[Trilinos-Users] MueLu setting block size warning (Tpetra+Belos+MueLu)

[Prokopenko, Andrey V.]

Hi Brian,

This is a minor warning, and it does not pose a problem in this case. It 
simply indicates that after your original operator was converted to 
Xpetra object (Xpetra matrix, specifically) that matrix was still 
considered scalar on the input. You did the right thing by indicating 
the number of equations, and this warning means that from now on we are 
going to treat a scalar matrix as a block matrix.

Tobias,

We could certainly do a better job on phrasing the warning and finding a 
situation where it is necessary. When do we really want this to be a 
warning, and when a simple Runtime0? I feel like if we switch from 
scalar to block matrices through a provided "number of equations", it 
should not be a warning. But I also recall that there was history behind 
this.


-Andrey

On 4/25/17 1:49 PM, Brian Granzow wrote:
> Hello!
>
> I have an application that uses Tpetra + MueLu + Belos to solve linear
> systems obtained via finite element assembly. To create the MueLu
> preconditioner, I am using the function:
> MueLu::CreateTpetraPreconditioner:
>
> https://github.com/trilinos/Trilinos/blob/master/packages/muelu/adapters/tpetra/MueLu_CreateTpetraPreconditioner.hpp#L49
>
> I am passing this function the required Tpetra::Operator (inA) and
> input Teuchos::ParameterList (inParamList), as well as the optional
> coordinate Tpetra::MultiVector (inCoords). For a problem with 3
> degrees of freedom per node (linear elasticity), I specify the MueLu
> input parameter "number of equations" to be equal to 3, and I see the
> warning below:
>
> ```
> ******* WARNING *******
> Setting matrix block size to 3 (value of the parameter in the list)
> instead of 1 (provided matrix).
> You may want to check "number of equations" (or "PDE equations" for
> factory style list) parameter.
> ```
>
> I am curious if anyone has experienced this before, and if the fix is
> immediately obvious.
>
> ---
>
> For more detail, the Tpetra::Operator (inA) that is passed to
> MueLu::CreateTpetraPreconditioner by dynamically casting an existing
> Tpetra::CrsMatrix to a Tpetra::Operator. The coordinate
> Tpetra::MultiVector is constructed from a Tpetra::Map that describes
> the parallel local to global distribution of nodes (in this case mesh
> vertices). The distinction here being that the number of rows of the
> coordinate MultiVector is smaller than the number of rows of my
> Tpetra::CrsMatrix by a factor of 3 (since there are 3 degrees of
> freedom per node). In fact, I've used this exact same approach in a
> small application in Albany where I did not see the above warning:
>
> https://github.com/gahansen/Albany/blob/master/src/CTM/CTM_LinearSolver.cpp#L50
>
> My current application is nearly identical to this Albany application,
> so I'm unsure where I am going wrong.  I am pretty confident that the
> Tpetra::Operator is formed correctly as I am able to solve my FEM
> problem with correct results when I simply omit passing the coordinate
> vector to MueLu::CreateTpetraPreconditioner and leave the "number of
> equations" parameter as the default value.
>
> Thanks for the consideration,
> Brian Granzow
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