[dune-pdelab] New istl MatrixBackend considered less user friendly

[Steffen Müthing]

Hi Oliver,

> Am 13.07.2015 um 11:58 schrieb Oliver Sander <sander at igpm.rwth-aachen.de>:
> 
> Dear pdelab,
> 
> I just switched from the old ISTLMatrixBackend to the new istl::BCRSMatrixBackend.
> Everything appears to work nicely, but I find the new implementation more cumbersome
> to use.
> 
> Before, I could simple instantiate GridOperator with the correct matrix
> backend type, and the corresponding object was created automatically for me.
> With the new interface, I have to create a backend object myself (that's acceptable),
> and additionally I have to give it information about the expected matrix density.
> But I am solving small problems on small grids.  Memory consumption and wall-clock
> time are not important to me.  Therefore I don't want to be forced to think about
> matrix occupation patterns.  Is it possible to make the backend object default
> constructible again (maybe choosing 3^d as a default value), so people like me
> don't have to think about issues that only affect HPC projects?

The new construction method is important not just for HPC, but also for people just using PDELab
on a small scale (laptops etc.). I don’t have the exact numbers right now (Dominic did the benchmarks
back when this was implemented), but for a simple Poisson problem with P1 elements, the new method
reduces the maximum memory consumption by a factor of 2 - 3 and can cut down the run time (of the
overall program) by an order of magnitude for larger problems. Unfortunately, it’s really not possible to
specify a sensible global default (your idea of using 3^d is e.g. way too small for non-blocked DG spaces,
where a good formula would be (average_number_of_neighbor_intersections + 1) * (k+1)^d ).

That said, we really need to document this a lot better somewhere and give people some good "cooking
recipes" for calculating these numbers. I’ll open a bug for that…

Steffen

> 
> Thanks,
> Oliver
> 
> 
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