[dune-pdelab] Matrix setup costs with StationaryLinearProblemSolver

[Christian Engwer]

Hi Eike,


> This reports matrix setup, matrix assembly and residual assembly times, which to me appear to be quite high (actually they are the dominant costs in my calculation).
> I am only using a 7 point stencil finite volume discretisation and the problem I am solving is always linear, so I'm wondering whether StationaryLinearProblemSolver produces a huge overhead in this case.
> 
> Are there any cheaper methods for setting up the matrix, in this case?

there are two problems here, some of them are solved in the upcoming
multi-index branch of pdelab.

a) the computation of the index mapping of a gfs in pdelab is quite
   expensive. The new branch will allow for optimized implementations
   for simple grid function spaces (e.g. FV).
b) the setup of the sprasity pattern is expensive, as the way pdelab
   can collect the necessary information and the way istl created the
   matrix structure don't fit well. This could be fixed by changes to
   the BCRSMatrix.

Christian