# Performance Analysis of a 3D Wireless Massively Parallel Computer
## 3 Simulation Results
### Link Utilisation Analysis
#### 3.2.3 Simulation Results and Overhead Effects  
The performance of the ABC can be affected by the task model and its packet-switching algorithm among other factors. To study the possible role the packet-switching algorithm plays in the performance, a comparison has been made between simulation results and ideal cases in which no traffic overhead exists. Anything other than real data (transferred once) is regarded as overhead. This includes the packet header, network control commands, ACK packets and packet retransmissions.  
Figure 15 makes this comparison in ABC platform (with 1000 and 2000 nodes). The black plot represents the ideal situation and the dots show the actual simulation measurements. The results are expressed in milliseconds rather than simulation iterations. This is to normalise simulation timescales represented in different experiments. The results should be rearranged based on the comm/comp ratio. The figure shows that in many experiments only a small share of communication is dedicated to the transmission of real data. Although the actual simulation results are close to the ideal (non-overhead) case for some data points, for other data points the difference between the ideal case and actual results is significant. This shows that the communication protocol is not efficient in some cases. Many data points in Figure 15a,d are a lot higher than the non-overhead values. In those experiments links are busy sending a lot of overhead information instead of real data. This is not surprising because the data size in Figure 15a,d is 100 KB. FFTTM splits the original data into a fixed number of packets regardless of the size of data; therefore, in Figure 15a,d the packet size is small and consequently the size of packet overhead is comparable to real data. Also, the graphs show that the transmission overhead rises when the comm/comp ratio rises. Data points at the right side of all plots (Figure 15a-f) represent this situation.  
The same comparison is made for a direct 5D network (Figure 16). In most experiments, the gap between the ideal and actual values is even wider compared with the ABC network. This helps with identifying how much of the power consumed by the transceivers is used for real data. For most data points with both topologies and with all data sizes, the real data are responsible for just a little of the transmission power and time. A comparison between Figures 15 and 16 indicates that ABC yields even better results than 5D topology in terms of the portion of transmission dedicated to real data.  
This implies that the performance can be improved by adopting a more flexible task model and, more importantly, a more effective packet-switching mechanism. This is predictable because the overall latency in all store-and-forward packet-switching algorithms (like the one used in these experiments) increases as the number of hops increases. This suggests that a cut-through or wormhole packet-switching algorithm may have a chance to improve the performance of the network through a significant decrease in both overall transmission time and power consumption.  
Figure 14: Link wait time (OLWT in ms) for FFTTM on a 5D grid with two sizes: 1000 nodes (**a**,**c**,**e**) and 2000 nodes (**b**,**d**,**f**). Data sizes are 100 KB (**a**,**b**), 1 MB (**c**,**d**) and 10 MB (**e**,**f**). Blue, red, green, orange and black graphs represent computational ability of 0.01, 0.1, 1, 10 and 100 GIPS, respectively.