[petsc-users] MatCopy question

[Junchao Zhang]

Is it possible to set the size of the COMPACT matrix as large as the WIDE
matrix, then do MatAYPX(WIDE, 0, COMPACT, SUBSET_NONZERO_PATTERN), i.e,
WIDE = 0*WIDE + COMPACT?  You need to build WIDE upfront once, but if you
repeat, the other MatAYPX will be fast.

--Junchao Zhang


On Thu, Sep 26, 2024 at 1:13 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>   Frank,
>
>    Sorry for the confusion. MatCopy() and MatAXPY() only work for
> identically sized matrices.
>
>    Even if the two matrices are the same size but one, say A, has a subset
> of nonzeros than the other, say B, what you want to do is non-trivial. This
> is because to do a copy you must MatAssembly the matrix first and this
> squeezes out the zero locations that you would like to fill in B.
>
>    A simple solution is in the code that computes the subset of entries,
> call MatSetValues() twice, once for A and once for B. Not entirely
> satisfying but it will work and be reasonably efficient.
>
>
>   Barry
>
>
> > On Sep 26, 2024, at 1:50 PM, Frank Bramkamp <bramkamp at nsc.liu.se> wrote:
> >
> > Dear PETSc team,
> >
> >
> > I would like to create two matrices. The first matrix is based on a
> compact stencil (COMPACT), and the second one has an extended stencil (WIDE)
> > So the non-zero elements of the  COMPACT matrix is contained in the WIDE
> matrix as well. But the WIDE matrix shall contain some additional elements.
> > I try to create the COMPACT matrix first, then copy it to the WIDE
> matrix and then add some more terms to the WIDE matrix.
> > We want to create a matrix that is based on a lower order approximation
> of our scheme which serves as basis matrix for preconditioning,
> > where we want to have a more accurate jacobian which has a larger
> stencil, that is based on the lower approximation plus some additional
> non-zero elements.
> > We want to avoid that we have to call the same routines twice to setup
> the more accurate matrix, but copy the elements from the lower order
> (compact) matrix.
> >
> >
> > I try to use MatCopy to copy the COMPACT matrix into the WIDE matrix.
> Somehow I cannot figure out to do this,
> > since there is always a problem wirh non conforming sizes
> >
> > [0]PETSC ERROR: Nonconforming object sizes
> > [0]PETSC ERROR: Mat A,Mat B: global dim (4,8) (4,8)
> >
> >
> > Here is a basic example that I try.
> > Adding additional elements to the WIDE matrix is disabled right now,
> since MatCopy already gives a problem.
> > I thought that in MatCopy(COMPACT, WIDE, DIFFERENT_NONZERO_PATTERN, ierr)
> > the option DIFFERENT_NONZERO_PATTERN should enable to copy different
> matrix sizes into each other.
> > Or is that not possible to do ?!
> >
> > Here the COMPACT matrix is a 4x4 matrix and the WIDE matrix 8x8, for a
> basic test.
> >
> > Thanks, Frank Bramkamp
> >
> >
> >
> >
> > program petsc_matrix_example
> > #include <petsc/finclude/petsc.h>
> >  use petsc
> >  implicit none
> >
> >  PetscErrorCode :: ierr
> >  Mat :: COMPACT, WIDE
> >  PetscInt :: m, n, MM, NN, d_nz
> >  PetscInt :: i, j
> >  PetscScalar :: additional_value
> >
> > ! Initialize PETSc
> >  call PetscInitialize(PETSC_NULL_CHARACTER, ierr)
> >
> > ! Set dimensions for COMPACT matrix
> >  m = 4  ! Local rows
> >  n = 4  ! Local columns
> >  MM = 4  ! Global rows
> >  NN = 4  ! Global columns
> >  d_nz = 4 ! Estimated non-zeros per row (diagonal) o_nz = 0 ! Estimated
> non-zeros per row (off-diagonal)
> >
> > ! Create COMPACT matrix
> >  call MatCreateSeqAIJ(PETSC_COMM_SELF, MM, NN, PETSC_DECIDE,
> PETSC_NULL_INTEGER, COMPACT, ierr)
> >
> > ! Set some values in COMPACT matrix (example) do i = 0, M-1
> >  do i = 0, NN-1
> >     !do j = max(0, i-1), min(NN-1, i+1)
> >     do j = 0,NN-1
> >      call MatSetValue(COMPACT, i, j, 1.0_PETSC_REAL_KIND, INSERT_VALUES,
> ierr)
> >    end do
> >  end do
> >
> > ! Assemble COMPACT matrix
> >  call MatAssemblyBegin(COMPACT, MAT_FINAL_ASSEMBLY, ierr)
> >  call MatAssemblyEnd(COMPACT, MAT_FINAL_ASSEMBLY, ierr)
> >
> > ! Set dimensions for WIDE matrix
> >  n = 8  ! Increase local columns
> >  NN = 8  ! Increase global columns
> >  m  = 8
> >  MM = 8
> >  d_nz = 8
> >
> > ! Create WIDE matrix
> >  call MatCreateSeqAIJ(PETSC_COMM_SELF, MM, NN, PETSC_DECIDE,
> PETSC_NULL_INTEGER, WIDE, ierr)
> >
> >  ! Assemble WIDE matrix: MUST WE ASSMBLE THE MATRIX BEFORE MatCOPY ?!
> >  call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr)
> >  call MatAssemblyEnd(WIDE, MAT_FINAL_ASSEMBLY, ierr)
> >
> > ! Copy elements from COMPACT to WIDE
> >  call MatCopy(COMPACT, WIDE, DIFFERENT_NONZERO_PATTERN, ierr)
> >
> >
> >
> >  !call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr)
> >
> >  ! Add additional elements to WIDE matrix
> > !!$  additional_value = 10.0
> > !!$
> > !!$  ! original
> > !!$  !do i = 0, MM-1
> > !!$  !  do j = NN/2, NN-1
> > !!$  !    call MatSetValue(WIDE, i, j, additional_value, INSERT_VALUES,
> ierr)
> > !!$  !  end do
> > !!$ !end do
> > !!$
> > !!$  do i = 0, NN-1
> > !!$    do j = max(0, i-1), min(NN-1, i+1)
> > !!$      call MatSetValue(WIDE, i, j, 10.0_PETSC_REAL_KIND, ADD_VALUES,
> ierr)
> > !!$    end do
> > !!$  end do
> >
> >
> > ! Assemble WIDE matrix again
> >  call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr)
> >  call MatAssemblyEnd(WIDE, MAT_FINAL_ASSEMBLY, ierr)
> >
> > ! View matrices (optional)
> >  call PetscPrintf(PETSC_COMM_WORLD, "COMPACT matrix:\n", ierr)
> >  call MatView(COMPACT, PETSC_VIEWER_STDOUT_WORLD, ierr)
> >  call PetscPrintf(PETSC_COMM_WORLD, "WIDE matrix:\n", ierr)
> >  call MatView(WIDE, PETSC_VIEWER_STDOUT_WORLD, ierr)
> >
> > ! Clean up
> >  call MatDestroy(COMPACT, ierr)
> >  call MatDestroy(WIDE, ierr)
> >
> > ! Finalize PETSc
> >  call PetscFinalize(ierr)
> >
> > end program petsc_matrix_example
> >
> >
>
>
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