[petsc-users] questions on BAIJ matrix

[Hong]

Xiangdong:
MatCreateMPIBAIJWithArrays() is obviously buggy, and not been tested.


> 1) In the remark of the function MatCreateMPIBAIJWithArrays, it says " bs -
> the block size, only a block size of 1 is supported". Why must the block
> size be 1? Is this a typo?
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/
> Mat/MatCreateMPIBAIJWithArrays.html
>

It seems only bs=1 was implemented. I would not trust it without a test
example.

>
> 2) In the Line 4040 of the implemention of MatCreateMPIBAIJWithArrays,
> would the matrix type be matmpibaij instead of matmpiSbaij?
>

This is an error. It should be matmpibaij.

>
> http://www.mcs.anl.gov/petsc/petsc-current/src/mat/impls/bai
> j/mpi/mpibaij.c.html#MatCreateMPIBAIJWithArrays
>
> 4031: PetscErrorCode  MatCreateMPIBAIJWithArrays(MPI_Comm comm,PetscInt
> bs,PetscInt m,PetscInt n,PetscInt M,PetscInt N,const PetscInt i[],const
> PetscInt j[],const PetscScalar a[],Mat *mat)
> 4032: {
>
> 4036:   if (i[0]) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_ARG_OUTOFRANGE,"i
> (row indices) must start with 0");
> 4037:   if (m < 0) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_ARG_OUTOFRANGE,"local
> number of rows (m) cannot be PETSC_DECIDE, or negative");
> 4038:   MatCreate(comm,mat);
> 4039:   MatSetSizes(*mat,m,n,M,N);
> 4040:   MatSetType(*mat,MATMPISBAIJ);
>

 It should be MATMPIBAIJ.

>
> 3) I want to create a petsc matrix M equivalent to the sum of two block
> csr matrix/array (M1csr, M2csr). What is the best way to achieve it? I am
> thinking of created two petsc baij matrix (M1baij and M2baij) by
> calling MatCreateMPIBAIJWithArrays twice and then call MATAXPY to get the
> sum M=M1baij + M2baij. Is there a better way to do it?
>

This is an approach. However  MatCreateMPIBAIJWithArrays() needs to be
fixed, tested and implemented with requested bs. What bs do you need?

Why not use MatCreate(), MatSetValuses() (set a block values at time) to
create two MPIBAIJ matrices, then call MATAXPY. Since petsc MPIBAIJ matrix
has different internal data structure than csr,
"The i, j, and a arrays ARE copied by MatCreateMPIBAIJWithArrays() into the
internal format used by PETSc;", so this approach would give similar
performance.

Hong
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