**A Parallel Rendezvous Algorithm for Interpolation Between Multiple
Grids**

S. J. Plimpton, B. Hendrickson, J. Stewart, J Parallel and Distributed Computing, 64, 266-276 (2004).

A number of computational procedures employ multiple grids on which solutions are computed. For example, in multi-physics simulations a primary grid may be used to compute mechanical deformation of an object while a secondary grid is used for thermal conduction calculations. When modeling coupled thermo-mechanical effects, solution data must be interpolated back and forth between the grids each timestep. On a parallel machine, this grid transfer operation can be challenging if the two grids are decomposed across processors differently for reasons of computational efficiency. If the grids move or adapt separately, the complexity of the operation is compounded. In this paper, we describe two grid transfer algorithms suitable for massively parallel simulations which use multiple grids. They use a rendezvous technique wherein a third decomposition is used to search for elements in one grid that contain nodal points of the other. This has the advantage of enabling the grid transfer operation to be load-balanced separately from the remainder of the computations. The algorithms are designed for use within the multi-physics code SIERRA, an object-oriented framework developed at Sandia. Performance and scalability results are given for the grid transfer operation running on up to 1024 processors of two large parallel machines, the Intel Tflops (ASCI Red) and DEC-Alpha CPlant cluster.

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