[Dune-devel] The importance of MPIHelper in sequential programs

[Oliver Sander]

Dear Dune,

I often find myself in the situation of writing purely sequential Dune programs.
They use grids like UGGrid or YaspGrid, but they are not supposed to be run in
a distributed setting ever.

I nevertheless have to begin each such program with

  MPIHelper::instance(argc, argv);

This is because I do have MPI installed on my machine, and Dune has picked it up.
Next, most grids have a CollectiveCommunication object somewhere, and the constructor
of this will throw an exception if MPI_Init hasn't been called previously.

While this is easy to understand from a developer point of view, it still strikes
me as odd from the user POV.  Why should I init MPI in a program that will never
actually use it (except for deep under the hood, of course)?  On the other hand
I have to admit that I do not know an obvious solution to this problem.
Have the constructor of CollectiveCommunication call MPI_Init if necessary?
Make it fall back to a sequential implementation if the communicator size is 1?

Anyhow, I'd be curious to hear other people's opinions on this.

Cheers,
Oliver

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