[petsc-users] SNES solve residual vector - Fortran

[Khaled Nabil Shar Abdelaziz]

Hello everyone,
I am running into an issue with SNES solver where the residual in the first iteration gets calculated fine, but on the 2nd iteration, even though it is getting calculated locally, it doesn't seem to be "assigned" to the residual vector correctly.
The residual vector here is referred to as petsc_PVector which is defined in a type (struct). It seems like the first iteration, it behaves as expected and that petsc_PVector gets updated, 2nd iteration however seems like there are 2 versions of it; one that is being passed into FormFunctionU, and another that exists in the type (struct). This result in the residual on the 2nd iteration to be exactly = 0 and convergence.

Any insights, suggestions, or experiences you can share regarding this issue would be immensely appreciated. I'm particularly interested in understanding why the residual vector behaves differently in the second iteration.
Best,
Khaled

The following is a snapshot of how I initialize and use SNES:

    subroutine set_function_jacobian_to_solver_u(nlU)
        type(variable_node_u),target,intent(inout)::nlU

        PetscErrorCode::ierr
        type(variable_node_part),pointer::var_nd_part
        integer::ipart
        ! external FormFunctionU,FormJacobianU

        do ipart=1,nlU%variable_node%npart
            var_nd_part=>nlU%variable_node%parts(ipart)
            PetscCallA(SNESSetFunction(var_nd_part%petsc_snes,var_nd_part%petsc_PVector,FormFunctionU,nlU,ierr))
            PetscCallA(SNESSetJacobian(var_nd_part%petsc_snes,var_nd_part%petsc_KMatrix,var_nd_part%petsc_KMatrix,FormJacobianU,nlU,ierr))
            PetscCallA(SNESSetFromOptions(var_nd_part%petsc_snes,ierr))
        end do
        var_nd_part=>null()
    end subroutine set_function_jacobian_to_solver_u

    subroutine FormFunctionU(petsc_snes,petsc_vector,petsc_PVector,nlU,ierr)
        type(tSNES)::petsc_snes
        type(tVec)::petsc_vector
        type(tVec)::petsc_PVector
        type(PetscScalar),pointer::vec_ptr_a(:),vec_ptr_b(:)
        PetscErrorCode::ierr
        type(variable_node_u),target::nlU
        type(PetscScalar),pointer::val_petsc_PVector(:)
        type(parameter),pointer::para_data
        type(mesh),pointer::mesh_data
        type(element),pointer::eles(:)
        integer::k,i,flag,pid
        PetscInt::its
        para_data=>nlU%para_data
        mesh_data=>nlU%mesh_data
        eles=>nlU%eles
        call MPI_Comm_rank(MPI_COMM_WORLD, pid, ierr)

        call update_data_by_petsc_vector(nlU%variable_node,petsc_vector)
        call set_petsc_PVector_to_zero(nlU%variable_node)
        call update_petsc_PVector(nlU%variable_node)
        flag = 0
        do k=1,mesh_data%nel
            call get_element_value(nlU%data_e, eles(k)%nnd_e, mesh_data%elements(:,k), nlU%nvar_total, nlU%data)
            call cal_element_ResidualU(nlU%PVector_e, eles(k), nlU%data_e, para_data)
            call set_petsc_PVector_values_by_element(nlU%variable_node, nlU%PVector_e, eles(k)%nnd_e, mesh_data%elements(:,k))
        end do

        call update_petsc_PVector(nlU%variable_node)
        para_data=>null()
        mesh_data=>null()
        eles=>null()
        return
    end subroutine FormFunctionU


                call SNESSolve(nlU%variable_node%petsc_snes,PETSC_NULL_VEC, nlU%variable_node%petsc_vector,error_petsc)

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