[petsc-users] IS Invert Not Permutation

[Florian Lindner]

Hey,

thanks for your quick reply!

As far as I understand how MatSetLocalToGlobalMapping works, there is no way to create such a mapping that can be used for MatSetValuesLocal?

What I need is basically

MatSetValuesLocal row/col argument = 3 / 4 / 6 maps to local row/col 0 / 1 / 2

I seems to me, that if I set an index set with MatSetLocalToGlobalMapping like {3, 4, 6} it does the opposite. i.e. maps local {0, 1, 2] to global {3, 4, 6}

Therefore I probably need to create my own translation of {3, 4, 6} to {0, 1, 2}

A first sketch looks like that

mapping[rank] = {3, 4, 6}

for (auto i : mapping[rank]) {
    for (auto j : mapping[rank]) {
      cout << "Setting " << i << ", " << j << endl;
      PetscScalar v[] = {i*10.0 + j};
      PetscInt rows[] = {i};
      PetscInt cols[] = {j};
      AOApplicationToPetsc(ao, 1, rows);
      AOApplicationToPetsc(ao, 1, cols);
      cout << "Really setting " << rows[0] << ", " << cols[0] << endl << endl;
      ierr = MatSetValues(matrix, 1, rows, 1, cols, v, INSERT_VALUES); CHKERRQ(ierr);
    }
  }

which seems to work so far.

Any comments from your side are appreciated!

Of course, I should set as much values as possible using MatSetValues...

Best,
Florian


Am 27.11.18 um 16:33 schrieb Matthew Knepley:
> On Tue, Nov 27, 2018 at 10:17 AM Florian Lindner via petsc-users <petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov>> wrote:
> 
>     Hello,
> 
>     I have a range of local input data indices that I want to use for row indexing, say { 3, 4, 6}.
> 
>     For that, I create a matrix with a local number of rows of 3 and map the indices {3, 4, 5} to these rows.
> 
> 
> It seems like you are trying to create a mapping and its inverse. We do that in 
> 
>   https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/AO/AOCreateMapping.html
> 
> Note that this is not really scalable. Usually there is a way to restructure the code to avoid this.
> 
>   Thanks,
> 
>     Matt
>  
> 
>     I create an index set:
> 
>     ISCreateGeneral(comm, myIndizes.size(), myIndizes.data(), PETSC_COPY_VALUES, &ISlocal);
>     ISSetPermutation(ISlocal);
>     ISInvertPermutation(ISlocal, myIndizes.size(), &ISlocalInv);
>     ISAllGather(ISlocalInv, &ISglobal); // Gather the IS from all processors
>     ISLocalToGlobalMappingCreateIS(ISglobal, &ISmapping); // Make it a mapping
> 
>     MatSetLocalToGlobalMapping(matrix, ISmapping, ISmapping); // Set mapping for rows and cols
> 
>     The InvertPermutation is required because, well, it seems that the direction the mapping stuff works in PETSc.
> 
>     Now I can use MatSetValues local to set rows {3, 4, 5} and actually set the local rows {1, 2, 3}.
> 
>     The problem is that the range of data vertices is not always contiguous, i.e. could be {3, 4, 6}. This seems to be not a permutation of PETSc, therefore ISSetPermutation fails and subsequently ISInvertPermutation.
> 
>     How can I still create such a mapping, so that:
> 
>     3 -> 1
>     4 -> 2
>     6 -> 6
> 
>     row 5 is unassigned and will never be addressed.
> 
>     Thanks!
>     Florian
> 
> 
> 
> 
> -- 
> 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
> 
> https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>