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

[rrossi at cimne.upc.edu]

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