1: #include <../src/mat/impls/aij/mpi/mpiaij.h>

  3: PetscErrorCode MatGetMultiProcBlock_MPIAIJ(Mat mat, MPI_Comm subComm, MatReuse scall, Mat *subMat)
  4: {
  5:   Mat_MPIAIJ  *aij  = (Mat_MPIAIJ *)mat->data;
  6:   Mat_SeqAIJ  *aijB = (Mat_SeqAIJ *)aij->B->data;
  7:   PetscMPIInt  subCommSize, subCommRank;
  8:   PetscMPIInt *commRankMap, subRank, rank, commRank;
  9:   PetscInt    *garrayCMap, col, i, j, *nnz, newRow, newCol;

 11:   PetscFunctionBegin;
 12:   PetscCallMPI(MPI_Comm_size(subComm, &subCommSize));
 13:   PetscCallMPI(MPI_Comm_rank(subComm, &subCommRank));
 14:   PetscCallMPI(MPI_Comm_rank(PetscObjectComm((PetscObject)mat), &commRank));

 16:   /* create subMat object with the relevant layout */
 17:   if (scall == MAT_INITIAL_MATRIX) {
 18:     PetscCall(MatCreate(subComm, subMat));
 19:     PetscCall(MatSetType(*subMat, MATMPIAIJ));
 20:     PetscCall(MatSetSizes(*subMat, mat->rmap->n, mat->cmap->n, PETSC_DECIDE, PETSC_DECIDE));
 21:     PetscCall(MatSetBlockSizesFromMats(*subMat, mat, mat));

 23:     /* need to setup rmap and cmap before Preallocation */
 24:     PetscCall(PetscLayoutSetUp((*subMat)->rmap));
 25:     PetscCall(PetscLayoutSetUp((*subMat)->cmap));
 26:   }

 28:   /* create a map of comm_rank from subComm to comm - should commRankMap and garrayCMap be kept for reused? */
 29:   PetscCall(PetscMalloc1(subCommSize, &commRankMap));
 30:   PetscCallMPI(MPI_Allgather(&commRank, 1, MPI_INT, commRankMap, 1, MPI_INT, subComm));

 32:   /* Traverse garray and identify column indices [of offdiag mat] that
 33:    should be discarded. For the ones not discarded, store the newCol+1
 34:    value in garrayCMap */
 35:   PetscCall(PetscCalloc1(aij->B->cmap->n, &garrayCMap));
 36:   for (i = 0; i < aij->B->cmap->n; i++) {
 37:     col = aij->garray[i];
 38:     for (subRank = 0; subRank < subCommSize; subRank++) {
 39:       rank = commRankMap[subRank];
 40:       if ((col >= mat->cmap->range[rank]) && (col < mat->cmap->range[rank + 1])) {
 41:         garrayCMap[i] = (*subMat)->cmap->range[subRank] + col - mat->cmap->range[rank] + 1;
 42:         break;
 43:       }
 44:     }
 45:   }

 47:   if (scall == MAT_INITIAL_MATRIX) {
 48:     /* Compute preallocation for the offdiag mat */
 49:     PetscCall(PetscCalloc1(aij->B->rmap->n, &nnz));
 50:     for (i = 0; i < aij->B->rmap->n; i++) {
 51:       for (j = aijB->i[i]; j < aijB->i[i + 1]; j++) {
 52:         if (garrayCMap[aijB->j[j]]) nnz[i]++;
 53:       }
 54:     }
 55:     PetscCall(MatMPIAIJSetPreallocation(*(subMat), 0, NULL, 0, nnz));

 57:     /* reuse diag block with the new submat */
 58:     PetscCall(MatDestroy(&((Mat_MPIAIJ *)((*subMat)->data))->A));
 59:     ((Mat_MPIAIJ *)((*subMat)->data))->A = aij->A;
 60:     PetscCall(PetscObjectReference((PetscObject)aij->A));
 61:   } else if (((Mat_MPIAIJ *)(*subMat)->data)->A != aij->A) {
 62:     PetscObject obj = (PetscObject)((Mat_MPIAIJ *)((*subMat)->data))->A;
 63:     PetscCall(PetscObjectReference(obj));
 64:     ((Mat_MPIAIJ *)((*subMat)->data))->A = aij->A;
 65:     PetscCall(PetscObjectReference((PetscObject)aij->A));
 66:   }

 68:   /* Traverse aij->B and insert values into subMat */
 69:   if ((*subMat)->assembled) {
 70:     (*subMat)->was_assembled = PETSC_TRUE;
 71:     (*subMat)->assembled     = PETSC_FALSE;
 72:   }
 73:   for (i = 0; i < aij->B->rmap->n; i++) {
 74:     newRow = (*subMat)->rmap->range[subCommRank] + i;
 75:     for (j = aijB->i[i]; j < aijB->i[i + 1]; j++) {
 76:       newCol = garrayCMap[aijB->j[j]];
 77:       if (newCol) {
 78:         newCol--; /* remove the increment */
 79:         PetscCall(MatSetValues_MPIAIJ(*subMat, 1, &newRow, 1, &newCol, aijB->a + j, INSERT_VALUES));
 80:       }
 81:     }
 82:   }
 83:   PetscCall(MatAssemblyBegin(*subMat, MAT_FINAL_ASSEMBLY));
 84:   PetscCall(MatAssemblyEnd(*subMat, MAT_FINAL_ASSEMBLY));

 86:   /* deallocate temporary data */
 87:   PetscCall(PetscFree(commRankMap));
 88:   PetscCall(PetscFree(garrayCMap));
 89:   if (scall == MAT_INITIAL_MATRIX) PetscCall(PetscFree(nnz));
 90:   PetscFunctionReturn(PETSC_SUCCESS);
 91: }