[petsc-dev] Getting "from" and "to" IS from a VecScatter

[Blaise Bourdin]

Hi,

I want to change the way I handle boundary conditions in my finite elements code:
Right now, I skip the rows and columns during the matrices assembly, using a flag (a SectionInt). Using an IS denoting all dof with a BC and MatZeroRowsColumns, MatZeroRowsColumnsIS or MatZeroRowsColumnsLocal would make much more sense.

The difficulty comes up when applying a Dirichlet boundary condition on only one of the dof associated with a vertex (roller boundary conditions in elasticity, for instance). What I can easily get from Sieve is the point number associated with a vertex, for instance, from which I can obtain the _local_ index of the dof associated with a given component of a field at this point. For vectors, I can then set values on the local Vec or the Section. For matrices, though I can't just use MatGetLocalSubMatrix since I need also to modify the out of diagonal block terms.

I guess I need the LocalToGlobalMapping or to renumber the IS describing my BC and convert it to global indices. I am sure that this information is somewhere in Sieve (in the overlap, I guess), but I was thinking that since getting a Local to Global scatter is trivial, there may be a way to obtain the mapping or renumbering the IS from the scatter.

Now that I think about it more, the right way is most likely to get the information I need from Sieve. 

Blaise 


On Jun 20, 2012, at 7:23 PM, Barry Smith wrote:

> 
>  Why do you need them? Maybe there is a better way?
> 
> 
> On Jun 20, 2012, at 6:19 PM, Blaise Bourdin wrote:
> 
>> Hi,
>> 
>> I need to reconstruct the IS that were used to build a VecScatter between a local and a global Vec. 
>> 
>> From what I see in VecScatterView_MPI (vpscat.c:13), it looks like I can get the indices from from->local.vslots[i],to->local.vslots[i], i=0 .. i<to->local.n where from and to are obtained with
>> VecScatter_MPI_General *to=(VecScatter_MPI_General*)ctx->todata;
>> VecScatter_MPI_General *from=(VecScatter_MPI_General*)ctx->fromdata;
>> 
>> From there, constructing the IS is trivial. Is there a way that does not involve poking inside the VecScatter data structure?
>> 
>> Blaise
>> 
>> 
>> -- 
>> Department of Mathematics and Center for Computation & Technology
>> Louisiana State University, Baton Rouge, LA 70803, USA
>> Tel. +1 (225) 578 1612, Fax  +1 (225) 578 4276 http://www.math.lsu.edu/~bourdin
>> 
>> 
>> 
>> 
>> 
>> 
>> 
> 

-- 
Department of Mathematics and Center for Computation & Technology
Louisiana State University, Baton Rouge, LA 70803, USA
Tel. +1 (225) 578 1612, Fax  +1 (225) 578 4276 http://www.math.lsu.edu/~bourdin