gather_scatter.f90

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module gather_scatter
  use neko_config, only : neko_bcknd_device, neko_bcknd_sx, neko_bcknd_hip, &
       neko_bcknd_cuda, neko_bcknd_opencl, neko_device_mpi
  use gs_bcknd, only : gs_bcknd_t, gs_bcknd_cpu, gs_bcknd_sx, gs_bcknd_dev
  use gs_device, only : gs_device_t
  use gs_sx, only : gs_sx_t
  use gs_cpu, only : gs_cpu_t
  use gs_ops, only : gs_op_add, gs_op_max, gs_op_min, gs_op_mul
  use gs_comm, only : gs_comm_t, gs_comm_mpi, gs_comm_mpigpu, gs_comm_nccl, &
       gs_comm_nvshmem, gs_comm_openshmem, gs_comm_caf
  use gs_mpi, only : gs_mpi_t
  use gs_shmem, only : gs_shmem_t
  use gs_caf, only : gs_caf_t
  use gs_device_mpi, only : gs_device_mpi_t
  use gs_device_nccl, only : gs_device_nccl_t
  use gs_device_shmem, only : gs_device_shmem_t
  use mesh, only : mesh_t
  use comm, only : pe_rank, pe_size, neko_comm
  use mpi_f08, only : mpi_reduce, mpi_allreduce, mpi_barrier, mpi_in_place, &
       mpi_wait, mpi_irecv, mpi_isend, mpi_wtime, mpi_sum, mpi_max, &
       mpi_integer, mpi_integer2, mpi_integer8, mpi_request, mpi_status, &
       mpi_status_ignore, mpi_get_count
  use dofmap, only : dofmap_t
  use field, only : field_t
  use num_types, only : rp, dp, i2, i8
  use htable, only : htable_i8_t, htable_iter_i8_t
  use stack, only : stack_i4_t
  use utils, only : neko_error, linear_index
  use logger, only : neko_log, log_size
  use profiler, only : profiler_start_region, profiler_end_region
  use device, only : device_memcpy, host_to_device, device_sync, device_free, &
       device_map, device_deassociate
  use, intrinsic :: iso_c_binding, only : c_ptr, c_null_ptr
  implicit none
  private
 
  type, public :: gs_t
     real(kind=rp), allocatable :: local_gs(:) 
     integer, allocatable :: local_dof_gs(:)
     integer, allocatable :: local_gs_dof(:)
     integer, allocatable :: local_blk_len(:)
     integer, allocatable :: local_blk_off(:)
     real(kind=rp), allocatable :: shared_gs(:) 
     integer, allocatable :: shared_dof_gs(:)
     integer, allocatable :: shared_gs_dof(:)
     integer, allocatable :: shared_blk_len(:)
     integer, allocatable :: shared_blk_off(:)
     type(dofmap_t), pointer ::dofmap
     type(htable_i8_t) :: shared_dofs
     integer :: nlocal
     integer :: nshared
     integer :: nlocal_blks
     integer :: nshared_blks
     integer :: local_facet_offset
     integer :: shared_facet_offset
     class(gs_bcknd_t), allocatable :: bcknd
     class(gs_comm_t), allocatable :: comm
   contains
     procedure, private, pass(gs) :: gs_op_fld
     procedure, private, pass(gs) :: gs_op_r4
     procedure, pass(gs) :: gs_op_vector
     procedure, pass(gs) :: init => gs_init
     procedure, pass(gs) :: free => gs_free
     generic :: op => gs_op_fld, gs_op_r4, gs_op_vector
  end type gs_t
 
  ! Expose available gather-scatter operation
  public :: gs_op_add, gs_op_mul, gs_op_min, gs_op_max
 
  ! Expose available gather-scatter backends
  public :: gs_bcknd_cpu, gs_bcknd_sx, gs_bcknd_dev
 
  ! Expose available gather-scatter comm. backends
  public :: gs_comm_mpi, gs_comm_mpigpu, gs_comm_nccl, gs_comm_nvshmem, &
       gs_comm_openshmem, gs_comm_caf
 
 
contains
 
  subroutine gs_init(gs, dofmap, bcknd, comm_bcknd)
    class(gs_t), intent(inout) :: gs
    type(dofmap_t), target, intent(inout) :: dofmap
    character(len=LOG_SIZE) :: log_buf
    character(len=20) :: bcknd_str
    integer, optional :: bcknd, comm_bcknd
    integer :: i, j, ierr, bcknd_, comm_bcknd_
    integer(i8) :: glb_nshared, glb_nlocal
    logical :: use_device_mpi, use_device_nccl, use_device_shmem, use_host_mpi
    logical :: use_host_shmem
    logical :: use_caf
    real(kind=rp), allocatable :: tmp(:)
    type(c_ptr) :: tmp_d = c_null_ptr
    integer :: strtgy(4) = [int(b'00'), int(b'01'), int(b'10'), int(b'11')]
    integer :: avg_strtgy, env_len
    character(len=255) :: env_strtgy, env_gscomm
    real(kind=dp) :: strtgy_time(4)
 
    call gs%free()
 
    call neko_log%section('Gather-Scatter')
    ! Currently this uses the dofmap which also contains geometric information
    ! Only connectivity/numbering of points is technically necessary for gs
    gs%dofmap => dofmap
 
    use_device_mpi = .false.
    use_device_nccl = .false.
    use_device_shmem = .false.
    use_host_mpi = .false.
    use_host_shmem = .false.
    use_caf = .false.
 
    ! Check if a comm-backend is requested via env. variables
    call get_environment_variable("NEKO_GS_COMM", env_gscomm, env_len)
    if (env_len .gt. 0) then
       if (env_gscomm(1:env_len) .eq. "MPI") then
          use_host_mpi = .true.
       else if (env_gscomm(1:env_len) .eq. "MPIGPU") then
          use_device_mpi = .true.
       else if (env_gscomm(1:env_len) .eq. "NCCL") then
          use_device_nccl = .true.
       else if (env_gscomm(1:env_len) .eq. "SHMEM") then
          if (neko_bcknd_device .eq. 1) then
             use_device_shmem = .true.
          else
             use_host_shmem = .true.
          end if
       else if (env_gscomm(1:env_len) .eq. "CAF") then
          use_caf = .true.
       else
          call neko_error('Unknown Gather-scatter comm. backend')
       end if
    end if
 
 
    if (present(comm_bcknd)) then
       comm_bcknd_ = comm_bcknd
    else if (use_host_mpi) then
       comm_bcknd_ = gs_comm_mpi
    else if (use_device_mpi) then
       comm_bcknd_ = gs_comm_mpigpu
    else if (use_device_nccl) then
       comm_bcknd_ = gs_comm_nccl
    else if (use_device_shmem) then
       comm_bcknd_ = gs_comm_nvshmem
    else if (use_host_shmem) then
       comm_bcknd_ = gs_comm_openshmem
    else if (use_caf) then
       comm_bcknd_ = gs_comm_caf
    else
       if (neko_device_mpi) then
          comm_bcknd_ = gs_comm_mpigpu
          use_device_mpi = .true.
       else
          comm_bcknd_ = gs_comm_mpi
       end if
    end if
 
    select case (comm_bcknd_)
    case (gs_comm_mpi)
       call neko_log%message('Comm         :          MPI')
       allocate(gs_mpi_t::gs%comm)
    case (gs_comm_mpigpu)
       call neko_log%message('Comm         :   Device MPI')
       allocate(gs_device_mpi_t::gs%comm)
    case (gs_comm_nccl)
       call neko_log%message('Comm         :         NCCL')
       allocate(gs_device_nccl_t::gs%comm)
    case (gs_comm_nvshmem)
       call neko_log%message('Comm         :      NVSHMEM')
       allocate(gs_device_shmem_t::gs%comm)
    case (gs_comm_openshmem)
       call neko_log%message('Comm         :    OpenSHMEM')
       allocate(gs_shmem_t::gs%comm)
    case (gs_comm_caf)
       call neko_log%message('Comm         :          CAF')
       allocate(gs_caf_t::gs%comm)
    case default
       call neko_error('Unknown Gather-scatter comm. backend')
    end select
    ! Initialize a stack for each rank containing which dofs to send/recv at
    ! that rank
    call gs%comm%init_dofs()
    ! Initialize mapping between local ids and gather-scatter ids
    ! based on the global numbering in dofmap
    call gs_init_mapping(gs)
    ! Setup buffers and which ranks to send/recv data from based on mapping
    ! and initializes gs%comm (sets up gs%comm%send_dof and gs%comm%recv_dof and
    ! recv_pe/send_pe)
    call gs_schedule(gs)
    ! Global number of points not needing to be sent over mpi for gs operations
    ! "Internal points"
    glb_nlocal = int(gs%nlocal, i8)
    ! Global number of points needing to be communicated with other pes/ranks
    ! "external points"
    glb_nshared = int(gs%nshared, i8)
    ! Can be thought of a measure of the volume of this rank (glb_nlocal) and
    ! the surface area (glb_nshared) that is shared with other ranks
    ! Lots of internal volume compared to surface that needs communication is
    ! good
 
    if (pe_rank .eq. 0) then
       call mpi_reduce(mpi_in_place, glb_nlocal, 1, &
            mpi_integer8, mpi_sum, 0, neko_comm, ierr)
 
       call mpi_reduce(mpi_in_place, glb_nshared, 1, &
            mpi_integer8, mpi_sum, 0, neko_comm, ierr)
    else
       call mpi_reduce(glb_nlocal, glb_nlocal, 1, &
            mpi_integer8, mpi_sum, 0, neko_comm, ierr)
 
       call mpi_reduce(glb_nshared, glb_nshared, 1, &
            mpi_integer8, mpi_sum, 0, neko_comm, ierr)
    end if
 
    write(log_buf, '(A,I12)') 'Avg. internal: ', glb_nlocal/pe_size
    call neko_log%message(log_buf)
    write(log_buf, '(A,I12)') 'Avg. external: ', glb_nshared/pe_size
    call neko_log%message(log_buf)
 
    if (present(bcknd)) then
       bcknd_ = bcknd
    else
       if (neko_bcknd_sx .eq. 1) then
          bcknd_ = gs_bcknd_sx
       else if (neko_bcknd_device .eq. 1) then
          bcknd_ = gs_bcknd_dev
       else
          bcknd_ = gs_bcknd_cpu
       end if
    end if
 
    ! Setup Gather-scatter backend
    select case (bcknd_)
    case (gs_bcknd_cpu)
       allocate(gs_cpu_t::gs%bcknd)
       bcknd_str = '         std'
    case (gs_bcknd_dev)
       allocate(gs_device_t::gs%bcknd)
       if (neko_bcknd_hip .eq. 1) then
          bcknd_str = '         hip'
       else if (neko_bcknd_cuda .eq. 1) then
          bcknd_str = '        cuda'
       else if (neko_bcknd_opencl .eq. 1) then
          bcknd_str = '      opencl'
       end if
    case (gs_bcknd_sx)
       allocate(gs_sx_t::gs%bcknd)
       bcknd_str = '          sx'
    case default
       call neko_error('Unknown Gather-scatter backend')
    end select
 
    write(log_buf, '(A)') 'Backend      : ' // trim(bcknd_str)
    call neko_log%message(log_buf)
 
 
    call gs%bcknd%init(gs%nlocal, gs%nshared, gs%nlocal_blks, gs%nshared_blks)
 
    if (use_device_mpi .or. use_device_nccl .or. use_device_shmem) then
       select type (b => gs%bcknd)
       type is (gs_device_t)
          b%shared_on_host = .false.
       end select
    end if
 
    if (use_device_mpi) then
       if (pe_size .gt. 1) then
          ! Select fastest device MPI strategy at runtime
          select type (c => gs%comm)
          type is (gs_device_mpi_t)
             call get_environment_variable("NEKO_GS_STRTGY", env_strtgy, &
                  env_len)
             if (env_len .eq. 0) then
                allocate(tmp(dofmap%size()))
                call device_map(tmp, tmp_d, dofmap%size())
                tmp = 1.0_rp
                call device_memcpy(tmp, tmp_d, dofmap%size(), &
                     host_to_device, sync = .false.)
                call gs_op_vector(gs, tmp, dofmap%size(), gs_op_add)
 
                do i = 1, size(strtgy)
                   c%nb_strtgy = strtgy(i)
                   call device_sync
                   call mpi_barrier(neko_comm)
                   strtgy_time(i) = mpi_wtime()
                   do j = 1, 100
                      call gs_op_vector(gs, tmp, dofmap%size(), gs_op_add)
                   end do
                   strtgy_time(i) = (mpi_wtime() - strtgy_time(i)) / 100d0
                end do
 
                call device_deassociate(tmp)
                call device_free(tmp_d)
                deallocate(tmp)
 
                c%nb_strtgy = strtgy(minloc(strtgy_time, 1))
 
                avg_strtgy = minloc(strtgy_time, 1)
                call mpi_allreduce(mpi_in_place, avg_strtgy, 1, &
                     mpi_integer, mpi_sum, neko_comm)
                avg_strtgy = avg_strtgy / pe_size
 
                write(log_buf, '(A,B0.2,A)') 'Avg. strtgy  :         [', &
                     strtgy(avg_strtgy), ']'
 
             else
                read(env_strtgy(1:env_len), *) i
 
                if (i .lt. 1 .or. i .gt. 4) then
                   call neko_error('Invalid gs sync strtgy')
                end if
 
                c%nb_strtgy = strtgy(i)
                avg_strtgy = i
 
                write(log_buf, '(A,B0.2,A)') 'Env. strtgy  :         [', &
                     strtgy(avg_strtgy), ']'
             end if
 
             call neko_log%message(log_buf)
 
          end select
       end if
    end if
 
    call neko_log%end_section()
 
  end subroutine gs_init
 
  subroutine gs_free(gs)
    class(gs_t), intent(inout) :: gs
 
    nullify(gs%dofmap)
 
    if (allocated(gs%local_gs)) then
       deallocate(gs%local_gs)
    end if
 
    if (allocated(gs%local_dof_gs)) then
       deallocate(gs%local_dof_gs)
    end if
 
    if (allocated(gs%local_gs_dof)) then
       deallocate(gs%local_gs_dof)
    end if
 
    if (allocated(gs%local_blk_len)) then
       deallocate(gs%local_blk_len)
    end if
 
    if (allocated(gs%local_blk_off)) then
       deallocate(gs%local_blk_off)
    end if
 
    if (allocated(gs%shared_gs)) then
       deallocate(gs%shared_gs)
    end if
 
    if (allocated(gs%shared_dof_gs)) then
       deallocate(gs%shared_dof_gs)
    end if
 
    if (allocated(gs%shared_gs_dof)) then
       deallocate(gs%shared_gs_dof)
    end if
 
    if (allocated(gs%shared_blk_len)) then
       deallocate(gs%shared_blk_len)
    end if
 
    if (allocated(gs%shared_blk_off)) then
       deallocate(gs%shared_blk_off)
    end if
 
    gs%nlocal = 0
    gs%nshared = 0
    gs%nlocal_blks = 0
    gs%nshared_blks = 0
 
    call gs%shared_dofs%free()
 
    if (allocated(gs%bcknd)) then
       call gs%bcknd%free()
       deallocate(gs%bcknd)
    end if
 
    if (allocated(gs%comm)) then
       call gs%comm%free()
       deallocate(gs%comm)
    end if
 
  end subroutine gs_free
 
  subroutine gs_init_mapping(gs)
    type(gs_t), target, intent(inout) :: gs
    type(mesh_t), pointer :: msh
    type(dofmap_t), pointer :: dofmap
    type(stack_i4_t), target :: local_dof, dof_local, shared_dof, dof_shared
    type(stack_i4_t), target :: local_face_dof, face_dof_local
    type(stack_i4_t), target :: shared_face_dof, face_dof_shared
    integer :: i, j, k, l, lx, ly, lz, max_id, max_sid, id, lid, dm_size
    type(htable_i8_t) :: dm
    type(htable_i8_t), pointer :: sdm
 
    dofmap => gs%dofmap
    msh => dofmap%msh
    sdm => gs%shared_dofs
 
    lx = dofmap%Xh%lx
    ly = dofmap%Xh%ly
    lz = dofmap%Xh%lz
    dm_size = dofmap%size()/lx
 
    call dm%init(dm_size, i)
    call sdm%init(dofmap%size(), i)
 
 
    call local_dof%init()
    call dof_local%init()
 
    call local_face_dof%init()
    call face_dof_local%init()
 
    call shared_dof%init()
    call dof_shared%init()
 
    call shared_face_dof%init()
    call face_dof_shared%init()
 
    !
    ! Setup mapping for dofs points
    !
 
    max_id = 0
    max_sid = 0
    do i = 1, msh%nelv
       ! Local id of vertices
       lid = linear_index(1, 1, 1, i, lx, ly, lz)
       ! Check if this dof is shared among ranks or not
       if (dofmap%shared_dof(1, 1, 1, i)) then
          id = gs_mapping_add_dof(sdm, dofmap%dof(1, 1, 1, i), max_sid)
          !If add unique gather-scatter id to shared_dof stack
          call shared_dof%push(id)
          !If add local id to dof_shared stack
          call dof_shared%push(lid)
          !Now we have the mapping of local id <-> gather scatter id!
       else
          ! Same here, only here we know the point is local
          ! It will as such not need to be sent to other ranks later
          id = gs_mapping_add_dof(dm, dofmap%dof(1, 1, 1, i), max_id)
          call local_dof%push(id)
          call dof_local%push(lid)
       end if
       ! This procedure is then repeated for all vertices and edges
       ! Facets can be treated a little bit differently since they only have one
       ! neighbor
 
       lid = linear_index(lx, 1, 1, i, lx, ly, lz)
       if (dofmap%shared_dof(lx, 1, 1, i)) then
          id = gs_mapping_add_dof(sdm, dofmap%dof(lx, 1, 1, i), max_sid)
          call shared_dof%push(id)
          call dof_shared%push(lid)
       else
          id = gs_mapping_add_dof(dm, dofmap%dof(lx, 1, 1, i), max_id)
          call local_dof%push(id)
          call dof_local%push(lid)
       end if
 
       lid = linear_index(1, ly, 1, i, lx, ly, lz)
       if (dofmap%shared_dof(1, ly, 1, i)) then
          id = gs_mapping_add_dof(sdm, dofmap%dof(1, ly, 1, i), max_sid)
          call shared_dof%push(id)
          call dof_shared%push(lid)
       else
          id = gs_mapping_add_dof(dm, dofmap%dof(1, ly, 1, i), max_id)
          call local_dof%push(id)
          call dof_local%push(lid)
       end if
 
       lid = linear_index(lx, ly, 1, i, lx, ly, lz)
       if (dofmap%shared_dof(lx, ly, 1, i)) then
          id = gs_mapping_add_dof(sdm, dofmap%dof(lx, ly, 1, i), max_sid)
          call shared_dof%push(id)
          call dof_shared%push(lid)
       else
          id = gs_mapping_add_dof(dm, dofmap%dof(lx, ly, 1, i), max_id)
          call local_dof%push(id)
          call dof_local%push(lid)
       end if
       if (lz .gt. 1) then
          lid = linear_index(1, 1, lz, i, lx, ly, lz)
          if (dofmap%shared_dof(1, 1, lz, i)) then
             id = gs_mapping_add_dof(sdm, dofmap%dof(1, 1, lz, i), max_sid)
             call shared_dof%push(id)
             call dof_shared%push(lid)
          else
             id = gs_mapping_add_dof(dm, dofmap%dof(1, 1, lz, i), max_id)
             call local_dof%push(id)
             call dof_local%push(lid)
          end if
 
          lid = linear_index(lx, 1, lz, i, lx, ly, lz)
          if (dofmap%shared_dof(lx, 1, lz, i)) then
             id = gs_mapping_add_dof(sdm, dofmap%dof(lx, 1, lz, i), max_sid)
             call shared_dof%push(id)
             call dof_shared%push(lid)
          else
             id = gs_mapping_add_dof(dm, dofmap%dof(lx, 1, lz, i), max_id)
             call local_dof%push(id)
             call dof_local%push(lid)
          end if
 
          lid = linear_index(1, ly, lz, i, lx, ly, lz)
          if (dofmap%shared_dof(1, ly, lz, i)) then
             id = gs_mapping_add_dof(sdm, dofmap%dof(1, ly, lz, i), max_sid)
             call shared_dof%push(id)
             call dof_shared%push(lid)
          else
             id = gs_mapping_add_dof(dm, dofmap%dof(1, ly, lz, i), max_id)
             call local_dof%push(id)
             call dof_local%push(lid)
          end if
 
          lid = linear_index(lx, ly, lz, i, lx, ly, lz)
          if (dofmap%shared_dof(lx, ly, lz, i)) then
             id = gs_mapping_add_dof(sdm, dofmap%dof(lx, ly, lz, i), max_sid)
             call shared_dof%push(id)
             call dof_shared%push(lid)
          else
             id = gs_mapping_add_dof(dm, dofmap%dof(lx, ly, lz, i), max_id)
             call local_dof%push(id)
             call dof_local%push(lid)
          end if
       end if
    end do
 
    ! Clear local dofmap table
    call dm%clear()
    ! Get gather scatter ids and local ids of edges
    if (lz .gt. 1) then
       !
       ! Setup mapping for dofs on edges
       !
       do i = 1, msh%nelv
 
          !
          ! dofs on edges in x-direction
          !
          if (dofmap%shared_dof(2, 1, 1, i)) then
             do j = 2, lx - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(j, 1, 1, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(j, 1, 1, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do j = 2, lx - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(j, 1, 1, i), max_id)
                call local_dof%push(id)
                id = linear_index(j, 1, 1, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          if (dofmap%shared_dof(2, 1, lz, i)) then
             do j = 2, lx - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(j, 1, lz, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(j, 1, lz, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do j = 2, lx - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(j, 1, lz, i), max_id)
                call local_dof%push(id)
                id = linear_index(j, 1, lz, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(2, ly, 1, i)) then
             do j = 2, lx - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(j, ly, 1, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(j, ly, 1, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
 
          else
             do j = 2, lx - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(j, ly, 1, i), max_id)
                call local_dof%push(id)
                id = linear_index(j, ly, 1, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          if (dofmap%shared_dof(2, ly, lz, i)) then
             do j = 2, lx - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(j, ly, lz, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(j, ly, lz, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do j = 2, lx - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(j, ly, lz, i), max_id)
                call local_dof%push(id)
                id = linear_index(j, ly, lz, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          !
          ! dofs on edges in y-direction
          !
          if (dofmap%shared_dof(1, 2, 1, i)) then
             do k = 2, ly - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(1, k, 1, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(1, k, 1, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do k = 2, ly - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(1, k, 1, i), max_id)
                call local_dof%push(id)
                id = linear_index(1, k, 1, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          if (dofmap%shared_dof(1, 2, lz, i)) then
             do k = 2, ly - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(1, k, lz, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(1, k, lz, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do k = 2, ly - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(1, k, lz, i), max_id)
                call local_dof%push(id)
                id = linear_index(1, k, lz, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(lx, 2, 1, i)) then
             do k = 2, ly - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(lx, k, 1, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(lx, k, 1, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do k = 2, ly - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(lx, k, 1, i), max_id)
                call local_dof%push(id)
                id = linear_index(lx, k, 1, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          if (dofmap%shared_dof(lx, 2, lz, i)) then
             do k = 2, ly - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(lx, k, lz, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(lx, k, lz, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do k = 2, ly - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(lx, k, lz, i), max_id)
                call local_dof%push(id)
                id = linear_index(lx, k, lz, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          !
          ! dofs on edges in z-direction
          !
          if (dofmap%shared_dof(1, 1, 2, i)) then
             do l = 2, lz - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(1, 1, l, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(1, 1, l, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do l = 2, lz - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(1, 1, l, i), max_id)
                call local_dof%push(id)
                id = linear_index(1, 1, l, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(lx, 1, 2, i)) then
             do l = 2, lz - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(lx, 1, l, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(lx, 1, l, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do l = 2, lz - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(lx, 1, l, i), max_id)
                call local_dof%push(id)
                id = linear_index(lx, 1, l, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(1, ly, 2, i)) then
             do l = 2, lz - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(1, ly, l, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(1, ly, l, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do l = 2, lz - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(1, ly, l, i), max_id)
                call local_dof%push(id)
                id = linear_index(1, ly, l, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(lx, ly, 2, i)) then
             do l = 2, lz - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(lx, ly, l, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(lx, ly, l, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do l = 2, lz - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(lx, ly, l, i), max_id)
                call local_dof%push(id)
                id = linear_index(lx, ly, l, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
       end do
    end if
 
    ! Clear local dofmap table
    call dm%clear()
 
    !
    ! Setup mapping for dofs on facets
    !
    ! This is for 2d
    if (lz .eq. 1) then
       do i = 1, msh%nelv
 
          !
          ! dofs on edges in x-direction
          !
          if (msh%facet_neigh(3, i) .ne. 0) then
             if (dofmap%shared_dof(2, 1, 1, i)) then
                do j = 2, lx - 1
                   id = gs_mapping_add_dof(sdm, dofmap%dof(j, 1, 1, i), max_sid)
                   call shared_face_dof%push(id)
                   id = linear_index(j, 1, 1, i, lx, ly, lz)
                   call face_dof_shared%push(id)
                end do
             else
                do j = 2, lx - 1
                   id = gs_mapping_add_dof(dm, dofmap%dof(j, 1, 1, i), max_id)
                   call local_face_dof%push(id)
                   id = linear_index(j, 1, 1, i, lx, ly, lz)
                   call face_dof_local%push(id)
                end do
             end if
          end if
 
          if (msh%facet_neigh(4, i) .ne. 0) then
             if (dofmap%shared_dof(2, ly, 1, i)) then
                do j = 2, lx - 1
                   id = gs_mapping_add_dof(sdm, dofmap%dof(j, ly, 1, i), &
                        max_sid)
                   call shared_face_dof%push(id)
                   id = linear_index(j, ly, 1, i, lx, ly, lz)
                   call face_dof_shared%push(id)
                end do
 
             else
                do j = 2, lx - 1
                   id = gs_mapping_add_dof(dm, dofmap%dof(j, ly, 1, i), &
                        max_id)
                   call local_face_dof%push(id)
                   id = linear_index(j, ly, 1, i, lx, ly, lz)
                   call face_dof_local%push(id)
                end do
             end if
          end if
 
          !
          ! dofs on edges in y-direction
          !
          if (msh%facet_neigh(1, i) .ne. 0) then
             if (dofmap%shared_dof(1, 2, 1, i)) then
                do k = 2, ly - 1
                   id = gs_mapping_add_dof(sdm, dofmap%dof(1, k, 1, i), max_sid)
                   call shared_face_dof%push(id)
                   id = linear_index(1, k, 1, i, lx, ly, lz)
                   call face_dof_shared%push(id)
                end do
             else
                do k = 2, ly - 1
                   id = gs_mapping_add_dof(dm, dofmap%dof(1, k, 1, i), max_id)
                   call local_face_dof%push(id)
                   id = linear_index(1, k, 1, i, lx, ly, lz)
                   call face_dof_local%push(id)
                end do
             end if
          end if
 
          if (msh%facet_neigh(2, i) .ne. 0) then
             if (dofmap%shared_dof(lx, 2, 1, i)) then
                do k = 2, ly - 1
                   id = gs_mapping_add_dof(sdm, dofmap%dof(lx, k, 1, i), &
                        max_sid)
                   call shared_face_dof%push(id)
                   id = linear_index(lx, k, 1, i, lx, ly, lz)
                   call face_dof_shared%push(id)
                end do
             else
                do k = 2, ly - 1
                   id = gs_mapping_add_dof(dm, dofmap%dof(lx, k, 1, i), &
                        max_id)
                   call local_face_dof%push(id)
                   id = linear_index(lx, k, 1, i, lx, ly, lz)
                   call face_dof_local%push(id)
                end do
             end if
          end if
       end do
    else
       do i = 1, msh%nelv
 
          ! Facets in x-direction (s, t)-plane
          if (msh%facet_neigh(1, i) .ne. 0) then
             if (dofmap%shared_dof(1, 2, 2, i)) then
                do l = 2, lz - 1
                   do k = 2, ly - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(1, k, l, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(1, k, l, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do l = 2, lz - 1
                   do k = 2, ly - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(1, k, l, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(1, k, l, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          if (msh%facet_neigh(2, i) .ne. 0) then
             if (dofmap%shared_dof(lx, 2, 2, i)) then
                do l = 2, lz - 1
                   do k = 2, ly - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(lx, k, l, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(lx, k, l, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do l = 2, lz - 1
                   do k = 2, ly - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(lx, k, l, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(lx, k, l, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          ! Facets in y-direction (r, t)-plane
          if (msh%facet_neigh(3, i) .ne. 0) then
             if (dofmap%shared_dof(2, 1, 2, i)) then
                do l = 2, lz - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(j, 1, l, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(j, 1, l, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do l = 2, lz - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(j, 1, l, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(j, 1, l, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          if (msh%facet_neigh(4, i) .ne. 0) then
             if (dofmap%shared_dof(2, ly, 2, i)) then
                do l = 2, lz - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(j, ly, l, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(j, ly, l, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do l = 2, lz - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(j, ly, l, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(j, ly, l, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          ! Facets in z-direction (r, s)-plane
          if (msh%facet_neigh(5, i) .ne. 0) then
             if (dofmap%shared_dof(2, 2, 1, i)) then
                do k = 2, ly - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(j, k, 1, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(j, k, 1, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do k = 2, ly - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(j, k, 1, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(j, k, 1, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          if (msh%facet_neigh(6, i) .ne. 0) then
             if (dofmap%shared_dof(2, 2, lz, i)) then
                do k = 2, ly - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(j, k, lz, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(j, k, lz, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do k = 2, ly - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(j, k, lz, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(j, k, lz, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
       end do
    end if
 
 
    call dm%free()
 
    gs%nlocal = local_dof%size() + local_face_dof%size()
    gs%local_facet_offset = local_dof%size() + 1
 
    ! Finalize local dof to gather-scatter index
    allocate(gs%local_dof_gs(gs%nlocal))
 
    ! Add dofs on points and edges
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => local_dof%data)
    type is (integer)
       j = local_dof%size()
       do i = 1, j
          gs%local_dof_gs(i) = dof_array(i)
       end do
    end select
    call local_dof%free()
 
    ! Add dofs on faces
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => local_face_dof%data)
    type is (integer)
       do i = 1, local_face_dof%size()
          gs%local_dof_gs(i + j) = dof_array(i)
       end do
    end select
    call local_face_dof%free()
 
    ! Finalize local gather-scatter index to dof
    allocate(gs%local_gs_dof(gs%nlocal))
 
    ! Add gather-scatter index on points and edges
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => dof_local%data)
    type is (integer)
       j = dof_local%size()
       do i = 1, j
          gs%local_gs_dof(i) = dof_array(i)
       end do
    end select
    call dof_local%free()
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => face_dof_local%data)
    type is (integer)
       do i = 1, face_dof_local%size()
          gs%local_gs_dof(i+j) = dof_array(i)
       end do
    end select
    call face_dof_local%free()
 
    call gs_qsort_dofmap(gs%local_dof_gs, gs%local_gs_dof, &
         gs%nlocal, 1, gs%nlocal)
 
    call gs_find_blks(gs%local_dof_gs, gs%local_blk_len, &
         gs%local_blk_off, gs%nlocal_blks, gs%nlocal, gs%local_facet_offset)
 
    ! Allocate buffer for local gs-ops
    allocate(gs%local_gs(gs%nlocal))
 
    gs%nshared = shared_dof%size() + shared_face_dof%size()
    gs%shared_facet_offset = shared_dof%size() + 1
 
    ! Finalize shared dof to gather-scatter index
    allocate(gs%shared_dof_gs(gs%nshared))
 
    ! Add shared dofs on points and edges
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => shared_dof%data)
    type is (integer)
       j = shared_dof%size()
       do i = 1, j
          gs%shared_dof_gs(i) = dof_array(i)
       end do
    end select
    call shared_dof%free()
 
    ! Add shared dofs on faces
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => shared_face_dof%data)
    type is (integer)
       do i = 1, shared_face_dof%size()
          gs%shared_dof_gs(i + j) = dof_array(i)
       end do
    end select
    call shared_face_dof%free()
 
    ! Finalize shared gather-scatter index to dof
    allocate(gs%shared_gs_dof(gs%nshared))
 
    ! Add dofs on points and edges
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => dof_shared%data)
    type is (integer)
       j = dof_shared%size()
       do i = 1, j
          gs%shared_gs_dof(i) = dof_array(i)
       end do
    end select
    call dof_shared%free()
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => face_dof_shared%data)
    type is (integer)
       do i = 1, face_dof_shared%size()
          gs%shared_gs_dof(i + j) = dof_array(i)
       end do
    end select
    call face_dof_shared%free()
 
    ! Allocate buffer for shared gs-ops
    allocate(gs%shared_gs(gs%nshared))
 
    if (gs%nshared .gt. 0) then
       call gs_qsort_dofmap(gs%shared_dof_gs, gs%shared_gs_dof, &
            gs%nshared, 1, gs%nshared)
 
       call gs_find_blks(gs%shared_dof_gs, gs%shared_blk_len, &
            gs%shared_blk_off, gs%nshared_blks, gs%nshared, &
            gs%shared_facet_offset)
    end if
 
  contains
 
    function gs_mapping_add_dof(map_, dof, max_id) result(id)
      type(htable_i8_t), intent(inout) :: map_
      integer(kind=i8), intent(inout) :: dof
      integer, intent(inout) :: max_id
      integer :: id
 
      if (map_%get(dof, id) .gt. 0) then
         max_id = max_id + 1
         call map_%set(dof, max_id)
         id = max_id
      end if
 
    end function gs_mapping_add_dof
 
    recursive subroutine gs_qsort_dofmap(dg, gd, n, lo, hi)
      integer, intent(inout) :: n
      integer, dimension(n), intent(inout) :: dg
      integer, dimension(n), intent(inout) :: gd
      integer :: lo, hi
      integer :: tmp, i, j, pivot
 
      i = lo - 1
      j = hi + 1
      pivot = dg((lo + hi) / 2)
      do
         do
            i = i + 1
            if (dg(i) .ge. pivot) exit
         end do
 
         do
            j = j - 1
            if (dg(j) .le. pivot) exit
         end do
 
         if (i .lt. j) then
            tmp = dg(i)
            dg(i) = dg(j)
            dg(j) = tmp
 
            tmp = gd(i)
            gd(i) = gd(j)
            gd(j) = tmp
         else if (i .eq. j) then
            i = i + 1
            exit
         else
            exit
         end if
      end do
      if (lo .lt. j) call gs_qsort_dofmap(dg, gd, n, lo, j)
      if (i .lt. hi) call gs_qsort_dofmap(dg, gd, n, i, hi)
 
    end subroutine gs_qsort_dofmap
 
    subroutine gs_find_blks(dg, blk_len, blk_off, nblks, n, m)
      integer, intent(in) :: n
      integer, intent(in) :: m
      integer, dimension(n), intent(inout) :: dg
      integer, allocatable, intent(inout) :: blk_len(:)
      integer, allocatable, intent(inout) :: blk_off(:)
      integer, intent(inout) :: nblks
      integer :: i, j
      integer :: id, count
      type(stack_i4_t), target :: blks
 
      call blks%init()
      i = 1
      do while (i .lt. m)
         id = dg(i)
         count = 1
         j = i
         do while ( j+1 .le. n .and. dg(j+1) .eq. id)
            j = j + 1
            count = count + 1
         end do
         call blks%push(count)
         i = j + 1
      end do
 
      select type (blk_array => blks%data)
      type is (integer)
         nblks = blks%size()
         allocate(blk_len(nblks))
         do i = 1, nblks
            blk_len(i) = blk_array(i)
         end do
         allocate(blk_off(nblks))
         blk_off(1) = 0
         do i = 2, nblks
            blk_off(i) = blk_off(i - 1) + blk_len(i - 1)
         end do
      end select
      call blks%free()
 
    end subroutine gs_find_blks
 
  end subroutine gs_init_mapping
 
  subroutine gs_schedule(gs)
    type(gs_t), target, intent(inout) :: gs
    integer(kind=i8), allocatable :: send_buf(:), recv_buf(:)
    integer(kind=i2), allocatable :: shared_flg(:), recv_flg(:)
    type(htable_iter_i8_t) :: it
    type(stack_i4_t) :: send_pe, recv_pe
    type(mpi_status) :: status
    type(mpi_request) :: send_req, recv_req
    integer :: i, j, max_recv, src, dst, ierr, n_recv
    integer :: tmp, shared_gs_id
    integer :: nshared_unique
 
    nshared_unique = gs%shared_dofs%num_entries()
 
    call it%init(gs%shared_dofs)
    allocate(send_buf(nshared_unique))
    i = 1
    do while (it%next())
       send_buf(i) = it%key()
       i = i + 1
    end do
 
    call send_pe%init()
    call recv_pe%init()
 
 
    !
    ! Schedule exchange of shared dofs
    !
 
    call mpi_allreduce(nshared_unique, max_recv, 1, &
         mpi_integer, mpi_max, neko_comm, ierr)
 
    allocate(recv_buf(max_recv))
    allocate(shared_flg(max_recv))
    allocate(recv_flg(max_recv))
 
    do i = 1, size(gs%dofmap%msh%neigh_order)
       src = modulo(pe_rank - gs%dofmap%msh%neigh_order(i) + pe_size, pe_size)
       dst = modulo(pe_rank + gs%dofmap%msh%neigh_order(i), pe_size)
 
       if (gs%dofmap%msh%neigh(src)) then
          call mpi_irecv(recv_buf, max_recv, mpi_integer8, &
               src, 0, neko_comm, recv_req, ierr)
       end if
 
       if (gs%dofmap%msh%neigh(dst)) then
          call mpi_isend(send_buf, nshared_unique, mpi_integer8, &
               dst, 0, neko_comm, send_req, ierr)
       end if
 
       if (gs%dofmap%msh%neigh(src)) then
          call mpi_wait(recv_req, status, ierr)
          call mpi_get_count(status, mpi_integer8, n_recv, ierr)
 
          do j = 1, n_recv
             shared_flg(j) = gs%shared_dofs%get(recv_buf(j), shared_gs_id)
             if (shared_flg(j) .eq. 0) then
                call gs%comm%recv_dof(src)%push(shared_gs_id)
             end if
          end do
 
          if (gs%comm%recv_dof(src)%size() .gt. 0) then
             call recv_pe%push(src)
          end if
       end if
 
       if (gs%dofmap%msh%neigh(dst)) then
          call mpi_wait(send_req, mpi_status_ignore, ierr)
          call mpi_irecv(recv_flg, max_recv, mpi_integer2, &
               dst, 0, neko_comm, recv_req, ierr)
       end if
 
       if (gs%dofmap%msh%neigh(src)) then
          call mpi_isend(shared_flg, n_recv, mpi_integer2, &
               src, 0, neko_comm, send_req, ierr)
       end if
 
       if (gs%dofmap%msh%neigh(dst)) then
          call mpi_wait(recv_req, status, ierr)
          call mpi_get_count(status, mpi_integer2, n_recv, ierr)
 
          do j = 1, n_recv
             if (recv_flg(j) .eq. 0) then
                tmp = gs%shared_dofs%get(send_buf(j), shared_gs_id)
                call gs%comm%send_dof(dst)%push(shared_gs_id)
             end if
          end do
 
          if (gs%comm%send_dof(dst)%size() .gt. 0) then
             call send_pe%push(dst)
          end if
       end if
 
       if (gs%dofmap%msh%neigh(src)) then
          call mpi_wait(send_req, mpi_status_ignore, ierr)
       end if
 
    end do
 
    call gs%comm%init(send_pe, recv_pe)
 
    call send_pe%free()
    call recv_pe%free()
 
    deallocate(send_buf)
    deallocate(recv_flg)
    deallocate(shared_flg)
    !This arrays seems to take massive amounts of memory...
    call gs%shared_dofs%free()
 
  end subroutine gs_schedule
 
  subroutine gs_op_fld(gs, u, op, event)
    class(gs_t), intent(inout) :: gs
    type(field_t), intent(inout) :: u
    type(c_ptr), optional, intent(inout) :: event
    integer :: n, op
 
    n = u%msh%nelv * u%Xh%lx * u%Xh%ly * u%Xh%lz
    if (present(event)) then
       call gs_op_vector(gs, u%x, n, op, event)
    else
       call gs_op_vector(gs, u%x, n, op)
    end if
 
  end subroutine gs_op_fld
 
  subroutine gs_op_r4(gs, u, n, op, event)
    class(gs_t), intent(inout) :: gs
    integer, intent(in) :: n
    real(kind=rp), contiguous, dimension(:,:,:,:), intent(inout) :: u
    type(c_ptr), optional, intent(inout) :: event
    integer :: op
 
    if (present(event)) then
       call gs_op_vector(gs, u, n, op, event)
    else
       call gs_op_vector(gs, u, n, op)
    end if
 
  end subroutine gs_op_r4
 
  subroutine gs_op_vector(gs, u, n, op, event)
    class(gs_t), intent(inout) :: gs
    integer, intent(in) :: n
    real(kind=rp), dimension(n), intent(inout) :: u
    type(c_ptr), optional, intent(inout) :: event
    integer :: m, l, op, lo, so
 
    lo = gs%local_facet_offset
    so = -gs%shared_facet_offset
    m = gs%nlocal
    l = gs%nshared
 
    call profiler_start_region("gather_scatter", 5)
    ! Gather shared dofs
    if (pe_size .gt. 1 .and. n .gt. 0) then
       call profiler_start_region("gs_nbrecv", 13)
       call gs%comm%nbrecv()
       call profiler_end_region("gs_nbrecv", 13)
       call profiler_start_region("gs_gather_shared", 14)
       call gs%bcknd%gather(gs%shared_gs, l, so, gs%shared_dof_gs, u, n, &
            gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
            gs%shared_blk_off, op, .true.)
       call profiler_end_region("gs_gather_shared", 14)
       call profiler_start_region("gs_nbsend", 6)
       call gs%comm%nbsend(gs%shared_gs, l, &
            gs%bcknd%gather_event, gs%bcknd%gs_stream)
       call profiler_end_region("gs_nbsend", 6)
 
    end if
 
    ! Gather-scatter local dofs
    call profiler_start_region("gs_local", 12)
    call gs%bcknd%gather(gs%local_gs, m, lo, gs%local_dof_gs, u, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         op, .false.)
    call gs%bcknd%scatter(gs%local_gs, m, gs%local_dof_gs, u, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         .false., c_null_ptr)
    call profiler_end_region("gs_local", 12)
    ! Scatter shared dofs
    if (pe_size .gt. 1 .and. n .gt. 0) then
       call profiler_start_region("gs_nbwait", 7)
       call gs%comm%nbwait(gs%shared_gs, l, op, gs%bcknd%gs_stream)
       call profiler_end_region("gs_nbwait", 7)
       call profiler_start_region("gs_scatter_shared", 15)
       if (present(event)) then
          call gs%bcknd%scatter(gs%shared_gs, l,&
               gs%shared_dof_gs, u, n, &
               gs%shared_gs_dof, gs%nshared_blks, &
               gs%shared_blk_len, gs%shared_blk_off, .true., event)
       else
          call gs%bcknd%scatter(gs%shared_gs, l,&
               gs%shared_dof_gs, u, n, &
               gs%shared_gs_dof, gs%nshared_blks, &
               gs%shared_blk_len, gs%shared_blk_off, .true., c_null_ptr)
       end if
       call profiler_end_region("gs_scatter_shared", 15)
    end if
 
    call profiler_end_region("gather_scatter", 5)
 
  end subroutine gs_op_vector
 
end module gather_scatter