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An Example: 3D Poisson CG Solver|
This code uses a conjugate gradient based method to solve a poisson equation in 3-dimensional space. A first version using blocking collective communication was written in collaboration with Peter Gottschling. We added support for non-blocking collective operations with the NBC library by changing only two lines in the source code.
The parallel code uses a standard domain decomposition technique to distribute a discretized 3D space among the processes. The corresponding three-dimensional process grid is built with MPI_Cart_create. Within each iteration, a matrix vector product takes a significant fraction of the execution time which can be accelerated by non-blocking communication. Each process performs three main steps in the calculation:
Step 1 is independent of all others, which allows to perform this task between step 2 and 3. The non-blocking implementation issues the non-blocking Alltoallv as first operation, calculates the inner part, waits for the Alltoallv to complete and calculates the outer part.
A detailed description and benchmark results are available in "Optimizing a Conjugate Gradient Solver with Non-Blocking Collective Operations" .The timed source code is available: CG Solver Source Code (pre-release) - (23.89 kb)
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