MPI.h

// Copyright (C) 2007-2014 Magnus Vikstrøm and Garth N. Wells
//
// This file is part of DOLFINx (https://www.fenicsproject.org)
//
// SPDX-License-Identifier:    LGPL-3.0-or-later
 
#pragma once
 
#include <array>
#include <cassert>
#include <complex>
#include <cstdint>
#include <dolfinx/graph/AdjacencyList.h>
#include <iostream>
#include <numeric>
#include <set>
#include <tuple>
#include <type_traits>
#include <utility>
#include <vector>
 
#define MPICH_IGNORE_CXX_SEEK 1
#include <mpi.h>
 
namespace dolfinx::MPI
{
 
class Comm
{
public:
  explicit Comm(MPI_Comm comm, bool duplicate = true);
 
  Comm(const Comm& comm) noexcept;
 
  Comm(Comm&& comm) noexcept;
 
  // Disable copy assignment operator
  Comm& operator=(const Comm& comm) = delete;
 
  Comm& operator=(Comm&& comm) noexcept;
 
  ~Comm();
 
  MPI_Comm comm() const noexcept;
 
private:
  // MPI communicator
  MPI_Comm _comm;
};
 
int rank(MPI_Comm comm);
 
int size(MPI_Comm comm);
 
template <typename T>
graph::AdjacencyList<T> all_to_all(MPI_Comm comm,
                                   const graph::AdjacencyList<T>& send_data);
 
std::vector<int> compute_graph_edges(MPI_Comm comm, const std::set<int>& edges);
 
template <typename T>
graph::AdjacencyList<T>
neighbor_all_to_all(MPI_Comm neighbor_comm,
                    const graph::AdjacencyList<T>& send_data);
 
std::tuple<std::vector<int>, std::vector<int>>
neighbors(MPI_Comm neighbor_comm);
 
constexpr std::array<std::int64_t, 2> local_range(int rank, std::int64_t N,
                                                  int size)
{
  assert(rank >= 0);
  assert(N >= 0);
  assert(size > 0);
 
  // Compute number of items per rank and remainder
  const std::int64_t n = N / size;
  const std::int64_t r = N % size;
 
  // Compute local range
  if (rank < r)
    return {{rank * (n + 1), rank * (n + 1) + n + 1}};
  else
    return {{rank * n + r, rank * n + r + n}};
}
 
constexpr int index_owner(int size, std::size_t index, std::size_t N)
{
  assert(index < N);
 
  // Compute number of items per rank and remainder
  const std::size_t n = N / size;
  const std::size_t r = N % size;
 
  // First r ranks own n + 1 indices
  if (index < r * (n + 1))
    return index / (n + 1);
 
  // Remaining ranks own n indices
  return r + (index - r * (n + 1)) / n;
}
 
template <typename T>
struct dependent_false : std::false_type
{
};
 
template <typename T>
constexpr MPI_Datatype mpi_type()
{
  if constexpr (std::is_same<T, float>::value)
    return MPI_FLOAT;
  else if constexpr (std::is_same<T, double>::value)
    return MPI_DOUBLE;
  else if constexpr (std::is_same<T, std::complex<double>>::value)
    return MPI_DOUBLE_COMPLEX;
  else if constexpr (std::is_same<T, short int>::value)
    return MPI_SHORT;
  else if constexpr (std::is_same<T, int>::value)
    return MPI_INT;
  else if constexpr (std::is_same<T, unsigned int>::value)
    return MPI_UNSIGNED;
  else if constexpr (std::is_same<T, long int>::value)
    return MPI_LONG;
  else if constexpr (std::is_same<T, unsigned long>::value)
    return MPI_UNSIGNED_LONG;
  else if constexpr (std::is_same<T, long long>::value)
    return MPI_LONG_LONG;
  else if constexpr (std::is_same<T, unsigned long long>::value)
    return MPI_UNSIGNED_LONG_LONG;
  else if constexpr (std::is_same<T, bool>::value)
    return MPI_C_BOOL;
}
 
//---------------------------------------------------------------------------
template <typename T>
graph::AdjacencyList<T> all_to_all(MPI_Comm comm,
                                   const graph::AdjacencyList<T>& send_data)
{
  const std::vector<std::int32_t>& send_offsets = send_data.offsets();
  const std::vector<T>& values_in = send_data.array();
 
  const int comm_size = MPI::size(comm);
  assert(send_data.num_nodes() == comm_size);
 
  // Data size per destination rank
  std::vector<int> send_size(comm_size);
  std::adjacent_difference(std::next(send_offsets.begin(), +1),
                           send_offsets.end(), send_size.begin());
 
  // Get received data sizes from each rank
  std::vector<int> recv_size(comm_size);
  MPI_Alltoall(send_size.data(), 1, mpi_type<int>(), recv_size.data(), 1,
               mpi_type<int>(), comm);
 
  // Compute receive offset
  std::vector<std::int32_t> recv_offset(comm_size + 1);
  recv_offset[0] = 0;
  std::partial_sum(recv_size.begin(), recv_size.end(),
                   std::next(recv_offset.begin()));
 
  // Send/receive data
  std::vector<T> recv_values(recv_offset[comm_size]);
  MPI_Alltoallv(values_in.data(), send_size.data(), send_offsets.data(),
                mpi_type<T>(), recv_values.data(), recv_size.data(),
                recv_offset.data(), mpi_type<T>(), comm);
 
  return graph::AdjacencyList<T>(std::move(recv_values),
                                 std::move(recv_offset));
}
//-----------------------------------------------------------------------------
template <typename T>
graph::AdjacencyList<T>
neighbor_all_to_all(MPI_Comm neighbor_comm,
                    const graph::AdjacencyList<T>& send_data)
{
  // Get neighbor processes
  int indegree(-1), outdegree(-2), weighted(-1);
  MPI_Dist_graph_neighbors_count(neighbor_comm, &indegree, &outdegree,
                                 &weighted);
 
  // Allocate memory (add '1' to handle empty case as OpenMPI fails for
  // null pointers
  std::vector<int> send_sizes(outdegree + 1, 0);
  std::vector<int> recv_sizes(indegree + 1);
  std::adjacent_difference(std::next(send_data.offsets().begin()),
                           send_data.offsets().end(), send_sizes.begin());
  // Get receive sizes
  MPI_Neighbor_alltoall(send_sizes.data(), 1, MPI::mpi_type<int>(),
                        recv_sizes.data(), 1, MPI::mpi_type<int>(),
                        neighbor_comm);
 
  // Work out recv offsets. Note use of std::prev to handle OpenMPI
  // issue mentioned above
  std::vector<int> recv_offsets(indegree + 1);
  recv_offsets[0] = 0;
  std::partial_sum(recv_sizes.begin(), std::prev(recv_sizes.end()),
                   std::next(recv_offsets.begin(), 1));
 
  std::vector<T> recv_data(recv_offsets[recv_offsets.size() - 1]);
  MPI_Neighbor_alltoallv(
      send_data.array().data(), send_sizes.data(), send_data.offsets().data(),
      MPI::mpi_type<T>(), recv_data.data(), recv_sizes.data(),
      recv_offsets.data(), MPI::mpi_type<T>(), neighbor_comm);
 
  return graph::AdjacencyList<T>(std::move(recv_data), std::move(recv_offsets));
}
//---------------------------------------------------------------------------
 
} // namespace dolfinx::MPI