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 <bsm...@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 <bramk...@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 > > > > > >
