[petsc-users] virtual nodes at processes boundary

[praveen kumar]

I didn't frame the question properly. Suppose we have grid on vertices
numbered 1 to N and we break it into two pieces (1,N/2) and (N/2+1,N). As
it is FVM code, DMboundary type is  DM_BOUNDARY_GHOSTED. nodes 0 and N+1
are to the left and right of nodes 1 and N at a distance of dx/2
respectively.  Let me call 0 and N+1 as virtual nodes where boundary
conditions are applied. As you know virtual nodes don't take part in
computation and are different from what we call ghost nodes in parallel
computing terminology. In serial code problem is solved by CALL TDMA(0,N+1).
I've decomposed the domain using PETSc, and I've replaced indices in serial
code DO Loops with the information from DMDAGetCorners .
if I want to solve the problem using TDMA on process0, it is not possible
as process0 doesn't contain virtual node at it's right boundary i.e CALL
TDMA(0,X) where X should be at a distance of dx/2 from N/2 but it is not
there. Similarly for process1 there is no virtual node at it's left
boundary. So, how can I create these virtual nodes at the processes
boundary. I want to set the variable value at X as previous
time-step/iteration value. I'm not sure whether my methodology is correct
or not. If you think it is very cumbersome, please suggest something else.

Thanks,
Praveen

On Fri, May 6, 2016 at 8:00 PM, praveen kumar <praveenpetsc at gmail.com>
wrote:

> Thanks Matt , Thanks Barry. I'll get back to you.
>
> Thanks,
> Praveen
>
> On Fri, May 6, 2016 at 7:48 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>> > On May 6, 2016, at 5:08 AM, praveen kumar <praveenpetsc at gmail.com>
>> wrote:
>> >
>> > Hi,
>> >
>> > I am trying to implement Petsc for DD in a serial fortran FVM code.  I
>> want to use solver from serial code itself. Solver consists of gauss seidel
>> + TDMA. BCs are given along with the solver at boundary virtual nodes. For
>> Ex: CALL TDMA(0,nx+1), where BCs are given at 0 and nx+1 which are virtual
>> nodes (which don't take part in computation). I partitioned the domain
>> using DMDACreate and got the ghost nodes information using DMDAGetcorners.
>> But how to create the virtual nodes at the processes boundary where BCs are
>> to be given. Please suggest all the possibilities to fix this other than
>> using PETSc for solver parallelization.
>>
>>    DMCreateGlobalVector(dm,gvector,ierr);
>>    DMCreateLocalVector(dm,lvector,ierr);
>>
>>     /* full up gvector with initial guess or whatever */
>>
>>    DMGlobalToLocalBegin(dm,gvector,INSERT_VALUES,lvector,ierr)
>>   DMGlobalToLocalEnd(dm,gvector,INSERT_VALUES,lvector,ierr)
>>
>>     Now the vector lvector has the ghost values you can use
>>
>>     DMDAVecGetArrayF90(dm,lvector,fortran_array_pointer_of_correct
>> dimension for your problem (1,2,3d))
>>
>>     Note that the indexing into the fortran_array_pointer uses the global
>> indexing, not the local indexing. You can use DMDAGetCorners() to get the
>> start and end indices for each process.
>>
>>    Barry
>>
>>
>>
>> >
>> > Thanks,
>> > Praveen
>> >
>>
>>
>
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