[Trilinos-Users] Epetra_FECrsGraph question

[Williams, Alan B]

Hello,

When NumIndicesPerRow is not set at construction time, the crsgraph does memory allocations each time InsertGlobalIndices is called. That is probably the cause of your slow performance.

We have finite-element applications which use CrsGraph and CrsMatrix, and the typical execution sequence is to first perform an initialization step where the matrix-graph structure is computed and the CrsGraph and CrsMatrix objects are constructed, and then repeatedly fill the linear system and solve it. (In a typical application there are many fill/solve steps per initialization step.) This produces good performance.
If the structure of the matrix changes, due to h-adaptivity or load-balancing etc., then a new initialization step must be performed before new fill/solve steps are performed.

Alan


> -----Original Message-----
> From: trilinos-users-bounces at software.sandia.gov
> [mailto:trilinos-users-bounces at software.sandia.gov] On Behalf
> Of Kent-Andre Mardal
> Sent: Monday, August 11, 2008 6:45 AM
> To: trilinos-users at software.sandia.gov
> Subject: [Trilinos-Users] Epetra_FECrsGraph question
>
> Hi,
>
>
> Lately we have been doing a set of benchmarks for comparing the
> efficiency of the (finite element) assembly routine in Dolfin with
> respect to its different backends, MTL, PETSC, uBlas, and Epetra.
> The benchmark results can be found here:
> http://www.fenics.org/wiki/Benchmark
>
> It turns out that for equations using vector elements, Epetra is
> somewhat slower than the other backends,  and most of this is
> due to the
> creation of the Epetra_FECrsGraph.
>
> We have implemented it roughly as follows:
>
> Epetra_Map row_map(dims[0], 0, Comm);
> Epetra_FECrsGraph* epetra_graph;
> epetra_graph = new Epetra_FECrsGraph(Copy, row_map, 0);
> int numRows, numCols;
> int *rows;
> int *cols;
> for each element:
>     epetra_graph->InsertGlobalIndices(numRows, rows, numCols, cols);
>
> I assume that the problem here is that NumIndicesPerRow is not set
> appropriately.
> However, this code is 'library code' and we do not know (with the
> current design in Dolfin) the number of indices per row when creating
> the Epetra_FECrsGraph.
>
> Do you know any ways to speed up the code (without knowing
> NumIndicesPerRow) ?
>
> Kent-Andre Mardal
> Simula Research Laboratory
>
>
>
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