[Trilinos-Users] optimal choice of maps for finite elements assembly and update

[Kevin Long]

Riccardo,

This is a good question whose solution needs to be documented better. Epetra 
handles it in a way that is very clean and clever, but not obvious. 

What you do is build *two* Epetra maps, one like your option 1, the other like 
your option 2. In the context of matrices, these two maps are distinguished 
explicitly: there are a pair of maps (called range/domain maps)  each 
containing only locally owned dofs (your option 1), and another pair (called 
row/column maps) containing both local and ghost dofs (your option 2). If you 
build an Epetra CRS or VBR matrix, the FillComplete() function will build the 
second map pair (row/column) for you automatically. 

So as far as matrices go, you basically do your option 1 and then option 2 
will be taken care of automatically. Vectors take a little more work. 
Suppose you have a vector containing only locally owned rows, i.e., without 
ghosts. In a nonlinear problem, you'll need to access the ghost rows when 
evaluating coefficients. You can get them by explicitly doing an import from 
the option 2 map. 

I'm not sure where to send you for an example; the examples I know of in 
packages/Sundance/src-solvers are probably pretty obscure as the Epetra code 
is wrapped in a representation-independent interface. If you want to look 
there, though, there are examples of dealing with ghosts and imports in 
packages/Sundance/src-solvers/Concrete/TSFEpetraGhostImporter.cpp
and 
packages/Sundance/src-solvers/Concrete/TSFEpetraGhostView.cpp.
Note that these are internal source files rather than examples, so they may 
not be easy to figure out, but it's better than nothing. 

Hope that helps. I'm sure Mike H or someone can point you to a more 
appropriate example in Epetra. 

Regards,

Kevin

On Sunday 26 October 2008 10:38:30 am rrossi at cimne.upc.edu wrote:
> Dear All
>
> I am trying to link my finite element code to the Trilinos and i would
> like to get some hint from the experts … on a quite standard subject:
>
> Let’s take a simple example with 6 nodes and two processors
>
> Nodes 1 2 3 are “owned” by processor 1 … but node 1 also has a copy of 4
> Nodes 4 5 6 are “owned” by processor 2 … but node 1 also has a copy of 3
>
>
> In our internal database nodes 3 and 4 exist in both domains. We wish of
> course that the nodal database is exactly replicated on the two nodes.
>
>
> Let’s suppose now I would like to define a map prior to assembling our
> Finite Element problem:
>
> We have two obvious options:
>
> OPTION1
> We define a linear map so that  the first processor owns exclusively 1 2 3
> and the second 4 5 6
>
> OPTION 2
> We define a map so that
> Node 1  has nodes 1 2 3 4
> Node 2  has nodes 3 4 5 6
>
> my problem is the following:
>
> ---- if I decide to use OPTION 1 at the moment of updating I need to
> gather the value of 4 to do the update of node 1 and of 3 to do the update
> of node 2 … how do I do this? I expect to use the “ExtractCopy” but having
> a look to the documentation it looks like it is not possible to get a non
> local entry by its GID… how should I do this? Shall I define an extra map
> and use the export operator?
>
> ---- OPTION 2 on the other hand would make the update trivial for me (I
> would have a local copy of all of the entries I need) … however … can I
> trust that the assembly and linear system solution will be done correctly?
>
>
>
> I am sure that those questions were asked once and again but I could
> figure out a clear answer through the didasko examples nor through the
> user manual…
> If someone could give me a short answer…or even a pointer to the correct
> documentation it would be of great help
>
> Thanks in advance
> Riccardo
>
>
>
>
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> Trilinos-Users at software.sandia.gov
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-- 
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Kevin Long
Associate Professor
Department of Mathematics and Statistics
Texas Tech University
Lubbock, TX 

Standardized tests are a great idea, provided 
you only have standardized students.
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