mpi.hpp

// SPDX-FileCopyrightText: 2017 - 2025 The Ginkgo authors
//
// SPDX-License-Identifier: BSD-3-Clause
 
#ifndef GKO_PUBLIC_CORE_BASE_MPI_HPP_
#define GKO_PUBLIC_CORE_BASE_MPI_HPP_
 
 
#include <memory>
#include <type_traits>
#include <utility>
 
#include <ginkgo/config.hpp>
#include <ginkgo/core/base/exception.hpp>
#include <ginkgo/core/base/exception_helpers.hpp>
#include <ginkgo/core/base/executor.hpp>
#include <ginkgo/core/base/half.hpp>
#include <ginkgo/core/base/types.hpp>
#include <ginkgo/core/base/utils_helper.hpp>
 
 
#if GINKGO_BUILD_MPI
 
 
#include <mpi.h>
 
 
namespace gko {
namespace experimental {
namespace mpi {
 
 
inline constexpr bool is_gpu_aware()
{
#if GINKGO_HAVE_GPU_AWARE_MPI
    return true;
#else
    return false;
#endif
}
 
 
int map_rank_to_device_id(MPI_Comm comm, int num_devices);
 
 
#define GKO_REGISTER_MPI_TYPE(input_type, mpi_type)         \
    template <>                                             \
    struct type_impl<input_type> {                          \
        static MPI_Datatype get_type() { return mpi_type; } \
    }
 
template <typename T>
struct type_impl {};
 
 
GKO_REGISTER_MPI_TYPE(char, MPI_CHAR);
GKO_REGISTER_MPI_TYPE(unsigned char, MPI_UNSIGNED_CHAR);
GKO_REGISTER_MPI_TYPE(unsigned, MPI_UNSIGNED);
GKO_REGISTER_MPI_TYPE(int, MPI_INT);
GKO_REGISTER_MPI_TYPE(unsigned short, MPI_UNSIGNED_SHORT);
GKO_REGISTER_MPI_TYPE(unsigned long, MPI_UNSIGNED_LONG);
GKO_REGISTER_MPI_TYPE(long, MPI_LONG);
GKO_REGISTER_MPI_TYPE(long long, MPI_LONG_LONG_INT);
GKO_REGISTER_MPI_TYPE(unsigned long long, MPI_UNSIGNED_LONG_LONG);
GKO_REGISTER_MPI_TYPE(float, MPI_FLOAT);
GKO_REGISTER_MPI_TYPE(double, MPI_DOUBLE);
GKO_REGISTER_MPI_TYPE(long double, MPI_LONG_DOUBLE);
#if GINKGO_ENABLE_HALF
// OpenMPI 5.0 have support from MPIX_C_FLOAT16 and MPICHv3.4a1 MPIX_C_FLOAT16
// Only OpenMPI support complex half
// TODO: use native type when mpi is configured with half feature
GKO_REGISTER_MPI_TYPE(half, MPI_UNSIGNED_SHORT);
GKO_REGISTER_MPI_TYPE(std::complex<half>, MPI_FLOAT);
#endif  // GKO_ENABLE_HALF
GKO_REGISTER_MPI_TYPE(std::complex<float>, MPI_C_FLOAT_COMPLEX);
GKO_REGISTER_MPI_TYPE(std::complex<double>, MPI_C_DOUBLE_COMPLEX);
 
 
class contiguous_type {
public:
    contiguous_type(int count, MPI_Datatype old_type) : type_(MPI_DATATYPE_NULL)
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Type_contiguous(count, old_type, &type_));
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Type_commit(&type_));
    }
 
    contiguous_type() : type_(MPI_DATATYPE_NULL) {}
 
    contiguous_type(const contiguous_type&) = delete;
 
    contiguous_type& operator=(const contiguous_type&) = delete;
 
    contiguous_type(contiguous_type&& other) noexcept : type_(MPI_DATATYPE_NULL)
    {
        *this = std::move(other);
    }
 
    contiguous_type& operator=(contiguous_type&& other) noexcept
    {
        if (this != &other) {
            this->type_ = std::exchange(other.type_, MPI_DATATYPE_NULL);
        }
        return *this;
    }
 
    ~contiguous_type()
    {
        if (type_ != MPI_DATATYPE_NULL) {
            MPI_Type_free(&type_);
        }
    }
 
    MPI_Datatype get() const { return type_; }
 
private:
    MPI_Datatype type_;
};
 
 
enum class thread_type {
    serialized = MPI_THREAD_SERIALIZED,
    funneled = MPI_THREAD_FUNNELED,
    single = MPI_THREAD_SINGLE,
    multiple = MPI_THREAD_MULTIPLE
};
 
 
class environment {
public:
    static bool is_finalized()
    {
        int flag = 0;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Finalized(&flag));
        return flag;
    }
 
    static bool is_initialized()
    {
        int flag = 0;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Initialized(&flag));
        return flag;
    }
 
    int get_provided_thread_support() const { return provided_thread_support_; }
 
    environment(int& argc, char**& argv,
                const thread_type thread_t = thread_type::serialized)
    {
        this->required_thread_support_ = static_cast<int>(thread_t);
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Init_thread(&argc, &argv, this->required_thread_support_,
                            &(this->provided_thread_support_)));
    }
 
    ~environment() { MPI_Finalize(); }
 
    environment(const environment&) = delete;
    environment(environment&&) = delete;
    environment& operator=(const environment&) = delete;
    environment& operator=(environment&&) = delete;
 
private:
    int required_thread_support_;
    int provided_thread_support_;
};
 
 
namespace {
 
 
class comm_deleter {
public:
    using pointer = MPI_Comm*;
    void operator()(pointer comm) const
    {
        GKO_ASSERT(*comm != MPI_COMM_NULL);
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Comm_free(comm));
        delete comm;
    }
};
 
 
}  // namespace
 
 
struct status {
    status() : status_(MPI_Status{}) {}
 
    MPI_Status* get() { return &this->status_; }
 
    template <typename T>
    int get_count(const T* data) const
    {
        int count;
        MPI_Get_count(&status_, type_impl<T>::get_type(), &count);
        return count;
    }
 
private:
    MPI_Status status_;
};
 
 
class request {
public:
    request() : req_(MPI_REQUEST_NULL) {}
 
    request(const request&) = delete;
 
    request& operator=(const request&) = delete;
 
    request(request&& o) noexcept { *this = std::move(o); }
 
    request& operator=(request&& o) noexcept
    {
        if (this != &o) {
            this->req_ = std::exchange(o.req_, MPI_REQUEST_NULL);
        }
        return *this;
    }
 
    ~request()
    {
        if (req_ != MPI_REQUEST_NULL) {
            if (MPI_Request_free(&req_) != MPI_SUCCESS) {
                std::terminate();  // since we can't throw in destructors, we
                                   // have to terminate the program
            }
        }
    }
 
    MPI_Request* get() { return &this->req_; }
 
    status wait()
    {
        status status;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Wait(&req_, status.get()));
        return status;
    }
 
 
private:
    MPI_Request req_;
};
 
 
inline std::vector<status> wait_all(std::vector<request>& req)
{
    std::vector<status> stat;
    for (std::size_t i = 0; i < req.size(); ++i) {
        stat.emplace_back(req[i].wait());
    }
    return stat;
}
 
 
class communicator {
public:
    communicator(const MPI_Comm& comm, bool force_host_buffer = false)
        : comm_(), force_host_buffer_(force_host_buffer)
    {
        this->comm_.reset(new MPI_Comm(comm));
    }
 
    communicator(const MPI_Comm& comm, int color, int key)
    {
        MPI_Comm comm_out;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Comm_split(comm, color, key, &comm_out));
        this->comm_.reset(new MPI_Comm(comm_out), comm_deleter{});
    }
 
    communicator(const communicator& comm, int color, int key)
    {
        MPI_Comm comm_out;
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Comm_split(comm.get(), color, key, &comm_out));
        this->comm_.reset(new MPI_Comm(comm_out), comm_deleter{});
    }
 
    const MPI_Comm& get() const { return *(this->comm_.get()); }
 
    bool force_host_buffer() const { return force_host_buffer_; }
 
    int size() const { return get_num_ranks(); }
 
    int rank() const { return get_my_rank(); };
 
    int node_local_rank() const { return get_node_local_rank(); };
 
    bool operator==(const communicator& rhs) const
    {
        return compare(rhs.get());
    }
 
    bool operator!=(const communicator& rhs) const { return !(*this == rhs); }
 
    void synchronize() const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Barrier(this->get()));
    }
 
    template <typename SendType>
    void send(std::shared_ptr<const Executor> exec, const SendType* send_buffer,
              const int send_count, const int destination_rank,
              const int send_tag) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Send(send_buffer, send_count, type_impl<SendType>::get_type(),
                     destination_rank, send_tag, this->get()));
    }
 
    template <typename SendType>
    request i_send(std::shared_ptr<const Executor> exec,
                   const SendType* send_buffer, const int send_count,
                   const int destination_rank, const int send_tag) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Isend(send_buffer, send_count, type_impl<SendType>::get_type(),
                      destination_rank, send_tag, this->get(), req.get()));
        return req;
    }
 
    template <typename RecvType>
    status recv(std::shared_ptr<const Executor> exec, RecvType* recv_buffer,
                const int recv_count, const int source_rank,
                const int recv_tag) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        status st;
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Recv(recv_buffer, recv_count, type_impl<RecvType>::get_type(),
                     source_rank, recv_tag, this->get(), st.get()));
        return st;
    }
 
    template <typename RecvType>
    request i_recv(std::shared_ptr<const Executor> exec, RecvType* recv_buffer,
                   const int recv_count, const int source_rank,
                   const int recv_tag) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Irecv(recv_buffer, recv_count, type_impl<RecvType>::get_type(),
                      source_rank, recv_tag, this->get(), req.get()));
        return req;
    }
 
    template <typename BroadcastType>
    void broadcast(std::shared_ptr<const Executor> exec, BroadcastType* buffer,
                   int count, int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Bcast(buffer, count,
                                           type_impl<BroadcastType>::get_type(),
                                           root_rank, this->get()));
    }
 
    template <typename BroadcastType>
    request i_broadcast(std::shared_ptr<const Executor> exec,
                        BroadcastType* buffer, int count, int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Ibcast(buffer, count, type_impl<BroadcastType>::get_type(),
                       root_rank, this->get(), req.get()));
        return req;
    }
 
    template <typename ReduceType>
    void reduce(std::shared_ptr<const Executor> exec,
                const ReduceType* send_buffer, ReduceType* recv_buffer,
                int count, MPI_Op operation, int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Reduce(send_buffer, recv_buffer, count,
                                            type_impl<ReduceType>::get_type(),
                                            operation, root_rank, this->get()));
    }
 
    template <typename ReduceType>
    request i_reduce(std::shared_ptr<const Executor> exec,
                     const ReduceType* send_buffer, ReduceType* recv_buffer,
                     int count, MPI_Op operation, int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Ireduce(
            send_buffer, recv_buffer, count, type_impl<ReduceType>::get_type(),
            operation, root_rank, this->get(), req.get()));
        return req;
    }
 
    template <typename ReduceType>
    void all_reduce(std::shared_ptr<const Executor> exec,
                    ReduceType* recv_buffer, int count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Allreduce(
            MPI_IN_PLACE, recv_buffer, count, type_impl<ReduceType>::get_type(),
            operation, this->get()));
    }
 
    template <typename ReduceType>
    request i_all_reduce(std::shared_ptr<const Executor> exec,
                         ReduceType* recv_buffer, int count,
                         MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Iallreduce(
            MPI_IN_PLACE, recv_buffer, count, type_impl<ReduceType>::get_type(),
            operation, this->get(), req.get()));
        return req;
    }
 
    template <typename ReduceType>
    void all_reduce(std::shared_ptr<const Executor> exec,
                    const ReduceType* send_buffer, ReduceType* recv_buffer,
                    int count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Allreduce(
            send_buffer, recv_buffer, count, type_impl<ReduceType>::get_type(),
            operation, this->get()));
    }
 
    template <typename ReduceType>
    request i_all_reduce(std::shared_ptr<const Executor> exec,
                         const ReduceType* send_buffer, ReduceType* recv_buffer,
                         int count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Iallreduce(
            send_buffer, recv_buffer, count, type_impl<ReduceType>::get_type(),
            operation, this->get(), req.get()));
        return req;
    }
 
    template <typename SendType, typename RecvType>
    void gather(std::shared_ptr<const Executor> exec,
                const SendType* send_buffer, const int send_count,
                RecvType* recv_buffer, const int recv_count,
                int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Gather(send_buffer, send_count, type_impl<SendType>::get_type(),
                       recv_buffer, recv_count, type_impl<RecvType>::get_type(),
                       root_rank, this->get()));
    }
 
    template <typename SendType, typename RecvType>
    request i_gather(std::shared_ptr<const Executor> exec,
                     const SendType* send_buffer, const int send_count,
                     RecvType* recv_buffer, const int recv_count,
                     int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Igather(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(), root_rank,
            this->get(), req.get()));
        return req;
    }
 
    template <typename SendType, typename RecvType>
    void gather_v(std::shared_ptr<const Executor> exec,
                  const SendType* send_buffer, const int send_count,
                  RecvType* recv_buffer, const int* recv_counts,
                  const int* displacements, int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Gatherv(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_counts, displacements,
            type_impl<RecvType>::get_type(), root_rank, this->get()));
    }
 
    template <typename SendType, typename RecvType>
    request i_gather_v(std::shared_ptr<const Executor> exec,
                       const SendType* send_buffer, const int send_count,
                       RecvType* recv_buffer, const int* recv_counts,
                       const int* displacements, int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Igatherv(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_counts, displacements,
            type_impl<RecvType>::get_type(), root_rank, this->get(),
            req.get()));
        return req;
    }
 
    template <typename SendType, typename RecvType>
    void all_gather(std::shared_ptr<const Executor> exec,
                    const SendType* send_buffer, const int send_count,
                    RecvType* recv_buffer, const int recv_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Allgather(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(),
            this->get()));
    }
 
    template <typename SendType, typename RecvType>
    request i_all_gather(std::shared_ptr<const Executor> exec,
                         const SendType* send_buffer, const int send_count,
                         RecvType* recv_buffer, const int recv_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Iallgather(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(),
            this->get(), req.get()));
        return req;
    }
 
    template <typename SendType, typename RecvType>
    void scatter(std::shared_ptr<const Executor> exec,
                 const SendType* send_buffer, const int send_count,
                 RecvType* recv_buffer, const int recv_count,
                 int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Scatter(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(), root_rank,
            this->get()));
    }
 
    template <typename SendType, typename RecvType>
    request i_scatter(std::shared_ptr<const Executor> exec,
                      const SendType* send_buffer, const int send_count,
                      RecvType* recv_buffer, const int recv_count,
                      int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Iscatter(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(), root_rank,
            this->get(), req.get()));
        return req;
    }
 
    template <typename SendType, typename RecvType>
    void scatter_v(std::shared_ptr<const Executor> exec,
                   const SendType* send_buffer, const int* send_counts,
                   const int* displacements, RecvType* recv_buffer,
                   const int recv_count, int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Scatterv(
            send_buffer, send_counts, displacements,
            type_impl<SendType>::get_type(), recv_buffer, recv_count,
            type_impl<RecvType>::get_type(), root_rank, this->get()));
    }
 
    template <typename SendType, typename RecvType>
    request i_scatter_v(std::shared_ptr<const Executor> exec,
                        const SendType* send_buffer, const int* send_counts,
                        const int* displacements, RecvType* recv_buffer,
                        const int recv_count, int root_rank) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Iscatterv(send_buffer, send_counts, displacements,
                          type_impl<SendType>::get_type(), recv_buffer,
                          recv_count, type_impl<RecvType>::get_type(),
                          root_rank, this->get(), req.get()));
        return req;
    }
 
    template <typename RecvType>
    void all_to_all(std::shared_ptr<const Executor> exec, RecvType* recv_buffer,
                    const int recv_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Alltoall(
            MPI_IN_PLACE, recv_count, type_impl<RecvType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(),
            this->get()));
    }
 
    template <typename RecvType>
    request i_all_to_all(std::shared_ptr<const Executor> exec,
                         RecvType* recv_buffer, const int recv_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Ialltoall(
            MPI_IN_PLACE, recv_count, type_impl<RecvType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(),
            this->get(), req.get()));
        return req;
    }
 
    template <typename SendType, typename RecvType>
    void all_to_all(std::shared_ptr<const Executor> exec,
                    const SendType* send_buffer, const int send_count,
                    RecvType* recv_buffer, const int recv_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Alltoall(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(),
            this->get()));
    }
 
    template <typename SendType, typename RecvType>
    request i_all_to_all(std::shared_ptr<const Executor> exec,
                         const SendType* send_buffer, const int send_count,
                         RecvType* recv_buffer, const int recv_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Ialltoall(
            send_buffer, send_count, type_impl<SendType>::get_type(),
            recv_buffer, recv_count, type_impl<RecvType>::get_type(),
            this->get(), req.get()));
        return req;
    }
 
    template <typename SendType, typename RecvType>
    void all_to_all_v(std::shared_ptr<const Executor> exec,
                      const SendType* send_buffer, const int* send_counts,
                      const int* send_offsets, RecvType* recv_buffer,
                      const int* recv_counts, const int* recv_offsets) const
    {
        this->all_to_all_v(std::move(exec), send_buffer, send_counts,
                           send_offsets, type_impl<SendType>::get_type(),
                           recv_buffer, recv_counts, recv_offsets,
                           type_impl<RecvType>::get_type());
    }
 
    void all_to_all_v(std::shared_ptr<const Executor> exec,
                      const void* send_buffer, const int* send_counts,
                      const int* send_offsets, MPI_Datatype send_type,
                      void* recv_buffer, const int* recv_counts,
                      const int* recv_offsets, MPI_Datatype recv_type) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Alltoallv(
            send_buffer, send_counts, send_offsets, send_type, recv_buffer,
            recv_counts, recv_offsets, recv_type, this->get()));
    }
 
    request i_all_to_all_v(std::shared_ptr<const Executor> exec,
                           const void* send_buffer, const int* send_counts,
                           const int* send_offsets, MPI_Datatype send_type,
                           void* recv_buffer, const int* recv_counts,
                           const int* recv_offsets,
                           MPI_Datatype recv_type) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Ialltoallv(
            send_buffer, send_counts, send_offsets, send_type, recv_buffer,
            recv_counts, recv_offsets, recv_type, this->get(), req.get()));
        return req;
    }
 
    template <typename SendType, typename RecvType>
    request i_all_to_all_v(std::shared_ptr<const Executor> exec,
                           const SendType* send_buffer, const int* send_counts,
                           const int* send_offsets, RecvType* recv_buffer,
                           const int* recv_counts,
                           const int* recv_offsets) const
    {
        return this->i_all_to_all_v(
            std::move(exec), send_buffer, send_counts, send_offsets,
            type_impl<SendType>::get_type(), recv_buffer, recv_counts,
            recv_offsets, type_impl<RecvType>::get_type());
    }
 
    template <typename ScanType>
    void scan(std::shared_ptr<const Executor> exec, const ScanType* send_buffer,
              ScanType* recv_buffer, int count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Scan(send_buffer, recv_buffer, count,
                                          type_impl<ScanType>::get_type(),
                                          operation, this->get()));
    }
 
    template <typename ScanType>
    request i_scan(std::shared_ptr<const Executor> exec,
                   const ScanType* send_buffer, ScanType* recv_buffer,
                   int count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Iscan(send_buffer, recv_buffer, count,
                                           type_impl<ScanType>::get_type(),
                                           operation, this->get(), req.get()));
        return req;
    }
 
private:
    std::shared_ptr<MPI_Comm> comm_;
    bool force_host_buffer_;
 
    int get_my_rank() const
    {
        int my_rank = 0;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Comm_rank(get(), &my_rank));
        return my_rank;
    }
 
    int get_node_local_rank() const
    {
        MPI_Comm local_comm;
        int rank;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Comm_split_type(
            this->get(), MPI_COMM_TYPE_SHARED, 0, MPI_INFO_NULL, &local_comm));
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Comm_rank(local_comm, &rank));
        MPI_Comm_free(&local_comm);
        return rank;
    }
 
    int get_num_ranks() const
    {
        int size = 1;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Comm_size(this->get(), &size));
        return size;
    }
 
    bool compare(const MPI_Comm& other) const
    {
        int flag;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Comm_compare(get(), other, &flag));
        return flag == MPI_IDENT;
    }
};
 
 
bool requires_host_buffer(const std::shared_ptr<const Executor>& exec,
                          const communicator& comm);
 
 
inline double get_walltime() { return MPI_Wtime(); }
 
 
template <typename ValueType>
class window {
public:
    enum class create_type { allocate = 1, create = 2, dynamic_create = 3 };
 
    enum class lock_type { shared = 1, exclusive = 2 };
 
    window() : window_(MPI_WIN_NULL) {}
 
    window(const window& other) = delete;
 
    window& operator=(const window& other) = delete;
 
    window(window&& other) : window_{std::exchange(other.window_, MPI_WIN_NULL)}
    {}
 
    window& operator=(window&& other)
    {
        window_ = std::exchange(other.window_, MPI_WIN_NULL);
    }
 
    window(std::shared_ptr<const Executor> exec, ValueType* base, int num_elems,
           const communicator& comm, const int disp_unit = sizeof(ValueType),
           MPI_Info input_info = MPI_INFO_NULL,
           create_type c_type = create_type::create)
    {
        auto guard = exec->get_scoped_device_id_guard();
        unsigned size = num_elems * sizeof(ValueType);
        if (c_type == create_type::create) {
            GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_create(
                base, size, disp_unit, input_info, comm.get(), &this->window_));
        } else if (c_type == create_type::dynamic_create) {
            GKO_ASSERT_NO_MPI_ERRORS(
                MPI_Win_create_dynamic(input_info, comm.get(), &this->window_));
        } else if (c_type == create_type::allocate) {
            GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_allocate(
                size, disp_unit, input_info, comm.get(), base, &this->window_));
        } else {
            GKO_NOT_IMPLEMENTED;
        }
    }
 
    MPI_Win get_window() const { return this->window_; }
 
    void fence(int assert = 0) const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_fence(assert, this->window_));
    }
 
    void lock(int rank, lock_type lock_t = lock_type::shared,
              int assert = 0) const
    {
        if (lock_t == lock_type::shared) {
            GKO_ASSERT_NO_MPI_ERRORS(
                MPI_Win_lock(MPI_LOCK_SHARED, rank, assert, this->window_));
        } else if (lock_t == lock_type::exclusive) {
            GKO_ASSERT_NO_MPI_ERRORS(
                MPI_Win_lock(MPI_LOCK_EXCLUSIVE, rank, assert, this->window_));
        } else {
            GKO_NOT_IMPLEMENTED;
        }
    }
 
    void unlock(int rank) const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_unlock(rank, this->window_));
    }
 
    void lock_all(int assert = 0) const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_lock_all(assert, this->window_));
    }
 
    void unlock_all() const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_unlock_all(this->window_));
    }
 
    void flush(int rank) const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_flush(rank, this->window_));
    }
 
    void flush_local(int rank) const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_flush_local(rank, this->window_));
    }
 
    void flush_all() const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_flush_all(this->window_));
    }
 
    void flush_all_local() const
    {
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_flush_local_all(this->window_));
    }
 
    void sync() const { GKO_ASSERT_NO_MPI_ERRORS(MPI_Win_sync(this->window_)); }
 
    ~window()
    {
        if (this->window_ && this->window_ != MPI_WIN_NULL) {
            MPI_Win_free(&this->window_);
        }
    }
 
    template <typename PutType>
    void put(std::shared_ptr<const Executor> exec, const PutType* origin_buffer,
             const int origin_count, const int target_rank,
             const unsigned int target_disp, const int target_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Put(origin_buffer, origin_count, type_impl<PutType>::get_type(),
                    target_rank, target_disp, target_count,
                    type_impl<PutType>::get_type(), this->get_window()));
    }
 
    template <typename PutType>
    request r_put(std::shared_ptr<const Executor> exec,
                  const PutType* origin_buffer, const int origin_count,
                  const int target_rank, const unsigned int target_disp,
                  const int target_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Rput(
            origin_buffer, origin_count, type_impl<PutType>::get_type(),
            target_rank, target_disp, target_count,
            type_impl<PutType>::get_type(), this->get_window(), req.get()));
        return req;
    }
 
    template <typename PutType>
    void accumulate(std::shared_ptr<const Executor> exec,
                    const PutType* origin_buffer, const int origin_count,
                    const int target_rank, const unsigned int target_disp,
                    const int target_count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Accumulate(
            origin_buffer, origin_count, type_impl<PutType>::get_type(),
            target_rank, target_disp, target_count,
            type_impl<PutType>::get_type(), operation, this->get_window()));
    }
 
    template <typename PutType>
    request r_accumulate(std::shared_ptr<const Executor> exec,
                         const PutType* origin_buffer, const int origin_count,
                         const int target_rank, const unsigned int target_disp,
                         const int target_count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Raccumulate(
            origin_buffer, origin_count, type_impl<PutType>::get_type(),
            target_rank, target_disp, target_count,
            type_impl<PutType>::get_type(), operation, this->get_window(),
            req.get()));
        return req;
    }
 
    template <typename GetType>
    void get(std::shared_ptr<const Executor> exec, GetType* origin_buffer,
             const int origin_count, const int target_rank,
             const unsigned int target_disp, const int target_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(
            MPI_Get(origin_buffer, origin_count, type_impl<GetType>::get_type(),
                    target_rank, target_disp, target_count,
                    type_impl<GetType>::get_type(), this->get_window()));
    }
 
    template <typename GetType>
    request r_get(std::shared_ptr<const Executor> exec, GetType* origin_buffer,
                  const int origin_count, const int target_rank,
                  const unsigned int target_disp, const int target_count) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Rget(
            origin_buffer, origin_count, type_impl<GetType>::get_type(),
            target_rank, target_disp, target_count,
            type_impl<GetType>::get_type(), this->get_window(), req.get()));
        return req;
    }
 
    template <typename GetType>
    void get_accumulate(std::shared_ptr<const Executor> exec,
                        GetType* origin_buffer, const int origin_count,
                        GetType* result_buffer, const int result_count,
                        const int target_rank, const unsigned int target_disp,
                        const int target_count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Get_accumulate(
            origin_buffer, origin_count, type_impl<GetType>::get_type(),
            result_buffer, result_count, type_impl<GetType>::get_type(),
            target_rank, target_disp, target_count,
            type_impl<GetType>::get_type(), operation, this->get_window()));
    }
 
    template <typename GetType>
    request r_get_accumulate(std::shared_ptr<const Executor> exec,
                             GetType* origin_buffer, const int origin_count,
                             GetType* result_buffer, const int result_count,
                             const int target_rank,
                             const unsigned int target_disp,
                             const int target_count, MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        request req;
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Rget_accumulate(
            origin_buffer, origin_count, type_impl<GetType>::get_type(),
            result_buffer, result_count, type_impl<GetType>::get_type(),
            target_rank, target_disp, target_count,
            type_impl<GetType>::get_type(), operation, this->get_window(),
            req.get()));
        return req;
    }
 
    template <typename GetType>
    void fetch_and_op(std::shared_ptr<const Executor> exec,
                      GetType* origin_buffer, GetType* result_buffer,
                      const int target_rank, const unsigned int target_disp,
                      MPI_Op operation) const
    {
        auto guard = exec->get_scoped_device_id_guard();
        GKO_ASSERT_NO_MPI_ERRORS(MPI_Fetch_and_op(
            origin_buffer, result_buffer, type_impl<GetType>::get_type(),
            target_rank, target_disp, operation, this->get_window()));
    }
 
private:
    MPI_Win window_;
};
 
 
}  // namespace mpi
}  // namespace experimental
}  // namespace gko
 
 
#endif  // GKO_HAVE_MPI
 
 
#endif  // GKO_PUBLIC_CORE_BASE_MPI_HPP_