## 4 Performance Evaluation and Comparative Analysis
### PF-ClusterCache Performance Analysis
However, this CS capacity is small to deliver great SHR for low Zipf popularity parameter values when a large number of traffic streams are deployed. A low Zipf parameter value amounts to a broadening of the range of popularity among data content. The CS size plays an important role, as was discussed earlier and illustrated in Figure 5.  
Figure 5: SHR as s a function of the Content Store size. Figure 6: SHR as a function of the Zipf popularity parameter. Table 2 provides the server hit reduction for different freshness period values and for different numbers of traffic streams. The content freshness impacts the server hit reduction ratio as cached popular content should be retrieved directly from its producers once it becomes stale (expired freshness). This is repeated every freshness period and for every piece of cached content. As such, the server hit reduction ratio decreases as the freshness period becomes lower. However, the impact of the freshness on the SHR remains relatively limited as the number of retrieved content due to freshness expiration is very small relative to the number of requested data content by consumers connected to the same cluster. For instance, for a CS size equal to 10, we obtain an increase of only 0.014 in the SHR when increasing the freshness period from 10 s to 250 s. The largest increase in the SHR is obtained when a CS size equal to 40 is used. Figure 7 depicts the content average retrieval delay as a function of CS size for different Zipf popularity parameter values. This figure clearly shows the great efficiency of the proposed PF-ClusterCache, which requires a very short average delay to retrieve popular content. The content average delay decreases as content popularity increases thanks to the underlying LPF that keeps the the most popular content in the cache. The content average delay also decreases as the CS size increases. Recall from Figure 5 that increasing the CS size provides better SHR. The PF-ClusterCache is very efficient for IoT and delay-sensitive applications, as it requires a very small content retrieval delay for popular content, even with a small CS size. Figure 8 depicts the average hop count to retrieve popular content as a function of CS size and for different values of the Zipf parameter. Recall that content may be retrieved from its surrogate node or ultimately from its producer. The strict minimum content average hop count is obtained when the most popular content has infinite freshness (i.e., the freshness value is greater than the simulation time). In such a case, the most popular content resides in the pool of caches of the cluster and therefore the average hop count is equal to the average path length from consumers to SNCs with the same cluster. Figures 7 and 8 clearly \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Freshness** & **CS = 5** & **T CS = 10** & **CS = 20** & **T CS = 30** & **CS = 40** \\ \hline
10 & 0.7055025 & 0.7893845 & 0.8299602 & 0.8364318 & 0.8366853 \\
50 & 0.7163088 & 0.8103767 & 0.8662949 & 0.8860598 & 0.8948450 \\
100 & 0.7182633 & 0.8139454 & 0.8726653 & 0.8944247 & 0.9051110 \\
250 & 0.7192372 & 0.8161867 & 0.8770879 & 0.9012621 & 0.9134426 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Server hit reduction for different freshness values and CS sizes. Figure 7: Content average delay as a function of CS size and Zipf parameter. illustrate the great efficiency of the proposed PF-ClusterCache. With a small but sufficient CS size, the most popular content is kept near the edge of the network, thus yielding a very small average retrieval delay. Figure 9 illustrates the eviction rate as a function of CS size and for different values of the Zipf parameter. First, we observe that the eviction rate is higher for lower values of the Zipf parameter for all considered values of the CS size. Recall that a lower value of the Zipf parameter allows a broader range of popular content and therefore requires more CS space; otherwise, it leads to more evictions.