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Computation of numerical method is inherently iterative: at each time step, the solution depends on the immediately preceding one.
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At each time step, Grav/MPGrav is called (several times as using Runge Kutta):
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For each particle i, one must sum over the forces due to all other particles j.
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Computation is O(N2) - potential for parallel computation in either i (Message Parallel) or j or both (Data parallel if needed).
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We will use 4th order Runge Kutta to integrate in time and the program is designed an overall routine looping over time with parallelism hidden in Grav/MPGrav routines
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We first analyse Data Parallel (starting with classic SIMD method) and then go through Message Parallel version
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