[Dune] Setting and optimising AMG parameters in ISTL

[Markus Blatt]

> >> As far as I can see the coarsen criterion is only used in the
> >> apply() method of the class ISTLBackend_AMG in backend.hh, where the
> >> AMG object is instantiated, so I guess I could always derive a class
> >> from ISTLBackend_AMG and overload the apply() method with the
> >> UnSymmetricCriterion (and then also derive ISTLBackend_AMG_SSOR
> >> etc. from this). Or even hack the ISTL_Backend_AMG class in my Dune
> >> installation... 
> > 
> > Yes, but you should probably use new names for ISTLBackend_AMG_SSOR,
> > etc. 

On second thought deriving and implementing new classes seems like
overkill to me.

We could simply add a flag to the amg backend, to select the criterion
and just instiate both in apply and the select the right one using a
simple if statement:

typedef
Dune::Amg::CoarsenCriterion<Dune::Amg::SymmetricCriterion<BCRSMat,Dune::Amg::FirstDiagonal> >
    SCriterion;
  typedef
  Dune::Amg::CoarsenCriterion<Dune::Amg::UnSymmetricCriterion<BCRSMat,Dune::Amg::FirstDiagonal> >
    UCriterion;
  SCriterion criterion(params);
  UCriterion ucriterion(params);
 
  .... 

  if (reuse==false || firstapply==true){
    if(useUnSymmetric)
       amg.reset(new AMG(oop, ucriterion, smootherArgs, oocc));
    else
       amg.reset(new AMG(oop, criterion, smootherArgs, oocc));

Markus
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