[Trilinos-Users] Xpetra wrappers around existing Tpetra objects

[Jonathan Hu]

Hi Martin,

     Yes, there are easier ways to use MueLu with an existing 
Tpetra::CrsMatrix.  You do not need to convert to Xpetra. Something like 
this will work in your case:

Teuchos::RCP<Tpetra::CrsMatrix<> > A;
ParameterList paramList;
//set the MueLu options
Teuchos::RCP<MueLu::TpetraOperator> M = 
MueLu::CreateTpetraPreconditioner(A, paramList);

M is a Tpetra::Operator and can be used directly with Belos.

This is described in more detail in the user's guide 
(muelu/doc/UsersGuide/mueluguide.pdf) on page 22.  Thank you for the 
feedback on the documentation ... I'll see if we can make the migration 
path clearer for first-time users.

Regards,
Jonathan

Martin Vymazal wrote on 06/03/2016 08:10 AM:
> Hello,
>
>  I have an RCP pointer to Tpetra::CrsMatrix called A and I would like 
> to pass this to MueLu Hierarchy object. It seems that the only option 
> is to wrap A, the right-hand side vector and the vector of unknows in 
> Xpetra RCP pointers. The following compiles (I found a similar code in 
> the MueLU tutorial):
>
>   Teuchos::RCP<Xpetra::CrsMatrix<>> exA = Teuchos::rcp(new 
> Xpetra::TpetraCrsMatrix<>(A));
>   Teuchos::RCP<Xpetra::CrsMatrixWrap<>> exAWrap = Teuchos::rcp(new 
> Xpetra::CrsMatrixWrap<>(exA));
>   RCP<Xpetra::Matrix<>> xpetra_A = 
> Teuchos::rcp_dynamic_cast<Xpetra::Matrix<>>(exAWrap);
>
>   MueLu::Hierarchy<Scalar, LocalOrdinal, GlobalOrdinal> H(xpetra_A);
>   H.SetDefaultVerbLevel(MueLu::Medium);
>
> Could you please confirm that the first line only creates a shallow 
> copy of A? Is there any easier/more correct/more elegant way of 
> constructing xpetra_A? I'm also stuck at this point, because now I 
> need Xpetra-style pointers to the map that was used to construct 
> (Tpetra) A in order to construct Xpetra-style right-hand side 
> multivector (or pointer to it). How do I get Xpetra vectors from 
> Tpetra vectors, given the fact that the map of A is a Tpetra map?
>
> Thank you for your help.
>
>  Martin
>
> P. S. MueLu looks interesting, but the documentation (especially the 
> Xpetra part) could be a bit more extensive ... It looks like there's a 
> lot of attention paid to users who migrate from Epetra to Tpetra, but 
> what about those, who already have codebase which is exclusively 
> Tpetra-based? Are they supposed to move everything to Xpetra in order 
> to be able to use MueLu?
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