[Trilinos-Users] Defining bordered problems efficiently: Epetra_Import

[Daniele Avitabile]

Mike,

I am back on the problem now. I think what I really need is a mix of what
you just said.

In some instances, I need to extract the last two values of xExtMap. I'll do
this now with a Broadcast.

In some other instances, though, I need to extract from the solution all but
the last two values of xExtMap. I'll do this wit the importer, as you
explained.

Thanks a lot for your help.

Daniele


On Tue, Nov 25, 2008 at 7:55 PM, Heroux, Michael A <maherou at sandia.gov>wrote:

>  Daniele,
>
> If I understand what you are doing, then one way to eliminate the extra
> copy of x values is to use View mode with the Epetra_Vector objects.  You
> will still have overlappedXExtMap and xExtMap, and the corresponding vectors
> overlappedXExt and xExt.  Furthermore, you would create overlappedXExt in
> the standard way.  However, when you create xExt, you should pass in the
> pointer to the first value in overlappedXExt using View mode.
>
> If you do this, Epetra's import function is smart enough to detect that all
> but the last value are the same address and it will not do a redundant copy.
>
> A further optimization would be to avoid the import, since you are really
> doing a broadcast.  You could use the Broadcast() method on Epetra_Comm to
> send to all processors and pass a pointer to the end of  overlappedXExt
> vector as the target buffer for the broadcast.
>
> I hope this makes sense.
>
> Mike
>
>
>
> On 11/24/08 6:53 AM, "Daniele Avitabile" <d.avitabile at gmail.com> wrote:
>
> Hi everybody,
>
> the solution to the PDEs I am solving is stored in an Epetra_Vector x,
> based on a certain linear Epetra_Map xMap. Now, I need to add an extra ODE
> to my system, defined on the boundary. At the present stage, whenever I want
> to compute the right-hand side of my PDE, I need to access a new "bordered"
> Epetra_Vector xExt, based on a new map Epetra_Map xExtMap.
>
> The problem is that, from each processor, I need access to the last
> component of xExt, the one on the boundary.
>
> The way I am doing this is
> 1. define an Epetra_Map overlappedXExtMap. All the GID of xExtMap are also
> contained in overlappedXExtMap, plus the last component of xExt is
> replicated on all processors.
> 2. define an importer with xExtMap as source and overlappedXExtMap as
> target
> 3. each time I evaluate the right-hand side of my equation, use the
> importer.
>
> I rely on the importer massively, because I also use it for printing
> purposes: I want to separate the solution in the interior of my PDE from the
> additional scalar component on the boundary.
>
> My questions:
> a. do you have a more efficient way to do this? It seems to me that I am
> copying a lot of unnecessary data, namely all the components relative to the
> GID of xEtxMap that are also in overlappedXExtMap.
> b. how does the importer Work? Does he really copy all the elements?
>
> Thanks in advance.
>
> Daniele
>
>
>
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