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[Gustav Magnus Vikstrøm <gustavv@xxxxxxxxxx>]

Hi

I'm working on parallel assembly and need to check if the results are  
equal to results from sequential assembly.

From src/sandbox/pmesh/main.cpp:

Regular assembly:
    Matrix A;
    Assembler assembler(m);
    assembler.assemble(A, a, true);

Parallel assembly:
    Matrix B;
    pAssembler passembler(mesh, partitions);
    passembler.assemble(B, a, true);
    B.rename("B", "Parallel matrix B");

The sequential assembly uses the PETSc function MatCreateSeqAIJ to create  
the matrix. Parallel assembly uses PETSc MatCreateMPIAIJ to create the  
matrix. In order to compare the matrices I tried writing them to file:

//Saving sequential matrix
    File file_a("matA.m");
    file_a << A;

//Saving parallel matrix
    File file_b(appendRank("matB", "m"));
    file_b << B;

Saving the matrix B results in PETSc error:

[1]PETSC ERROR: --------------------- Error Message  
------------------------------------
[1]PETSC ERROR: Argument out of range!
[1]PETSC ERROR: Only local rows!

I guess this is because each processor has a part of the matrix. Is there  
a way to "merge" the local matrices?

- Magnus