiant routing for various port utilization threshold with reference to ECMP.  
The improvement in the performance of the butterfly topology is attained at the cost of the increased average port load, which may eventually lead to data loss in case of a sudden burst of traffic. It must be noted, however, that the observed average port load (around 40% in Figure 8) is consistent with the used device activation threshold and cannot be considered excessive.  
To some extent, this result may be mitigated by using a restricted pivot set. As can be observed in Figure 9 for the fat-tree topology, the cost of activating the switches did not change, but the transmission costs were significantly lower. Specifics of the fat-tree topology cause such behavior. In fact, most (or even all) switches must be actively reducing the possibility of disabling any of them for intensive load scenarios. However, some paths may be shorter when the selected pivot belongs to lower layers, reducing energy consumption. For the butterfly topology, narrowing the search depth to determine the pivots enabled both reducing energy costs associated with data transmission and switching on devices. This is a natural consequence of higher redundancy. The choice of pivots located closer to the servers contributes to shorter paths to such an extent that it is possible to use fewer nodes. Essentially, reducing pivot search depth allows for improving traffic distribution. As can be seen in Figure 10, both the average port load and its standard deviation are slightly lower for depths between 2 and 3 as the result of shortening some of the excessively long transmission paths.  
Figure 8: Reliability indicators of Valiant routing for various port utilization threshold regarding ECMP.  
Figure 9: Energy cost of Valiant routing with reduced pivot search depth.