MueLu_CoalesceDropFactory_kokkos_def.hpp

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#ifndef MUELU_COALESCEDROPFACTORY_KOKKOS_DEF_HPP
#define MUELU_COALESCEDROPFACTORY_KOKKOS_DEF_HPP

#include <Kokkos_Core.hpp>
#include <KokkosSparse_CrsMatrix.hpp>

#include "Xpetra_Matrix.hpp"

#include "MueLu_CoalesceDropFactory_kokkos_decl.hpp"

#include "MueLu_AmalgamationInfo.hpp"
#include "MueLu_Exceptions.hpp"
#include "MueLu_Level.hpp"
#include "MueLu_LWGraph_kokkos.hpp"
#include "MueLu_MasterList.hpp"
#include "MueLu_Monitor.hpp"
#include "MueLu_Utilities.hpp"

namespace MueLu {

namespace CoalesceDrop_Kokkos_Details {  // anonymous

template <class LO, class RowType>
class ScanFunctor {
 public:
  ScanFunctor(RowType rows_)
    : rows(rows_) {}

  KOKKOS_INLINE_FUNCTION
  void operator()(const LO i, LO& upd, const bool& final) const {
    upd += rows(i);
    if (final)
      rows(i) = upd;
  }

 private:
  RowType rows;
};

template <class LO, class GhostedViewType>
class ClassicalDropFunctor {
 private:
  typedef typename GhostedViewType::value_type SC;
  typedef Kokkos::ArithTraits<SC> ATS;
  typedef typename ATS::magnitudeType magnitudeType;

  GhostedViewType diag;  // corresponds to overlapped diagonal multivector (2D View)
  magnitudeType eps;

 public:
  ClassicalDropFunctor(GhostedViewType ghostedDiag, magnitudeType threshold)
    : diag(ghostedDiag)
    , eps(threshold) {}

  // Return true if we drop, false if not
  KOKKOS_FORCEINLINE_FUNCTION
  bool operator()(LO row, LO col, SC val) const {
    // We avoid square root by using squared values
    auto aiiajj = ATS::magnitude(diag(row, 0)) * ATS::magnitude(diag(col, 0));  // |a_ii|*|a_jj|
    auto aij2   = ATS::magnitude(val) * ATS::magnitude(val);                    // |a_ij|^2

    return (aij2 <= eps * eps * aiiajj);
  }
};

template <class LO, class CoordsType>
class DistanceFunctor {
 private:
  typedef typename CoordsType::value_type SC;
  typedef Kokkos::ArithTraits<SC> ATS;
  typedef typename ATS::magnitudeType magnitudeType;

 public:
  typedef SC value_type;

 public:
  DistanceFunctor(CoordsType coords_)
    : coords(coords_) {}

  KOKKOS_INLINE_FUNCTION
  magnitudeType distance2(LO row, LO col) const {
    SC d = ATS::zero(), s;
    for (size_t j = 0; j < coords.extent(1); j++) {
      s = coords(row, j) - coords(col, j);
      d += s * s;
    }
    return ATS::magnitude(d);
  }

 private:
  CoordsType coords;
};

template <class LO, class GhostedViewType, class DistanceFunctor>
class DistanceLaplacianDropFunctor {
 private:
  typedef typename GhostedViewType::value_type SC;
  typedef Kokkos::ArithTraits<SC> ATS;
  typedef typename ATS::magnitudeType magnitudeType;

 public:
  DistanceLaplacianDropFunctor(GhostedViewType ghostedLaplDiag, DistanceFunctor distFunctor_, magnitudeType threshold)
    : diag(ghostedLaplDiag)
    , distFunctor(distFunctor_)
    , eps(threshold) {}

  // Return true if we drop, false if not
  KOKKOS_INLINE_FUNCTION
  bool operator()(LO row, LO col, SC /* val */) const {
    // We avoid square root by using squared values

    // We ignore incoming value of val as we operate on an auxiliary
    // distance Laplacian matrix
    typedef typename DistanceFunctor::value_type dSC;
    typedef Kokkos::ArithTraits<dSC> dATS;
    auto fval = dATS::one() / distFunctor.distance2(row, col);

    auto aiiajj = ATS::magnitude(diag(row, 0)) * ATS::magnitude(diag(col, 0));  // |a_ii|*|a_jj|
    auto aij2   = ATS::magnitude(fval) * ATS::magnitude(fval);                  // |a_ij|^2

    return (aij2 <= eps * eps * aiiajj);
  }

 private:
  GhostedViewType diag;  // corresponds to overlapped diagonal multivector (2D View)
  DistanceFunctor distFunctor;
  magnitudeType eps;
};

template <class SC, class LO, class MatrixType, class BndViewType, class DropFunctorType>
class ScalarFunctor {
 private:
  typedef typename MatrixType::StaticCrsGraphType graph_type;
  typedef typename graph_type::row_map_type rows_type;
  typedef typename graph_type::entries_type cols_type;
  typedef typename MatrixType::values_type vals_type;
  typedef Kokkos::ArithTraits<SC> ATS;
  typedef typename ATS::val_type impl_Scalar;
  typedef Kokkos::ArithTraits<impl_Scalar> impl_ATS;
  typedef typename ATS::magnitudeType magnitudeType;

 public:
  ScalarFunctor(MatrixType A_, BndViewType bndNodes_, DropFunctorType dropFunctor_,
                typename rows_type::non_const_type rows_,
                typename cols_type::non_const_type colsAux_,
                typename vals_type::non_const_type valsAux_,
                bool reuseGraph_, bool lumping_, SC /* threshold_ */,
                bool aggregationMayCreateDirichlet_)
    : A(A_)
    , bndNodes(bndNodes_)
    , dropFunctor(dropFunctor_)
    , rows(rows_)
    , colsAux(colsAux_)
    , valsAux(valsAux_)
    , reuseGraph(reuseGraph_)
    , lumping(lumping_)
    , aggregationMayCreateDirichlet(aggregationMayCreateDirichlet_) {
    rowsA = A.graph.row_map;
    zero  = impl_ATS::zero();
  }

  KOKKOS_INLINE_FUNCTION
  void operator()(const LO row, LO& nnz) const {
    auto rowView = A.rowConst(row);
    auto length  = rowView.length;
    auto offset  = rowsA(row);

    impl_Scalar diag = zero;
    LO rownnz        = 0;
    LO diagID        = -1;
    for (decltype(length) colID = 0; colID < length; colID++) {
      LO col          = rowView.colidx(colID);
      impl_Scalar val = rowView.value(colID);

      if (!dropFunctor(row, col, rowView.value(colID)) || row == col) {
        colsAux(offset + rownnz) = col;

        LO valID                = (reuseGraph ? colID : rownnz);
        valsAux(offset + valID) = val;
        if (row == col)
          diagID = valID;

        rownnz++;

      } else {
        // Rewrite with zeros (needed for reuseGraph)
        valsAux(offset + colID) = zero;
        diag += val;
      }
    }
    // How to assert on the device?
    // assert(diagIndex != -1);
    rows(row + 1) = rownnz;
    // if (lumping && diagID != -1) {
    if (lumping) {
      // Add diag to the diagonal

      // NOTE_KOKKOS: valsAux was allocated with
      // ViewAllocateWithoutInitializing. This is not a problem here
      // because we explicitly set this value above.
      valsAux(offset + diagID) += diag;
    }

    // If the only element remaining after filtering is diagonal, mark node as boundary
    // FIXME: this should really be replaced by the following
    //    if (indices.size() == 1 && indices[0] == row)
    //        boundaryNodes[row] = true;
    // We do not do it this way now because there is no framework for distinguishing isolated
    // and boundary nodes in the aggregation algorithms
    bndNodes(row) = (rownnz == 1 && aggregationMayCreateDirichlet);

    nnz += rownnz;
  }

 private:
  MatrixType A;
  BndViewType bndNodes;
  DropFunctorType dropFunctor;

  rows_type rowsA;

  typename rows_type::non_const_type rows;
  typename cols_type::non_const_type colsAux;
  typename vals_type::non_const_type valsAux;

  bool reuseGraph;
  bool lumping;
  bool aggregationMayCreateDirichlet;
  impl_Scalar zero;
};

// collect number nonzeros of blkSize rows in nnz_(row+1)
template <class MatrixType, class NnzType, class blkSizeType>
class Stage1aVectorFunctor {
 private:
  typedef typename MatrixType::ordinal_type LO;

 public:
  Stage1aVectorFunctor(MatrixType kokkosMatrix_, NnzType nnz_, blkSizeType blkSize_)
    : kokkosMatrix(kokkosMatrix_)
    , nnz(nnz_)
    , blkSize(blkSize_) {}

  KOKKOS_INLINE_FUNCTION
  void operator()(const LO row, LO& totalnnz) const {
    // the following code is more or less what MergeRows is doing
    // count nonzero entries in all dof rows associated with node row
    LO nodeRowMaxNonZeros = 0;
    for (LO j = 0; j < blkSize; j++) {
      auto rowView = kokkosMatrix.row(row * blkSize + j);
      nodeRowMaxNonZeros += rowView.length;
    }
    nnz(row + 1) = nodeRowMaxNonZeros;
    totalnnz += nodeRowMaxNonZeros;
  }

 private:
  MatrixType kokkosMatrix;  //< local matrix part
  NnzType nnz;              //< View containing number of nonzeros for current row
  blkSizeType blkSize;      //< block size (or partial block size in strided maps)
};

// build the dof-based column map containing the local dof ids belonging to blkSize rows in matrix
// sort column ids
// translate them into (unique) node ids
// count the node column ids per node row
template <class MatrixType, class NnzType, class blkSizeType, class ColDofType, class Dof2NodeTranslationType, class BdryNodeTypeConst, class BdryNodeType, class boolType>
class Stage1bcVectorFunctor {
 private:
  typedef typename MatrixType::ordinal_type LO;

 private:
  MatrixType kokkosMatrix;           //< local matrix part
  NnzType coldofnnz;                 //< view containing start and stop indices for subviews
  blkSizeType blkSize;               //< block size (or partial block size in strided maps)
  ColDofType coldofs;                //< view containing the local dof ids associated with columns for the blkSize rows (not sorted)
  Dof2NodeTranslationType dof2node;  //< view containing the local node id associated with the local dof id
  NnzType colnodennz;                //< view containing number of column nodes for each node row
  BdryNodeTypeConst dirichletdof;    //< view containing with num dofs booleans. True if dof (not necessarily entire node) is dirichlet boundardy dof.
  BdryNodeType bdrynode;             //< view containing with numNodes booleans. True if node is (full) dirichlet boundardy node.
  boolType usegreedydirichlet;       //< boolean for use of greedy Dirichlet (if any dof is Dirichlet, entire node is dirichlet) default false (need all dofs in node to be Dirichlet for node to be Dirichlet)

 public:
  Stage1bcVectorFunctor(MatrixType kokkosMatrix_,
                        NnzType coldofnnz_,
                        blkSizeType blkSize_,
                        ColDofType coldofs_,
                        Dof2NodeTranslationType dof2node_,
                        NnzType colnodennz_,
                        BdryNodeTypeConst dirichletdof_,
                        BdryNodeType bdrynode_,
                        boolType usegreedydirichlet_)
    : kokkosMatrix(kokkosMatrix_)
    , coldofnnz(coldofnnz_)
    , blkSize(blkSize_)
    , coldofs(coldofs_)
    , dof2node(dof2node_)
    , colnodennz(colnodennz_)
    , dirichletdof(dirichletdof_)
    , bdrynode(bdrynode_)
    , usegreedydirichlet(usegreedydirichlet_) {
  }

  KOKKOS_INLINE_FUNCTION
  void operator()(const LO rowNode, LO& nnz) const {
    LO pos = coldofnnz(rowNode);
    if (usegreedydirichlet) {
      bdrynode(rowNode) = false;
      for (LO j = 0; j < blkSize; j++) {
        auto rowView    = kokkosMatrix.row(rowNode * blkSize + j);
        auto numIndices = rowView.length;

        // if any dof in the node is Dirichlet
        if (dirichletdof(rowNode * blkSize + j))
          bdrynode(rowNode) = true;

        for (decltype(numIndices) k = 0; k < numIndices; k++) {
          auto dofID   = rowView.colidx(k);
          coldofs(pos) = dofID;
          pos++;
        }
      }
    } else {
      bdrynode(rowNode) = true;
      for (LO j = 0; j < blkSize; j++) {
        auto rowView    = kokkosMatrix.row(rowNode * blkSize + j);
        auto numIndices = rowView.length;

        // if any dof in the node is not Dirichlet
        if (dirichletdof(rowNode * blkSize + j) == false)
          bdrynode(rowNode) = false;

        for (decltype(numIndices) k = 0; k < numIndices; k++) {
          auto dofID   = rowView.colidx(k);
          coldofs(pos) = dofID;
          pos++;
        }
      }
    }

    // sort coldofs
    LO begin = coldofnnz(rowNode);
    LO end   = coldofnnz(rowNode + 1);
    LO n     = end - begin;
    for (LO i = 0; i < (n - 1); i++) {
      for (LO j = 0; j < (n - i - 1); j++) {
        if (coldofs(j + begin) > coldofs(j + begin + 1)) {
          LO temp                = coldofs(j + begin);
          coldofs(j + begin)     = coldofs(j + begin + 1);
          coldofs(j + begin + 1) = temp;
        }
      }
    }
    size_t cnt    = 0;
    LO lastNodeID = -1;
    for (LO i = 0; i < n; i++) {
      LO dofID  = coldofs(begin + i);
      LO nodeID = dof2node(dofID);
      if (nodeID != lastNodeID) {
        lastNodeID           = nodeID;
        coldofs(begin + cnt) = nodeID;
        cnt++;
      }
    }
    colnodennz(rowNode + 1) = cnt;
    nnz += cnt;
  }
};

// fill column node id view
template <class MatrixType, class ColDofNnzType, class ColDofType, class ColNodeNnzType, class ColNodeType>
class Stage1dVectorFunctor {
 private:
  typedef typename MatrixType::ordinal_type LO;
  typedef typename MatrixType::value_type SC;

 private:
  ColDofType coldofs;         //< view containing mixed node and dof indices (only input)
  ColDofNnzType coldofnnz;    //< view containing the start and stop indices for subviews (dofs)
  ColNodeType colnodes;       //< view containing the local node ids associated with columns
  ColNodeNnzType colnodennz;  //< view containing start and stop indices for subviews

 public:
  Stage1dVectorFunctor(ColDofType coldofs_, ColDofNnzType coldofnnz_, ColNodeType colnodes_, ColNodeNnzType colnodennz_)
    : coldofs(coldofs_)
    , coldofnnz(coldofnnz_)
    , colnodes(colnodes_)
    , colnodennz(colnodennz_) {
  }

  KOKKOS_INLINE_FUNCTION
  void operator()(const LO rowNode) const {
    auto dofbegin  = coldofnnz(rowNode);
    auto nodebegin = colnodennz(rowNode);
    auto nodeend   = colnodennz(rowNode + 1);
    auto n         = nodeend - nodebegin;

    for (decltype(nodebegin) i = 0; i < n; i++) {
      colnodes(nodebegin + i) = coldofs(dofbegin + i);
    }
  }
};

}  // namespace CoalesceDrop_Kokkos_Details

template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
RCP<const ParameterList> CoalesceDropFactory_kokkos<Scalar, LocalOrdinal, GlobalOrdinal, Node>::GetValidParameterList() const {
  RCP<ParameterList> validParamList = rcp(new ParameterList());

#define SET_VALID_ENTRY(name) validParamList->setEntry(name, MasterList::getEntry(name))
  SET_VALID_ENTRY("aggregation: drop tol");
  SET_VALID_ENTRY("aggregation: Dirichlet threshold");
  SET_VALID_ENTRY("aggregation: drop scheme");
  SET_VALID_ENTRY("aggregation: dropping may create Dirichlet");
  SET_VALID_ENTRY("aggregation: greedy Dirichlet");
  SET_VALID_ENTRY("filtered matrix: use lumping");
  SET_VALID_ENTRY("filtered matrix: reuse graph");
  SET_VALID_ENTRY("filtered matrix: reuse eigenvalue");
  SET_VALID_ENTRY("aggregation: use ml scaling of drop tol");
  {
    validParamList->getEntry("aggregation: drop scheme").setValidator(rcp(new Teuchos::StringValidator(Teuchos::tuple<std::string>("classical", "distance laplacian"))));
  }
#undef SET_VALID_ENTRY
  validParamList->set<RCP<const FactoryBase>>("A", Teuchos::null, "Generating factory of the matrix A");
  validParamList->set<RCP<const FactoryBase>>("UnAmalgamationInfo", Teuchos::null, "Generating factory for UnAmalgamationInfo");
  validParamList->set<RCP<const FactoryBase>>("Coordinates", Teuchos::null, "Generating factory for Coordinates");

  return validParamList;
}

template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
void CoalesceDropFactory_kokkos<Scalar, LocalOrdinal, GlobalOrdinal, Node>::DeclareInput(Level& currentLevel) const {
  Input(currentLevel, "A");
  Input(currentLevel, "UnAmalgamationInfo");

  const ParameterList& pL = GetParameterList();
  if (pL.get<std::string>("aggregation: drop scheme") == "distance laplacian")
    Input(currentLevel, "Coordinates");
}

template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
void CoalesceDropFactory_kokkos<Scalar, LocalOrdinal, GlobalOrdinal, Node>::
    Build(Level& currentLevel) const {
  FactoryMonitor m(*this, "Build", currentLevel);

  typedef Teuchos::ScalarTraits<SC> STS;
  typedef typename STS::magnitudeType MT;
  const MT zero = Teuchos::ScalarTraits<MT>::zero();

  auto A = Get<RCP<Matrix>>(currentLevel, "A");

  /* NOTE: storageblocksize (from GetStorageBlockSize()) is the size of a block in the chosen storage scheme.
     blkSize is the number of storage blocks that must kept together during the amalgamation process.

     Both of these quantities may be different than numPDEs (from GetFixedBlockSize()), but the following must always hold:

     numPDEs = blkSize * storageblocksize.

     If numPDEs==1
       Matrix is point storage (classical CRS storage).  storageblocksize=1 and  blkSize=1
       No other values makes sense.

     If numPDEs>1
       If matrix uses point storage, then storageblocksize=1  and blkSize=numPDEs.
       If matrix uses block storage, with block size of n, then storageblocksize=n, and blkSize=numPDEs/n.
       Thus far, only storageblocksize=numPDEs and blkSize=1 has been tested.
    */

  TEUCHOS_TEST_FOR_EXCEPTION(A->GetFixedBlockSize() % A->GetStorageBlockSize() != 0, Exceptions::RuntimeError, "A->GetFixedBlockSize() needs to be a multiple of A->GetStorageBlockSize()");
  LO blkSize = A->GetFixedBlockSize() / A->GetStorageBlockSize();

  auto amalInfo = Get<RCP<AmalgamationInfo>>(currentLevel, "UnAmalgamationInfo");

  const ParameterList& pL = GetParameterList();

  // Sanity Checking: ML drop tol scaling is not supported in UncoupledAggregation_Kokkos
  TEUCHOS_TEST_FOR_EXCEPTION(pL.get<bool>("aggregation: use ml scaling of drop tol"), std::invalid_argument, "Option: 'aggregation: use ml scaling of drop tol' is not supported in the Kokkos version of CoalesceDroPFactory");

  std::string algo = pL.get<std::string>("aggregation: drop scheme");

  double threshold = pL.get<double>("aggregation: drop tol");
  GetOStream(Runtime0) << "algorithm = \"" << algo << "\": threshold = " << threshold
                       << ", blocksize = " << A->GetFixedBlockSize() << std::endl;

  const typename STS::magnitudeType dirichletThreshold =
      STS::magnitude(as<SC>(pL.get<double>("aggregation: Dirichlet threshold")));

  GO numDropped = 0, numTotal = 0;

  RCP<LWGraph_kokkos> graph;
  LO dofsPerNode = -1;

  typedef typename LWGraph_kokkos::boundary_nodes_type boundary_nodes_type;
  boundary_nodes_type boundaryNodes;

  RCP<Matrix> filteredA;
  if (blkSize == 1 && threshold == zero) {
    // Scalar problem without dropping

    // Detect and record rows that correspond to Dirichlet boundary conditions
    boundaryNodes = Utilities::DetectDirichletRows_kokkos(*A, dirichletThreshold);

    // Trivial LWGraph construction
    graph = rcp(new LWGraph_kokkos(A->getCrsGraph()->getLocalGraphDevice(), A->getRowMap(), A->getColMap(), "graph of A"));
    graph->SetBoundaryNodeMap(boundaryNodes);

    numTotal    = A->getLocalNumEntries();
    dofsPerNode = 1;

    filteredA = A;

  } else if (blkSize == 1 && threshold != zero) {
    // Scalar problem with dropping

    typedef typename Matrix::local_matrix_type local_matrix_type;
    typedef typename LWGraph_kokkos::local_graph_type kokkos_graph_type;
    typedef typename kokkos_graph_type::row_map_type::non_const_type rows_type;
    typedef typename kokkos_graph_type::entries_type::non_const_type cols_type;
    typedef typename local_matrix_type::values_type::non_const_type vals_type;

    LO numRows                     = A->getLocalNumRows();
    local_matrix_type kokkosMatrix = A->getLocalMatrixDevice();
    auto nnzA                      = kokkosMatrix.nnz();
    auto rowsA                     = kokkosMatrix.graph.row_map;

    typedef Kokkos::ArithTraits<SC> ATS;
    typedef typename ATS::val_type impl_Scalar;
    typedef Kokkos::ArithTraits<impl_Scalar> impl_ATS;

    bool reuseGraph = pL.get<bool>("filtered matrix: reuse graph");
    bool lumping    = pL.get<bool>("filtered matrix: use lumping");
    if (lumping)
      GetOStream(Runtime0) << "Lumping dropped entries" << std::endl;

    const bool aggregationMayCreateDirichlet = pL.get<bool>("aggregation: dropping may create Dirichlet");

    // FIXME_KOKKOS: replace by ViewAllocateWithoutInitializing + setting a single value
    rows_type rows("FA_rows", numRows + 1);
    cols_type colsAux(Kokkos::ViewAllocateWithoutInitializing("FA_aux_cols"), nnzA);
    vals_type valsAux;
    if (reuseGraph) {
      SubFactoryMonitor m2(*this, "CopyMatrix", currentLevel);

      // Share graph with the original matrix
      filteredA = MatrixFactory::Build(A->getCrsGraph());

      // Do a no-op fill-complete
      RCP<ParameterList> fillCompleteParams(new ParameterList);
      fillCompleteParams->set("No Nonlocal Changes", true);
      filteredA->fillComplete(fillCompleteParams);

      // No need to reuseFill, just modify in place
      valsAux = filteredA->getLocalMatrixDevice().values;

    } else {
      // Need an extra array to compress
      valsAux = vals_type(Kokkos::ViewAllocateWithoutInitializing("FA_aux_vals"), nnzA);
    }

    typename boundary_nodes_type::non_const_type bndNodes(Kokkos::ViewAllocateWithoutInitializing("boundaryNodes"), numRows);

    LO nnzFA = 0;
    {
      if (algo == "classical") {
        // Construct overlapped matrix diagonal
        RCP<Vector> ghostedDiag;
        {
          kokkosMatrix = local_matrix_type();
          SubFactoryMonitor m2(*this, "Ghosted diag construction", currentLevel);
          ghostedDiag  = Utilities::GetMatrixOverlappedDiagonal(*A);
          kokkosMatrix = A->getLocalMatrixDevice();
        }

        // Filter out entries
        {
          SubFactoryMonitor m2(*this, "MainLoop", currentLevel);

          auto ghostedDiagView = ghostedDiag->getDeviceLocalView(Xpetra::Access::ReadWrite);

          CoalesceDrop_Kokkos_Details::ClassicalDropFunctor<LO, decltype(ghostedDiagView)> dropFunctor(ghostedDiagView, threshold);
          CoalesceDrop_Kokkos_Details::ScalarFunctor<typename ATS::val_type, LO, local_matrix_type, decltype(bndNodes), decltype(dropFunctor)>
              scalarFunctor(kokkosMatrix, bndNodes, dropFunctor, rows, colsAux, valsAux, reuseGraph, lumping, threshold, aggregationMayCreateDirichlet);

          Kokkos::parallel_reduce("MueLu:CoalesceDropF:Build:scalar_filter:main_loop", range_type(0, numRows),
                                  scalarFunctor, nnzFA);
        }

      } else if (algo == "distance laplacian") {
        typedef Xpetra::MultiVector<typename Teuchos::ScalarTraits<Scalar>::magnitudeType, LO, GO, NO> doubleMultiVector;
        auto coords = Get<RCP<doubleMultiVector>>(currentLevel, "Coordinates");

        auto uniqueMap    = A->getRowMap();
        auto nonUniqueMap = A->getColMap();

        // Construct ghosted coordinates
        RCP<const Import> importer;
        {
          SubFactoryMonitor m2(*this, "Coords Import construction", currentLevel);
          importer = ImportFactory::Build(uniqueMap, nonUniqueMap);
        }
        RCP<doubleMultiVector> ghostedCoords;
        {
          SubFactoryMonitor m2(*this, "Ghosted coords construction", currentLevel);
          ghostedCoords = Xpetra::MultiVectorFactory<typename Teuchos::ScalarTraits<Scalar>::magnitudeType, LO, GO, NO>::Build(nonUniqueMap, coords->getNumVectors());
          ghostedCoords->doImport(*coords, *importer, Xpetra::INSERT);
        }

        auto ghostedCoordsView = ghostedCoords->getDeviceLocalView(Xpetra::Access::ReadWrite);
        CoalesceDrop_Kokkos_Details::DistanceFunctor<LO, decltype(ghostedCoordsView)> distFunctor(ghostedCoordsView);

        // Construct Laplacian diagonal
        RCP<Vector> localLaplDiag;
        {
          SubFactoryMonitor m2(*this, "Local Laplacian diag construction", currentLevel);

          localLaplDiag = VectorFactory::Build(uniqueMap);

          auto localLaplDiagView = localLaplDiag->getDeviceLocalView(Xpetra::Access::OverwriteAll);
          auto kokkosGraph       = kokkosMatrix.graph;

          Kokkos::parallel_for(
              "MueLu:CoalesceDropF:Build:scalar_filter:laplacian_diag", range_type(0, numRows),
              KOKKOS_LAMBDA(const LO row) {
                auto rowView = kokkosGraph.rowConst(row);
                auto length  = rowView.length;

                impl_Scalar d = impl_ATS::zero();
                for (decltype(length) colID = 0; colID < length; colID++) {
                  auto col = rowView(colID);
                  if (row != col)
                    d += impl_ATS::one() / distFunctor.distance2(row, col);
                }
                localLaplDiagView(row, 0) = d;
              });
        }

        // Construct ghosted Laplacian diagonal
        RCP<Vector> ghostedLaplDiag;
        {
          SubFactoryMonitor m2(*this, "Ghosted Laplacian diag construction", currentLevel);
          ghostedLaplDiag = VectorFactory::Build(nonUniqueMap);
          ghostedLaplDiag->doImport(*localLaplDiag, *importer, Xpetra::INSERT);
        }

        // Filter out entries
        {
          SubFactoryMonitor m2(*this, "MainLoop", currentLevel);

          auto ghostedLaplDiagView = ghostedLaplDiag->getDeviceLocalView(Xpetra::Access::ReadWrite);

          CoalesceDrop_Kokkos_Details::DistanceLaplacianDropFunctor<LO, decltype(ghostedLaplDiagView), decltype(distFunctor)>
              dropFunctor(ghostedLaplDiagView, distFunctor, threshold);
          CoalesceDrop_Kokkos_Details::ScalarFunctor<SC, LO, local_matrix_type, decltype(bndNodes), decltype(dropFunctor)>
              scalarFunctor(kokkosMatrix, bndNodes, dropFunctor, rows, colsAux, valsAux, reuseGraph, lumping, threshold, true);

          Kokkos::parallel_reduce("MueLu:CoalesceDropF:Build:scalar_filter:main_loop", range_type(0, numRows),
                                  scalarFunctor, nnzFA);
        }
      }
    }
    numDropped = nnzA - nnzFA;

    boundaryNodes = bndNodes;

    {
      SubFactoryMonitor m2(*this, "CompressRows", currentLevel);

      // parallel_scan (exclusive)
      Kokkos::parallel_scan(
          "MueLu:CoalesceDropF:Build:scalar_filter:compress_rows", range_type(0, numRows + 1),
          KOKKOS_LAMBDA(const LO i, LO& update, const bool& final_pass) {
            update += rows(i);
            if (final_pass)
              rows(i) = update;
          });
    }

    // Compress cols (and optionally vals)
    // We use a trick here: we moved all remaining elements to the beginning
    // of the original row in the main loop, so we don't need to check for
    // INVALID here, and just stop when achieving the new number of elements
    // per row.
    cols_type cols(Kokkos::ViewAllocateWithoutInitializing("FA_cols"), nnzFA);
    vals_type vals;
    if (reuseGraph) {
      GetOStream(Runtime1) << "reuse matrix graph for filtering (compress matrix columns only)" << std::endl;
      // Only compress cols
      SubFactoryMonitor m2(*this, "CompressColsAndVals", currentLevel);

      Kokkos::parallel_for(
          "MueLu:TentativePF:Build:compress_cols", range_type(0, numRows),
          KOKKOS_LAMBDA(const LO i) {
            // Is there Kokkos memcpy?
            LO rowStart   = rows(i);
            LO rowAStart  = rowsA(i);
            size_t rownnz = rows(i + 1) - rows(i);
            for (size_t j = 0; j < rownnz; j++)
              cols(rowStart + j) = colsAux(rowAStart + j);
          });
    } else {
      // Compress cols and vals
      GetOStream(Runtime1) << "new matrix graph for filtering (compress matrix columns and values)" << std::endl;
      SubFactoryMonitor m2(*this, "CompressColsAndVals", currentLevel);

      vals = vals_type(Kokkos::ViewAllocateWithoutInitializing("FA_vals"), nnzFA);

      Kokkos::parallel_for(
          "MueLu:TentativePF:Build:compress_cols", range_type(0, numRows),
          KOKKOS_LAMBDA(const LO i) {
            LO rowStart   = rows(i);
            LO rowAStart  = rowsA(i);
            size_t rownnz = rows(i + 1) - rows(i);
            for (size_t j = 0; j < rownnz; j++) {
              cols(rowStart + j) = colsAux(rowAStart + j);
              vals(rowStart + j) = valsAux(rowAStart + j);
            }
          });
    }

    kokkos_graph_type kokkosGraph(cols, rows);

    {
      SubFactoryMonitor m2(*this, "LWGraph construction", currentLevel);

      graph = rcp(new LWGraph_kokkos(kokkosGraph, A->getRowMap(), A->getColMap(), "filtered graph of A"));
      graph->SetBoundaryNodeMap(boundaryNodes);
    }

    numTotal = A->getLocalNumEntries();

    dofsPerNode = 1;

    if (!reuseGraph) {
      SubFactoryMonitor m2(*this, "LocalMatrix+FillComplete", currentLevel);

      local_matrix_type localFA = local_matrix_type("A", numRows, A->getLocalMatrixDevice().numCols(), nnzFA, vals, rows, cols);
      auto filteredACrs         = CrsMatrixFactory::Build(localFA, A->getRowMap(), A->getColMap(), A->getDomainMap(), A->getRangeMap(),
                                                          A->getCrsGraph()->getImporter(), A->getCrsGraph()->getExporter());
      filteredA                 = rcp(new CrsMatrixWrap(filteredACrs));
    }

    filteredA->SetFixedBlockSize(A->GetFixedBlockSize());

    if (pL.get<bool>("filtered matrix: reuse eigenvalue")) {
      // Reuse max eigenvalue from A
      // It is unclear what eigenvalue is the best for the smoothing, but we already may have
      // the D^{-1}A estimate in A, may as well use it.
      // NOTE: ML does that too
      filteredA->SetMaxEigenvalueEstimate(A->GetMaxEigenvalueEstimate());
    } else {
      filteredA->SetMaxEigenvalueEstimate(-Teuchos::ScalarTraits<SC>::one());
    }

  } else if (blkSize > 1 && threshold == zero) {
    // Case 3:  block problem without filtering
    //
    // FIXME_KOKKOS: this code is completely unoptimized. It really should do
    // a very simple thing: merge rows and produce nodal graph. But the code
    // seems very complicated. Can we do better?

    TEUCHOS_TEST_FOR_EXCEPTION(A->getRowMap()->getLocalNumElements() % blkSize != 0, MueLu::Exceptions::RuntimeError, "MueLu::CoalesceDropFactory: Number of local elements is " << A->getRowMap()->getLocalNumElements() << " but should be a multiply of " << blkSize);

    const RCP<const Map> rowMap = A->getRowMap();
    const RCP<const Map> colMap = A->getColMap();

    // build a node row map (uniqueMap = non-overlapping) and a node column map
    // (nonUniqueMap = overlapping). The arrays rowTranslation and colTranslation
    // stored in the AmalgamationInfo class container contain the local node id
    // given a local dof id. The data is calculated in the AmalgamationFactory and
    // stored in the variable "UnAmalgamationInfo" (which is of type AmalagamationInfo)
    const RCP<const Map> uniqueMap    = amalInfo->getNodeRowMap();
    const RCP<const Map> nonUniqueMap = amalInfo->getNodeColMap();
    Array<LO> rowTranslationArray     = *(amalInfo->getRowTranslation());  // TAW should be transform that into a View?
    Array<LO> colTranslationArray     = *(amalInfo->getColTranslation());

    Kokkos::View<LO*, Kokkos::MemoryUnmanaged>
        rowTranslationView(rowTranslationArray.getRawPtr(), rowTranslationArray.size());
    Kokkos::View<LO*, Kokkos::MemoryUnmanaged>
        colTranslationView(colTranslationArray.getRawPtr(), colTranslationArray.size());

    // get number of local nodes
    LO numNodes = Teuchos::as<LocalOrdinal>(uniqueMap->getLocalNumElements());
    typedef typename Kokkos::View<LocalOrdinal*, typename Node::device_type> id_translation_type;
    id_translation_type rowTranslation("dofId2nodeId", rowTranslationArray.size());
    id_translation_type colTranslation("ov_dofId2nodeId", colTranslationArray.size());
    Kokkos::deep_copy(rowTranslation, rowTranslationView);
    Kokkos::deep_copy(colTranslation, colTranslationView);

    // extract striding information
    blkSize                  = A->GetFixedBlockSize();  //< the full block size (number of dofs per node in strided map)
    LocalOrdinal blkId       = -1;                      //< the block id within a strided map or -1 if it is a full block map
    LocalOrdinal blkPartSize = A->GetFixedBlockSize();  //< stores block size of part blkId (or the full block size)
    if (A->IsView("stridedMaps") == true) {
      const RCP<const Map> myMap         = A->getRowMap("stridedMaps");
      const RCP<const StridedMap> strMap = Teuchos::rcp_dynamic_cast<const StridedMap>(myMap);
      TEUCHOS_TEST_FOR_EXCEPTION(strMap.is_null() == true, Exceptions::RuntimeError, "Map is not of type stridedMap");
      blkSize = Teuchos::as<const LocalOrdinal>(strMap->getFixedBlockSize());
      blkId   = strMap->getStridedBlockId();
      if (blkId > -1)
        blkPartSize = Teuchos::as<LocalOrdinal>(strMap->getStridingData()[blkId]);
    }
    auto kokkosMatrix = A->getLocalMatrixDevice();  // access underlying kokkos data

    //
    typedef typename LWGraph_kokkos::local_graph_type kokkos_graph_type;
    typedef typename kokkos_graph_type::row_map_type row_map_type;
    // typedef typename row_map_type::HostMirror           row_map_type_h;
    typedef typename kokkos_graph_type::entries_type entries_type;

    // Stage 1c: get number of dof-nonzeros per blkSize node rows
    typename row_map_type::non_const_type dofNnz("nnz_map", numNodes + 1);
    LO numDofCols = 0;
    CoalesceDrop_Kokkos_Details::Stage1aVectorFunctor<decltype(kokkosMatrix), decltype(dofNnz), decltype(blkPartSize)> stage1aFunctor(kokkosMatrix, dofNnz, blkPartSize);
    Kokkos::parallel_reduce("MueLu:CoalesceDropF:Build:scalar_filter:stage1a", range_type(0, numNodes), stage1aFunctor, numDofCols);
    // parallel_scan (exclusive)
    CoalesceDrop_Kokkos_Details::ScanFunctor<LO, decltype(dofNnz)> scanFunctor(dofNnz);
    Kokkos::parallel_scan("MueLu:CoalesceDropF:Build:scalar_filter:stage1_scan", range_type(0, numNodes + 1), scanFunctor);

    // Detect and record dof rows that correspond to Dirichlet boundary conditions
    boundary_nodes_type singleEntryRows = Utilities::DetectDirichletRows_kokkos(*A, dirichletThreshold);

    typename entries_type::non_const_type dofcols("dofcols", numDofCols /*dofNnz(numNodes)*/);  // why does dofNnz(numNodes) work? should be a parallel reduce, i guess

    // we have dofcols and dofids from Stage1dVectorFunctor
    LO numNodeCols = 0;
    typename row_map_type::non_const_type rows("nnz_nodemap", numNodes + 1);
    typename boundary_nodes_type::non_const_type bndNodes("boundaryNodes", numNodes);

    CoalesceDrop_Kokkos_Details::Stage1bcVectorFunctor<decltype(kokkosMatrix), decltype(dofNnz), decltype(blkPartSize), decltype(dofcols), decltype(colTranslation), decltype(singleEntryRows), decltype(bndNodes), bool> stage1bcFunctor(kokkosMatrix, dofNnz, blkPartSize, dofcols, colTranslation, rows, singleEntryRows, bndNodes, pL.get<bool>("aggregation: greedy Dirichlet"));
    Kokkos::parallel_reduce("MueLu:CoalesceDropF:Build:scalar_filter:stage1c", range_type(0, numNodes), stage1bcFunctor, numNodeCols);

    // parallel_scan (exclusive)
    CoalesceDrop_Kokkos_Details::ScanFunctor<LO, decltype(rows)> scanNodeFunctor(rows);
    Kokkos::parallel_scan("MueLu:CoalesceDropF:Build:scalar_filter:stage1_scan", range_type(0, numNodes + 1), scanNodeFunctor);

    // create column node view
    typename entries_type::non_const_type cols("nodecols", numNodeCols);

    CoalesceDrop_Kokkos_Details::Stage1dVectorFunctor<decltype(kokkosMatrix), decltype(dofNnz), decltype(dofcols), decltype(rows), decltype(cols)> stage1dFunctor(dofcols, dofNnz, cols, rows);
    Kokkos::parallel_for("MueLu:CoalesceDropF:Build:scalar_filter:stage1c", range_type(0, numNodes), stage1dFunctor);
    kokkos_graph_type kokkosGraph(cols, rows);

    // create LW graph
    graph = rcp(new LWGraph_kokkos(kokkosGraph, uniqueMap, nonUniqueMap, "amalgamated graph of A"));

    boundaryNodes = bndNodes;
    graph->SetBoundaryNodeMap(boundaryNodes);
    numTotal = A->getLocalNumEntries();

    dofsPerNode = blkSize;

    filteredA = A;

  } else {
    TEUCHOS_TEST_FOR_EXCEPTION(true, Exceptions::RuntimeError, "MueLu: CoalesceDropFactory_kokkos: Block filtering is not implemented");
  }

  if (GetVerbLevel() & Statistics1) {
    GO numLocalBoundaryNodes  = 0;
    GO numGlobalBoundaryNodes = 0;

    Kokkos::parallel_reduce(
        "MueLu:CoalesceDropF:Build:bnd", range_type(0, boundaryNodes.extent(0)),
        KOKKOS_LAMBDA(const LO i, GO& n) {
          if (boundaryNodes(i))
            n++;
        },
        numLocalBoundaryNodes);

    auto comm = A->getRowMap()->getComm();
    MueLu_sumAll(comm, numLocalBoundaryNodes, numGlobalBoundaryNodes);
    GetOStream(Statistics1) << "Detected " << numGlobalBoundaryNodes << " Dirichlet nodes" << std::endl;
  }

  if ((GetVerbLevel() & Statistics1) && threshold != zero) {
    auto comm = A->getRowMap()->getComm();

    GO numGlobalTotal, numGlobalDropped;
    MueLu_sumAll(comm, numTotal, numGlobalTotal);
    MueLu_sumAll(comm, numDropped, numGlobalDropped);

    if (numGlobalTotal != 0) {
      GetOStream(Statistics1) << "Number of dropped entries: "
                              << numGlobalDropped << "/" << numGlobalTotal
                              << " (" << 100 * Teuchos::as<double>(numGlobalDropped) / Teuchos::as<double>(numGlobalTotal) << "%)" << std::endl;
    }
  }

  Set(currentLevel, "DofsPerNode", dofsPerNode);
  Set(currentLevel, "Graph", graph);
  Set(currentLevel, "A", filteredA);
}
}  // namespace MueLu
#endif  // MUELU_COALESCEDROPFACTORY_KOKKOS_DEF_HPP