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Use MPI_Alltoall instead of MPI_Alltoallv if able. #77

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Merged
romerojosh merged 2 commits into from
Jul 11, 2025
Merged

Use MPI_Alltoall instead of MPI_Alltoallv if able. #77

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5bd0f0d
Use MPI_Alltoall instead of MPI_Alltoallv if able.
romerojosh Jul 11, 2024
c40cab8
Minor update.
romerojosh Jul 11, 2025
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59 changes: 50 additions & 9 deletions include/internal/comm_routines.h
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Original file line number Diff line number Diff line change
Expand Up @@ -53,6 +53,40 @@ static inline MPI_Datatype getMpiDataType(cuda::std::complex<float>) { return MP
static inline MPI_Datatype getMpiDataType(cuda::std::complex<double>) { return MPI_C_DOUBLE_COMPLEX; }
template <typename T> static inline MPI_Datatype getMpiDataType() { return getMpiDataType(T(0)); }

static inline bool canUseMpiAlltoall(const std::vector<comm_count_t>& send_counts,
const std::vector<comm_count_t>& send_offsets,
const std::vector<comm_count_t>& recv_counts,
const std::vector<comm_count_t>& recv_offsets) {
auto scount = send_counts[0];
auto rcount = recv_counts[0];
// Check that send and recv counts are constants
for (int i = 1; i < send_counts.size(); ++i) {
if (send_counts[i] != scount) {
return false;
}
}
for (int i = 1; i < recv_counts.size(); ++i) {
if (recv_counts[i] != rcount) {
return false;
}
}

// Check that offsets are contiguous and equal to counts
for (int i = 0; i < send_offsets.size(); ++i) {
if (send_offsets[i] != i * scount) {
return false;
}
}
for (int i = 0; i < recv_offsets.size(); ++i) {
if (recv_offsets[i] != i * rcount) {
return false;
}
}

return true;
}


#ifdef ENABLE_NVSHMEM
#define CUDECOMP_NVSHMEM_CHUNK_SZ (static_cast<size_t>(1024 * 1024 * 1024))
template <typename T>
Expand Down Expand Up @@ -217,20 +251,27 @@ cudecompAlltoall(const cudecompHandle_t& handle, const cudecompGridDesc_t& grid_
}
case CUDECOMP_TRANSPOSE_COMM_MPI_A2A: {
CHECK_CUDA(cudaStreamSynchronize(stream));
auto send_counts_mod = send_counts;
auto recv_counts_mod = recv_counts;
auto comm =
(comm_axis == CUDECOMP_COMM_ROW) ? grid_desc->row_comm_info.mpi_comm : grid_desc->col_comm_info.mpi_comm;
int self_rank = (comm_axis == CUDECOMP_COMM_ROW) ? grid_desc->row_comm_info.rank : grid_desc->col_comm_info.rank;

// Self-copy with cudaMemcpy
CHECK_CUDA(cudaMemcpyAsync(recv_buff + recv_offsets[self_rank], send_buff + send_offsets[self_rank],
send_counts[self_rank] * sizeof(T), cudaMemcpyDeviceToDevice, stream));
bool use_alltoall = canUseMpiAlltoall(send_counts, send_offsets, recv_counts, recv_offsets);

send_counts_mod[self_rank] = 0;
recv_counts_mod[self_rank] = 0;
CHECK_MPI(MPI_Alltoallv(send_buff, send_counts_mod.data(), send_offsets.data(), getMpiDataType<T>(), recv_buff,
recv_counts_mod.data(), recv_offsets.data(), getMpiDataType<T>(), comm));
if (use_alltoall) {
CHECK_MPI(MPI_Alltoall(send_buff, send_counts[0], getMpiDataType<T>(), recv_buff, recv_counts[0],
getMpiDataType<T>(), comm));
} else {
// Self-copy with cudaMemcpy
CHECK_CUDA(cudaMemcpyAsync(recv_buff + recv_offsets[self_rank], send_buff + send_offsets[self_rank],
send_counts[self_rank] * sizeof(T), cudaMemcpyDeviceToDevice, stream));

auto send_counts_mod = send_counts;
auto recv_counts_mod = recv_counts;
send_counts_mod[self_rank] = 0;
recv_counts_mod[self_rank] = 0;
CHECK_MPI(MPI_Alltoallv(send_buff, send_counts_mod.data(), send_offsets.data(), getMpiDataType<T>(), recv_buff,
recv_counts_mod.data(), recv_offsets.data(), getMpiDataType<T>(), comm));
}
break;
}
default: {
Expand Down