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Figure 4: MPI results on Cluster 2. The whole execution time is depicted with a blue line; the time required for data communication is indicated as MPI and depicted with a red line; the stencil part of the solver is indicated as Boltzmann and depicted with a black line. When this figure is compared with Figure 3, InfiniBand proved to be crucial. Its absence meant that more time was spent on data communication rather than doing calculations.