[Dune] Performace of ISTL

[Andreas Dedner]

Patrick Leidenberger wrote:
> 
> And there is an other question:
> Is there no easy way, to get a global, consecutive integer id in a
> parallel DUNE code? I know, there is the global id, which I can compare
> and sort, but if I have such an integer index I can feed my problem to a
> parallel Trilinos solver in a very easy way. An other application for
> such an index will be, to write the solution from different processes in
> the same hdf5 file.
> I have a look at the global id and I saw, that it consists of 4
> integers. Is any functionality to send such a global id object via MPI?
> Then can I build my own integer index.

On a locally refined and distributed grid (like ALU) it is very time
consuming to construct a consecutive global index set. This cost
is only reasonable if e.g. an expensive linear solver has to be used
between grid refinement steps - or only globally refined grids are used.
I would suggest an additional index set.  This could be
default implemented for every grid, could have the same functionality
as normal index sets and does not require any impl. aspects of the
underlying grids, i.e., does not require a lot of dune knowhow -
more mpi knowhow seems required....
In fact the normal cons. index set can be used and only the offsets
e.g. grid sizes need to be communicated through mpi -
the global ids are probably not required...

My thoughts
Andreas