[petsc-dev] Multigrid is confusing

[Barry Smith]

On May 24, 2012, at 6:45 PM, Jed Brown wrote:

> On Thu, May 24, 2012 at 6:26 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> > 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.
> 
> Haven't we already identified most of these primitives? Doesn't it generally come down to selecting a (block) row (upper, lower, or the entire thing) and applying a blocked sparse-dense-{minus,plus}-dot?

   Sarcasm on --
   Yes, of course. It is essentially done, write it up and I want it on my desk by 8 AM Friday.
  -- Sarcasm off

   So what if we know the primatives. How does it give us the "language" to express these kernels and the tools to do the processing?

   Barry