[petsc-dev] Multigrid is confusing

[Barry Smith]

On May 24, 2012, at 3:39 PM, Matthew Knepley wrote:

> On Thu, May 24, 2012 at 4:16 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> On Thu, May 24, 2012 at 3:10 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> Absolutely. And if it turns out to be too much of a pain to write and maintain such a kernel there is something wrong with our programming model and code development system. The right system should make all the complexity fall away; when the complexity becomes too much of a bear you know you have the wrong system.
> 
> So do we manage blocks using internal C++ templates, "templates in C", C generated using some other system (m4 anyone?), or something else entirely?
> 
> Yes! Finally, we acknowledge that this a problem.
> 
> 1) C++ templates are not a solution to anything. ANYTHING.
> 
> 2) I am assuming "templates in C" would work somewhat like a templating engine.
>     I tried this for the last TOMS paper with Andy. Its was just not a big payoff for
>     the work put in, and definitely did not justify incorporating another package.
> 
> 3) I prefer C generated from another system, like the one I use for FEM (which I am
>     not attached to). We will definitely need this for GPU kernels, and I am guessing
>     thread kernels if they are going to be worth something.
> 

    For this particular example (and perhaps many others for sparse matrices) it is a matter of writing "the same algorithm" for a different data structure (the split storage). Perhaps what is needed is a "sparse matrix/graph/mesh" language, for which one can write kernels/code fragments "independent of the data structure"  from which C/whatever is generated? But I don't have a clue what the language would look like.  I think a general purpose tool (for example templates) is unlikely to be useful for us.

   Barry


>     Matt
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener