LA::d::Vector<double> for component wise reordered FESystem

[ghost]

In a current application program I work with TrilinosWrappers::MPI::Vector. However in a subroutine I do a L2 projection of the gradient of the TrilinosWrappers::MPI::Vector. Therefore I looked into VectorTools::project and set up the mass matrix as MatrixFreeOperator. However, I need to transfer the TrilinosWrappers::MPI::Vector to a LinearAlgebra::distributed::Vector<double>. The problem is, that I consider a vector-valued problem and reorder the DOFs component wise. The assertion which is thrown requires locally_owned_indices.is_contiguous() == true. Is there a chance to transfer a TrilinosWrappers::MPI::Vector into a LinearAlgebra::distributed::Vector<double> overcomming this issue? If I turn of the component wise reordering it works, but this is not the way I can proceed, as I want to later develop a block-preconditioner for the problem.

Any suggestions how to proceed?

[kronbichler]

The assumption of a contiguous index range for the locally owned dofs is to render certain operations more efficient.

Whichever way we put it, I think there will be some need for adaptations in the deal.II source code to support your case. Since you mention the desire to work on a block preconditioner later, would it be an option to fill a LinearAlgebra::distributed::BlockVector instead? In that case, we would need to make sure to be able to interface with a Trilinos vector from the block vector, and probably to detect the case in the VectorTools::project function. I think most interfaces are already prepared for that, so there are probably only small adaptations necessary. Do you want to give it a try?

[ghost]

Many thanks for your suggestion! Yesterday evening I was thinking a lot on this problem and using a LinearAlgebra::distributed::BlockVector<double> was also my final thought. I will work on this as I need it for my own project_gradient function. I have also #12699 open. Should I do the major revision of VectorTools::project there or open different minor PRs for this.