xref: /petsc/src/mat/graphops/coarsen/impls/misk/misk.c (revision e0b7e82fd3cf27fce84cc3e37e8d70a5c36a2d4e)

#include <petsc/private/matimpl.h> /*I "petscmat.h" I*/
#include <../src/mat/impls/aij/seq/aij.h>
#include <../src/mat/impls/aij/mpi/mpiaij.h>
#include <petscsf.h>

#define MIS_NOT_DONE       -2
#define MIS_DELETED        -1
#define MIS_REMOVED        -3
#define MIS_IS_SELECTED(s) (s >= 0)

/* edge for priority queue */
typedef struct edge_tag {
  PetscReal weight;
  PetscInt  lid0, gid1, cpid1;
} Edge;

static PetscErrorCode PetscCoarsenDataView_private(PetscCoarsenData *agg_lists, PetscViewer viewer)
{
  PetscCDIntNd *pos, *pos2;

  PetscFunctionBegin;
  for (PetscInt kk = 0; kk < agg_lists->size; kk++) {
    PetscCall(PetscCDGetHeadPos(agg_lists, kk, &pos));
    if ((pos2 = pos)) PetscCall(PetscViewerASCIISynchronizedPrintf(viewer, "selected local %d: ", (int)kk));
    while (pos) {
      PetscInt gid1;
      PetscCall(PetscCDIntNdGetID(pos, &gid1));
      PetscCall(PetscCDGetNextPos(agg_lists, kk, &pos));
      PetscCall(PetscViewerASCIISynchronizedPrintf(viewer, " %d ", (int)gid1));
    }
    if (pos2) PetscCall(PetscViewerASCIISynchronizedPrintf(viewer, "\n"));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
  MatCoarsenApply_MISK_private - parallel heavy edge matching

  Input Parameter:
   . perm - permutation
   . Gmat - global matrix of graph (data not defined)

  Output Parameter:
   . a_locals_llist - array of list of local nodes rooted at local node
*/
static PetscErrorCode MatCoarsenApply_MISK_private(IS perm, const PetscInt misk, Mat Gmat, PetscCoarsenData **a_locals_llist)
{
  PetscBool   isMPI;
  MPI_Comm    comm;
  PetscMPIInt rank, size;
  Mat         cMat, Prols[5], Rtot;
  PetscScalar one = 1;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(perm, IS_CLASSID, 1);
  PetscValidHeaderSpecific(Gmat, MAT_CLASSID, 3);
  PetscAssertPointer(a_locals_llist, 4);
  PetscCheck(misk < 5 && misk > 0, PETSC_COMM_SELF, PETSC_ERR_SUP, "too many/few levels: %d", (int)misk);
  PetscCall(PetscObjectBaseTypeCompare((PetscObject)Gmat, MATMPIAIJ, &isMPI));
  PetscCall(PetscObjectGetComm((PetscObject)Gmat, &comm));
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  PetscCallMPI(MPI_Comm_size(comm, &size));
  PetscCall(PetscInfo(Gmat, "misk %d\n", (int)misk));
  /* make a copy of the graph, this gets destroyed in iterates */
  if (misk > 1) PetscCall(MatDuplicate(Gmat, MAT_COPY_VALUES, &cMat));
  else cMat = Gmat;
  for (PetscInt iterIdx = 0; iterIdx < misk; iterIdx++) {
    Mat_SeqAIJ       *matA, *matB = NULL;
    Mat_MPIAIJ       *mpimat = NULL;
    const PetscInt   *perm_ix;
    const PetscInt    nloc_inner = cMat->rmap->n;
    PetscCoarsenData *agg_lists;
    PetscInt         *cpcol_gid = NULL, *cpcol_state, *lid_cprowID, *lid_state, *lid_parent_gid = NULL;
    PetscInt          num_fine_ghosts, kk, n, ix, j, *idx, *ai, Iend, my0, nremoved, gid, cpid, lidj, sgid, t1, t2, slid, nDone, nselected = 0, state;
    PetscBool        *lid_removed, isOK;
    PetscLayout       layout;
    PetscSF           sf;

    if (isMPI) {
      mpimat = (Mat_MPIAIJ *)cMat->data;
      matA   = (Mat_SeqAIJ *)mpimat->A->data;
      matB   = (Mat_SeqAIJ *)mpimat->B->data;
      /* force compressed storage of B */
      PetscCall(MatCheckCompressedRow(mpimat->B, matB->nonzerorowcnt, &matB->compressedrow, matB->i, cMat->rmap->n, -1.0));
    } else {
      PetscBool isAIJ;
      PetscCall(PetscObjectBaseTypeCompare((PetscObject)cMat, MATSEQAIJ, &isAIJ));
      PetscCheck(isAIJ, PETSC_COMM_SELF, PETSC_ERR_USER, "Require AIJ matrix.");
      matA = (Mat_SeqAIJ *)cMat->data;
    }
    PetscCall(MatGetOwnershipRange(cMat, &my0, &Iend));
    if (mpimat) {
      PetscInt *lid_gid;
      PetscCall(PetscMalloc1(nloc_inner, &lid_gid)); /* explicit array needed */
      for (kk = 0, gid = my0; kk < nloc_inner; kk++, gid++) lid_gid[kk] = gid;
      PetscCall(VecGetLocalSize(mpimat->lvec, &num_fine_ghosts));
      PetscCall(PetscMalloc1(num_fine_ghosts, &cpcol_gid));
      PetscCall(PetscMalloc1(num_fine_ghosts, &cpcol_state));
      PetscCall(PetscSFCreate(PetscObjectComm((PetscObject)cMat), &sf));
      PetscCall(MatGetLayouts(cMat, &layout, NULL));
      PetscCall(PetscSFSetGraphLayout(sf, layout, num_fine_ghosts, NULL, PETSC_COPY_VALUES, mpimat->garray));
      PetscCall(PetscSFBcastBegin(sf, MPIU_INT, lid_gid, cpcol_gid, MPI_REPLACE));
      PetscCall(PetscSFBcastEnd(sf, MPIU_INT, lid_gid, cpcol_gid, MPI_REPLACE));
      for (kk = 0; kk < num_fine_ghosts; kk++) cpcol_state[kk] = MIS_NOT_DONE;
      PetscCall(PetscFree(lid_gid));
    } else num_fine_ghosts = 0;

    PetscCall(PetscMalloc1(nloc_inner, &lid_cprowID));
    PetscCall(PetscMalloc1(nloc_inner, &lid_removed)); /* explicit array needed */
    PetscCall(PetscMalloc1(nloc_inner, &lid_parent_gid));
    PetscCall(PetscMalloc1(nloc_inner, &lid_state));

    /* the data structure */
    PetscCall(PetscCDCreate(nloc_inner, &agg_lists));
    /* need an inverse map - locals */
    for (kk = 0; kk < nloc_inner; kk++) {
      lid_cprowID[kk]    = -1;
      lid_removed[kk]    = PETSC_FALSE;
      lid_parent_gid[kk] = -1.0;
      lid_state[kk]      = MIS_NOT_DONE;
    }
    /* set index into cmpressed row 'lid_cprowID' */
    if (matB) {
      for (ix = 0; ix < matB->compressedrow.nrows; ix++) {
        const PetscInt lid = matB->compressedrow.rindex[ix];
        if (lid >= 0) lid_cprowID[lid] = ix;
      }
    }
    /* MIS */
    nremoved = nDone = 0;
    if (!iterIdx) PetscCall(ISGetIndices(perm, &perm_ix)); // use permutation on first MIS
    else perm_ix = NULL;
    while (nDone < nloc_inner || PETSC_TRUE) { /* asynchronous not implemented */
      /* check all vertices */
      for (kk = 0; kk < nloc_inner; kk++) {
        const PetscInt lid = perm_ix ? perm_ix[kk] : kk;
        state              = lid_state[lid];
        if (iterIdx == 0 && lid_removed[lid]) continue;
        if (state == MIS_NOT_DONE) {
          /* parallel test, delete if selected ghost */
          isOK = PETSC_TRUE;
          /* parallel test */
          if ((ix = lid_cprowID[lid]) != -1) { /* if I have any ghost neighbors */
            ai  = matB->compressedrow.i;
            n   = ai[ix + 1] - ai[ix];
            idx = matB->j + ai[ix];
            for (j = 0; j < n; j++) {
              cpid = idx[j]; /* compressed row ID in B mat */
              gid  = cpcol_gid[cpid];
              if (cpcol_state[cpid] == MIS_NOT_DONE && gid >= Iend) { /* or pe>rank */
                isOK = PETSC_FALSE;                                   /* can not delete */
                break;
              }
            }
          }
          if (isOK) { /* select or remove this vertex if it is a true singleton like a BC */
            nDone++;
            /* check for singleton */
            ai = matA->i;
            n  = ai[lid + 1] - ai[lid];
            if (n < 2) {
              /* if I have any ghost adj then not a singleton */
              ix = lid_cprowID[lid];
              if (ix == -1 || !(matB->compressedrow.i[ix + 1] - matB->compressedrow.i[ix])) {
                if (iterIdx == 0) {
                  lid_removed[lid] = PETSC_TRUE;
                  nremoved++; // let it get selected
                }
                // PetscCall(PetscCDAppendID(agg_lists, lid, lid + my0));
                // lid_state[lid] = nselected; // >= 0  is selected, cache for ordering coarse grid
                /* should select this because it is technically in the MIS but lets not */
                continue; /* one local adj (me) and no ghost - singleton */
              }
            }
            /* SELECTED state encoded with global index */
            lid_state[lid] = nselected; // >= 0  is selected, cache for ordering coarse grid
            nselected++;
            PetscCall(PetscCDAppendID(agg_lists, lid, lid + my0));
            /* delete local adj */
            idx = matA->j + ai[lid];
            for (j = 0; j < n; j++) {
              lidj = idx[j];
              if (lid_state[lidj] == MIS_NOT_DONE) {
                nDone++;
                PetscCall(PetscCDAppendID(agg_lists, lid, lidj + my0));
                lid_state[lidj] = MIS_DELETED; /* delete this */
              }
            }
          } /* selected */
        } /* not done vertex */
      } /* vertex loop */

      /* update ghost states and count todos */
      if (mpimat) {
        /* scatter states, check for done */
        PetscCall(PetscSFBcastBegin(sf, MPIU_INT, lid_state, cpcol_state, MPI_REPLACE));
        PetscCall(PetscSFBcastEnd(sf, MPIU_INT, lid_state, cpcol_state, MPI_REPLACE));
        ai = matB->compressedrow.i;
        for (ix = 0; ix < matB->compressedrow.nrows; ix++) {
          const int lidj = matB->compressedrow.rindex[ix]; /* local boundary node */
          state          = lid_state[lidj];
          if (state == MIS_NOT_DONE) {
            /* look at ghosts */
            n   = ai[ix + 1] - ai[ix];
            idx = matB->j + ai[ix];
            for (j = 0; j < n; j++) {
              cpid = idx[j];                            /* compressed row ID in B mat */
              if (MIS_IS_SELECTED(cpcol_state[cpid])) { /* lid is now deleted by ghost */
                nDone++;
                lid_state[lidj]      = MIS_DELETED; /* delete this */
                sgid                 = cpcol_gid[cpid];
                lid_parent_gid[lidj] = sgid; /* keep track of proc that I belong to */
                break;
              }
            }
          }
        }
        /* all done? */
        t1 = nloc_inner - nDone;
        PetscCall(MPIU_Allreduce(&t1, &t2, 1, MPIU_INT, MPI_SUM, comm)); /* synchronous version */
        if (!t2) break;
      } else break; /* no mpi - all done */
    } /* outer parallel MIS loop */
    if (!iterIdx) PetscCall(ISRestoreIndices(perm, &perm_ix));
    PetscCall(PetscInfo(Gmat, "\t removed %" PetscInt_FMT " of %" PetscInt_FMT " vertices.  %" PetscInt_FMT " selected.\n", nremoved, nloc_inner, nselected));

    /* tell adj who my lid_parent_gid vertices belong to - fill in agg_lists selected ghost lists */
    if (matB) {
      PetscInt *cpcol_sel_gid, *icpcol_gid;
      /* need to copy this to free buffer -- should do this globally */
      PetscCall(PetscMalloc1(num_fine_ghosts, &cpcol_sel_gid));
      PetscCall(PetscMalloc1(num_fine_ghosts, &icpcol_gid));
      for (cpid = 0; cpid < num_fine_ghosts; cpid++) icpcol_gid[cpid] = cpcol_gid[cpid];
      /* get proc of deleted ghost */
      PetscCall(PetscSFBcastBegin(sf, MPIU_INT, lid_parent_gid, cpcol_sel_gid, MPI_REPLACE));
      PetscCall(PetscSFBcastEnd(sf, MPIU_INT, lid_parent_gid, cpcol_sel_gid, MPI_REPLACE));
      for (cpid = 0; cpid < num_fine_ghosts; cpid++) {
        sgid = cpcol_sel_gid[cpid];
        gid  = icpcol_gid[cpid];
        if (sgid >= my0 && sgid < Iend) { /* I own this deleted */
          slid = sgid - my0;
          PetscCall(PetscCDAppendID(agg_lists, slid, gid));
        }
      }
      // done - cleanup
      PetscCall(PetscFree(icpcol_gid));
      PetscCall(PetscFree(cpcol_sel_gid));
      PetscCall(PetscSFDestroy(&sf));
      PetscCall(PetscFree(cpcol_gid));
      PetscCall(PetscFree(cpcol_state));
    }
    PetscCall(PetscFree(lid_cprowID));
    PetscCall(PetscFree(lid_removed));
    PetscCall(PetscFree(lid_parent_gid));
    PetscCall(PetscFree(lid_state));

    /* MIS done - make projection matrix - P */
    MatType jtype;
    PetscCall(MatGetType(Gmat, &jtype));
    PetscCall(MatCreate(comm, &Prols[iterIdx]));
    PetscCall(MatSetType(Prols[iterIdx], jtype));
    PetscCall(MatSetSizes(Prols[iterIdx], nloc_inner, nselected, PETSC_DETERMINE, PETSC_DETERMINE));
    PetscCall(MatSeqAIJSetPreallocation(Prols[iterIdx], 1, NULL));
    PetscCall(MatMPIAIJSetPreallocation(Prols[iterIdx], 1, NULL, 1, NULL));
    {
      PetscCDIntNd *pos, *pos2;
      PetscInt      colIndex, Iend, fgid;
      PetscCall(MatGetOwnershipRangeColumn(Prols[iterIdx], &colIndex, &Iend));
      // TODO - order with permutation in lid_selected (reversed)
      for (PetscInt lid = 0; lid < agg_lists->size; lid++) {
        PetscCall(PetscCDGetHeadPos(agg_lists, lid, &pos));
        pos2 = pos;
        while (pos) {
          PetscCall(PetscCDIntNdGetID(pos, &fgid));
          PetscCall(PetscCDGetNextPos(agg_lists, lid, &pos));
          PetscCall(MatSetValues(Prols[iterIdx], 1, &fgid, 1, &colIndex, &one, INSERT_VALUES));
        }
        if (pos2) colIndex++;
      }
      PetscCheck(Iend == colIndex, PETSC_COMM_SELF, PETSC_ERR_SUP, "Iend!=colIndex: %d %d", (int)Iend, (int)colIndex);
    }
    PetscCall(MatAssemblyBegin(Prols[iterIdx], MAT_FINAL_ASSEMBLY));
    PetscCall(MatAssemblyEnd(Prols[iterIdx], MAT_FINAL_ASSEMBLY));
    /* project to make new graph for next MIS, skip if last */
    if (iterIdx < misk - 1) {
      Mat new_mat;
      PetscCall(MatPtAP(cMat, Prols[iterIdx], MAT_INITIAL_MATRIX, PETSC_DEFAULT, &new_mat));
      PetscCall(MatDestroy(&cMat));
      cMat = new_mat; // next iter
    } else if (cMat != Gmat) PetscCall(MatDestroy(&cMat));
    // cleanup
    PetscCall(PetscCDDestroy(agg_lists));
  } /* MIS-k iteration */
  /* make total prolongator Rtot = P_0 * P_1 * ... */
  Rtot = Prols[misk - 1]; // compose P then transpose to get R
  for (PetscInt iterIdx = misk - 1; iterIdx > 0; iterIdx--) {
    Mat P;
    PetscCall(MatMatMult(Prols[iterIdx - 1], Rtot, MAT_INITIAL_MATRIX, PETSC_DEFAULT, &P));
    PetscCall(MatDestroy(&Prols[iterIdx - 1]));
    PetscCall(MatDestroy(&Rtot));
    Rtot = P;
  }
  PetscCall(MatTranspose(Rtot, MAT_INPLACE_MATRIX, &Rtot)); // R now
  PetscCall(MatViewFromOptions(Rtot, NULL, "-misk_aggregation_view"));
  /* make aggregates with Rtot - could use Rtot directly in theory but have to go through the aggregate list data structure */
  {
    PetscInt          Istart, Iend, ncols, NN, MM, jj = 0, max_osz = 0;
    const PetscInt    nloc = Gmat->rmap->n;
    PetscCoarsenData *agg_lists;
    Mat               mat;
    PetscCall(PetscCDCreate(nloc, &agg_lists));
    *a_locals_llist = agg_lists; // return
    PetscCall(MatGetOwnershipRange(Rtot, &Istart, &Iend));
    for (int grow = Istart, lid = 0; grow < Iend; grow++, lid++) {
      const PetscInt *idx;
      PetscCall(MatGetRow(Rtot, grow, &ncols, &idx, NULL));
      for (int jj = 0; jj < ncols; jj++) {
        PetscInt gcol = idx[jj];
        PetscCall(PetscCDAppendID(agg_lists, lid, gcol)); // local row, global column
      }
      PetscCall(MatRestoreRow(Rtot, grow, &ncols, &idx, NULL));
    }
    PetscCall(MatDestroy(&Rtot));

    /* make fake matrix, get largest nnz */
    for (int lid = 0; lid < nloc; lid++) {
      PetscCall(PetscCDCountAt(agg_lists, lid, &jj));
      if (jj > max_osz) max_osz = jj;
    }
    PetscCall(MatGetSize(Gmat, &MM, &NN));
    if (max_osz > MM - nloc) max_osz = MM - nloc;
    PetscCall(MatGetOwnershipRange(Gmat, &Istart, NULL));
    /* matrix of ghost adj for square graph */
    PetscCall(MatCreateAIJ(comm, nloc, nloc, PETSC_DETERMINE, PETSC_DETERMINE, 0, NULL, max_osz, NULL, &mat));
    for (PetscInt lid = 0, gidi = Istart; lid < nloc; lid++, gidi++) {
      PetscCDIntNd *pos;
      PetscCall(PetscCDGetHeadPos(agg_lists, lid, &pos));
      while (pos) {
        PetscInt gidj;
        PetscCall(PetscCDIntNdGetID(pos, &gidj));
        PetscCall(PetscCDGetNextPos(agg_lists, lid, &pos));
        if (gidj < Istart || gidj >= Istart + nloc) PetscCall(MatSetValues(mat, 1, &gidi, 1, &gidj, &one, ADD_VALUES));
      }
    }
    PetscCall(MatAssemblyBegin(mat, MAT_FINAL_ASSEMBLY));
    PetscCall(MatAssemblyEnd(mat, MAT_FINAL_ASSEMBLY));
    PetscCall(PetscCDSetMat(agg_lists, mat));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
   Distance k MIS. k is in 'subctx'
*/
static PetscErrorCode MatCoarsenApply_MISK(MatCoarsen coarse)
{
  Mat      mat = coarse->graph;
  PetscInt k;

  PetscFunctionBegin;
  PetscCall(MatCoarsenMISKGetDistance(coarse, &k));
  PetscCheck(k > 0, PETSC_COMM_SELF, PETSC_ERR_SUP, "too few levels: %d", (int)k);
  if (!coarse->perm) {
    IS       perm;
    PetscInt n, m;

    PetscCall(MatGetLocalSize(mat, &m, &n));
    PetscCall(ISCreateStride(PetscObjectComm((PetscObject)mat), m, 0, 1, &perm));
    PetscCall(MatCoarsenApply_MISK_private(perm, (PetscInt)k, mat, &coarse->agg_lists));
    PetscCall(ISDestroy(&perm));
  } else {
    PetscCall(MatCoarsenApply_MISK_private(coarse->perm, (PetscInt)k, mat, &coarse->agg_lists));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatCoarsenView_MISK(MatCoarsen coarse, PetscViewer viewer)
{
  PetscMPIInt       rank;
  PetscBool         iascii;
  PetscViewerFormat format;

  PetscFunctionBegin;
  PetscCallMPI(MPI_Comm_rank(PetscObjectComm((PetscObject)coarse), &rank));
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii));
  PetscCall(PetscViewerGetFormat(viewer, &format));
  if (iascii && format == PETSC_VIEWER_ASCII_INFO_DETAIL) {
    PetscCall(PetscViewerASCIIPushSynchronized(viewer));
    PetscCall(PetscViewerASCIISynchronizedPrintf(viewer, "  [%d] MISK aggregator\n", rank));
    if (!rank) PetscCall(PetscCoarsenDataView_private(coarse->agg_lists, viewer));
    PetscCall(PetscViewerFlush(viewer));
    PetscCall(PetscViewerASCIIPopSynchronized(viewer));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode MatCoarsenSetFromOptions_MISK(MatCoarsen coarse, PetscOptionItems *PetscOptionsObject)
{
  PetscInt  k = 1;
  PetscBool flg;

  PetscFunctionBegin;
  PetscOptionsHeadBegin(PetscOptionsObject, "MatCoarsen-MISk options");
  PetscCall(PetscOptionsInt("-mat_coarsen_misk_distance", "k distance for MIS", "", k, &k, &flg));
  if (flg) coarse->subctx = (void *)(size_t)k;
  PetscOptionsHeadEnd();
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*MC
   MATCOARSENMISK - A coarsener that uses MISK, a simple greedy coarsener

   Level: beginner

   Options Database Key:
.   -mat_coarsen_misk_distance <k> - distance for MIS

   Note:
   When the coarsening is used inside `PCGAMG` then the options database key is `-pc_gamg_mat_coarsen_misk_distance`

.seealso: `MatCoarsen`, `MatCoarsenMISKSetDistance()`, `MatCoarsenApply()`, `MatCoarsenSetType()`, `MatCoarsenType`, `MatCoarsenCreate()`
M*/

PETSC_EXTERN PetscErrorCode MatCoarsenCreate_MISK(MatCoarsen coarse)
{
  PetscFunctionBegin;
  coarse->ops->apply          = MatCoarsenApply_MISK;
  coarse->ops->view           = MatCoarsenView_MISK;
  coarse->subctx              = (void *)(size_t)1;
  coarse->ops->setfromoptions = MatCoarsenSetFromOptions_MISK;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatCoarsenMISKSetDistance - the distance to be used by MISK

  Collective

  Input Parameters:
+ crs - the coarsen
- k   - the distance

  Options Database Key:
. -mat_coarsen_misk_distance <k> - distance for MIS

  Level: advanced

  Note:
  When the coarsening is used inside `PCGAMG` then the options database key is `-pc_gamg_mat_coarsen_misk_distance`

.seealso: `MATCOARSENMISK`, `MatCoarsen`, `MatCoarseSetFromOptions()`, `MatCoarsenSetType()`, `MatCoarsenRegister()`, `MatCoarsenCreate()`,
          `MatCoarsenDestroy()`, `MatCoarsenSetAdjacency()`, `MatCoarsenMISKGetDistance()`
          `MatCoarsenGetData()`
@*/
PetscErrorCode MatCoarsenMISKSetDistance(MatCoarsen crs, PetscInt k)
{
  PetscFunctionBegin;
  crs->subctx = (void *)(size_t)k;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  MatCoarsenMISKGetDistance - gets the distance to be used by MISK

  Collective

  Input Parameter:
. crs - the coarsen

  Output Parameter:
. k - the distance

  Level: advanced

.seealso: `MATCOARSENMISK`, `MatCoarsen`, `MatCoarseSetFromOptions()`, `MatCoarsenSetType()`,
`MatCoarsenRegister()`, `MatCoarsenCreate()`, `MatCoarsenDestroy()`,
`MatCoarsenSetAdjacency()`, `MatCoarsenGetData()`
@*/
PetscErrorCode MatCoarsenMISKGetDistance(MatCoarsen crs, PetscInt *k)
{
  PetscFunctionBegin;
  *k = (PetscInt)(size_t)crs->subctx;
  PetscFunctionReturn(PETSC_SUCCESS);
}