[dune-pdelab] Howto updat backend?

[Steffen Müthing]

Hi Christian,

Am 18.05.2013 um 22:03 schrieb Christian Engwer:

> Hi Steffen,
> 
> could you give some hints on how to update en existing backend
> (vector) or is it best to just strt from scratch again?

I don't think updating is a very viable strategy, I'm afraid. Your best bet right
now is probably to just take the ISTLBlockVectorContainer as a template and replace
the few bits that are actually ISTL-specific (and throw out all of the helper code around the
actual class that makes up more than half of that header file). Make sure to keep the nested
View classes, those are required by the interface. The default DOF infrastructure will always
give you MultiIndices for your ContainerIndices, so even if you use flat indexing, you will need to
index into position 0 when extracting the index value to pass on to the underlying LA library.

After that, you need to write a working set of Backend descriptors (the types / objects you stuff
into the GridFunctionSpaces, like ISTLVectorBackend). What these things look like really depends
on your backend, but they have to export a typedef 'size_type' and need a public method blocked()
- that method will be called by the ordering constructor to figure out whether your vector/matrix is
blocked at the current level in the GFS hierarchy (so for simple cases, returning false is always a
good bet ;-) ). The descriptors also need to be either default-constructible, or you have to pass an
explicitly instantiated object to the GFS constructor. I can't remember off of my head whether they
need to be copy-constructible as well, but I think they don't. Finally, you need to provide a nested 
Traits struct with a static const member max_blocking_depth. As long as you don't need nested
indexing, set that constant to 0. Again, the ISTL descriptor in backend/istl/tags.hh is a good template.
The max_blocking_depth constant is used by the ordering transformation to figure out the compile-time
maximum size of the ContainerIndex.

Finally, adjust the specialization of the BackendVectorSelector if you changed the signature of
the backend descriptors. As long as you don't want to calculate the correct vector blocking structure
from the GFS tree (the way the new ISTL backend does), you shouldn't have to do anything else there.


I hope that helps a little,
Steffen

> 
> Ciao
> Christian
> 
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Steffen Müthing
Universität Stuttgart
Institut für Parallele und Verteilte Systeme
Universitätsstr. 38
70569 Stuttgart
Tel: +49 711 685 88429
Fax: +49 711 685 88340
Email: steffen.muething at ipvs.uni-stuttgart.de

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