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Epetra Lesson 05: Redistribution

Parallel data redistribution of Epetra objects

Lesson topics

The Epetra_Lesson02_Vector example introduces and describes the Epetra_Map class, which is Epetra's representation of a data distribution. This example builds on that by showing how to use Maps and the Epetra_Export class to redistribute data. In this case, we build a sparse matrix on one MPI process, and redistribute it to a sparse matrix stored in block row fashion, with an equal number of rows per process.

Epetra data redistribution: Import and Export

The Epetra_Map class describes a data distribution over one or more distributed-memory parallel processes. It "maps" global indices (unique labels for the elements of a data structure) to parallel processes. This ability to describe a data distribution calls for a redistribution capability, that is, to reorganize or remap data from one distribution to another. Epetra provides this capability through the Epetra_Import and Epetra_Export classes.

Import redistributes from a uniquely owned (one-to-one) Map to a possibly not uniquely owned Map. Export redistributes from a possibly not uniquely owned to a uniquely owned Map. We distinguish between these cases both for historical reasons and for performance reasons.

Import and Export objects encapsulate and remember a communication pattern for reuse. Computing the computation pattern requires nontrivial work, but keeping around the Import or Export object lets you reuse that work. This is very important for operations that are performed frequently, such as the Import and Export operations in Epetra's sparse matrix-vector multiply.

In both cases, Import and Export let the user specify how to combine incoming new data with existing data that has the same global index. For example, one may replace old data with new data or sum them together.

Code example

This example shows how to migrate the data in Epetra objects (sparse matrices and vectors) between two different parallel distributions.