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

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

 13:   MPI_Comm_size(PetscObjectComm((PetscObject)mat),&commsize);
 14:   MPI_Comm_size(subComm,&subCommSize);

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

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

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

 34:   /* Traverse garray and identify column indices [of offdiag mat] that
 35:    should be discarded. For the ones not discarded, store the newCol+1
 36:    value in garrayCMap */
 37:   PetscCalloc1(aij->B->cmap->n,&garrayCMap);
 38:   for (i=0; i<aij->B->cmap->n; i++) {
 39:     col = aij->garray[i];
 40:     for (subRank=0; subRank<subCommSize; subRank++) {
 41:       rank = commRankMap[subRank];
 42:       if ((col >= mat->cmap->range[rank]) && (col < mat->cmap->range[rank+1])) {
 43:         garrayCMap[i] = (*subMat)->cmap->range[subRank] + col - mat->cmap->range[rank]+1;
 44:         break;
 45:       }
 46:     }
 47:   }

 49:   if (scall == MAT_INITIAL_MATRIX) {
 50:     /* Now compute preallocation for the offdiag mat */
 51:     PetscCalloc1(aij->B->rmap->n,&nnz);
 52:     for (i=0; i<aij->B->rmap->n; i++) {
 53:       for (j=aijB->i[i]; j<aijB->i[i+1]; j++) {
 54:         if (garrayCMap[aijB->j[j]]) nnz[i]++;
 55:       }
 56:     }
 57:     MatMPIAIJSetPreallocation(*(subMat),0,NULL,0,nnz);

 59:     /* reuse diag block with the new submat */
 60:     MatDestroy(&((Mat_MPIAIJ*)((*subMat)->data))->A);

 62:     ((Mat_MPIAIJ*)((*subMat)->data))->A = aij->A;

 64:     PetscObjectReference((PetscObject)aij->A);
 65:   } else if (((Mat_MPIAIJ*)(*subMat)->data)->A != aij->A) {
 66:     PetscObject obj = (PetscObject)((Mat_MPIAIJ*)((*subMat)->data))->A;

 68:     PetscObjectReference((PetscObject)obj);

 70:     ((Mat_MPIAIJ*)((*subMat)->data))->A = aij->A;

 72:     PetscObjectReference((PetscObject)aij->A);
 73:   }

 75:   /* Now traverse aij->B and insert values into subMat */
 76:   for (i=0; i<aij->B->rmap->n; i++) {
 77:     newRow = (*subMat)->rmap->range[subCommRank] + i;
 78:     for (j=aijB->i[i]; j<aijB->i[i+1]; j++) {
 79:       newCol = garrayCMap[aijB->j[j]];
 80:       if (newCol) {
 81:         newCol--; /* remove the increment */
 82:         MatSetValues(*subMat,1,&newRow,1,&newCol,(aijB->a+j),INSERT_VALUES);
 83:       }
 84:     }
 85:   }

 87:   /* assemble the submat */
 88:   MatAssemblyBegin(*subMat,MAT_FINAL_ASSEMBLY);
 89:   MatAssemblyEnd(*subMat,MAT_FINAL_ASSEMBLY);

 91:   /* deallocate temporary data */
 92:   PetscFree(commRankMap);
 93:   PetscFree(garrayCMap);
 94:   if (scall == MAT_INITIAL_MATRIX) {
 95:     PetscFree(nnz);
 96:   }
 97:   return(0);
 98: }