[petsc-users] Output question for ex29.

[Matthew Knepley]

On Thu, Jun 9, 2011 at 6:18 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Jun 9, 2011 at 6:01 PM, <zhenglun.wei at gmail.com> wrote:
>
>> Dear Sir/Madam,
>> I'm still studying on the ex29 of /src/ksp/ksp/example/tutorial. Before I
>> met a problem on VecView_VTK in parallel computation. I'm trying to modify
>> it in order to output some data from the computation.
>>
>
Here is a better answer. If you want output, throw away this old function
which is broken, and use

  PetscViewerASCIIOpen()
  PetscViewerASCIISetFormat(PETSC_VIEWER_ASCII_VTK)
  DMView()
  VecView()
  PetscViewerDestroy()

    Thanks,

       Matt


> 1) My first questions is that what does this section do in VecView_VTK:
>>
>> 272: MPI_Comm_rank(comm, &rank);
>> 273: MPI_Comm_size(comm, &size);
>> 274: MPI_Reduce(&n, &maxn, 1, MPIU_INT, MPI_MAX, 0, comm);
>> 275: tag = ((PetscObject) viewer)->tag;
>> 276: if (!rank) {
>> 277: PetscMalloc((maxn+1) * sizeof(PetscScalar), &values);
>> 278: for(i = 0; i < n; i++) {
>> 279: PetscViewerASCIIPrintf(viewer, "%G\n", PetscRealPart(array[i]));
>> 280: }
>> 281: for(p = 1; p < size; p++) {
>> 282: MPI_Recv(values, (PetscMPIInt) n, MPIU_SCALAR, p, tag, comm,
>> &status);
>> 283: MPI_Get_count(&status, MPIU_SCALAR, &nn);
>> 284: for(i = 0; i < nn; i++) {
>> 285: PetscViewerASCIIPrintf(viewer, "%G\n", PetscRealPart(array[i]));
>> 286: }
>> 287: }
>> 288: PetscFree(values);
>> 289: } else {
>> 290: MPI_Send(array, n, MPIU_SCALAR, 0, tag, comm);
>> 291: }
>>
>> What I understand is: it gather all the data from different process in
>> parallel computation, and output it to the 'viewer'. I comment out
>> everything in VecView_VTK except this part, there is no error message coming
>> up in my parallel computation so far.
>>
>> 2) however, I really don't know how did it split the domain for parallel
>> computation. For example, if I use 4 processes, is the domain split like:
>>
>
> The DMDA describes the domain splitting.
>
>    Matt
>
>
>> a)
>> |
>> 0 | 1
>> |
>> ---------------|--------------
>> |
>> 2 | 3
>> |
>>
>> b)
>> |
>> 0 | 2
>> |
>> ---------------|--------------
>> |
>> 1 | 3
>> |
>>
>> c)
>> | | | |
>> | | | |
>> 0 | | 2 | |
>> | 1 | | 3 |
>> | | | |
>> | | | |
>>
>> d)
>> 0
>> ------------------------
>> 1
>> ------------------------
>> 2
>> ------------------------
>> 3
>>
>> thanks in advance,
>> Alan
>
>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>



-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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