## 4 Performance Evaluation of ROMS
### Ideal Experiment: ROMS-Benchmark  
The ideal experiment consisted of an \(8192\times 1024\) grid in the global region in the range of 0\({}^{\circ}\)E \(\sim\) 360\({}^{\circ}\)E longitude and 70\({}^{\circ}\)S \(\sim\) 70\({}^{\circ}\)N latitude (Figure 9). The grid interval of the longitude/latitude direction is approximately 2.6/8.2 minute, respectively.  
The overall depth of the model was set to 4000 m, and the depth of the Antarctic boundary was set to 500 m to represent a simple version of Antarctica. The lateral boundaries of the east and west side were set to be cyclic conditions and the vertical grid was divided into 30 layers. The initial sea surface height (SSH) and currents (U, V, W) were all set at zero, and salinity (S) was set to 35 psu. The vertical profile for temperature was set to decrease linearly from 3.5 \({}^{\circ}\)C at the surface to 0 \({}^{\circ}\)C at the bottom to represent density stratification. The time interval of model integration was 15 s.  
Table 3 shows the configuration of benchmark experiments for the allocated cores and nodes. The benchmark experiment for the ideal case is suitable for comparing the performances of computational resources due to the smaller I/O loads of data. We used 128, 256, and 512 cores to compare the performance of parallel systems according to the experimental results. In addition, a 1024 ultra-high resolution experiment was performed on the KNL system to test its performance limits.  
The results of each experiment are summarized in Figure 10. The KNL system performed at an overall rate of approximately 47% compared to the performances of SKL and CLS. This performance is slightly higher than the result (approximately three times) shown in the white paper [2] described in \"Chapter 1\".  
The 1024 cores experiment shows a continuous increase in the parallel performance of the KNL system. The CLS system shows a stable improvement in the performance depending on the number of cores used compared to the SKL system. The CLS system can run the model exclusively and consists of a simple network of 16 nodes, whereas the SKL system consists of a complex network of 132 nodes with many users.  
The KNL system's performance was also analyzed according to the number of cores. Figure 11 shows the differences in performance according to the number of cores, with 8, 16, 32, and 64 cores per node. In the 128 cores experiment (red line), the case using only 8 or 16 cores per node shows the highest performance, and increasing the number of cores per node while reducing the number of nodes gradually decreased the performance. However, the performance difference was small enough to  
Figure 9: Ultra-high resolution experimental design using ROMS for an ideal case.  
ignore when the total number of cores was more than 256. This means that the most economical method is to use all the cores of the node and reduce the number of nodes for an ultra-high parallel model.  
Figure 11: ROMS benchmark performances using 8, 16, 32 and 64 cores per node while reducing node usage in the KNL system.  
Figure 10: Performances of the benchmark experiment by the number of cores using ROMS: (SKL+CLS)/(2*KNL) = 47%.