mlx/python/src/distributed.cpp

// Copyright  © 2024 Apple Inc.

#include <nanobind/nanobind.h>
#include <nanobind/stl/optional.h>
#include <nanobind/stl/shared_ptr.h>
#include <nanobind/stl/string.h>
#include <nanobind/stl/variant.h>
#include <nanobind/stl/vector.h>

#include "mlx/distributed/distributed.h"
#include "mlx/distributed/ops.h"
#include "python/src/small_vector.h"
#include "python/src/utils.h"

namespace mx = mlx::core;
namespace nb = nanobind;
using namespace nb::literals;

void init_distributed(nb::module_& parent_module) {
  auto m = parent_module.def_submodule(
      "distributed", "mlx.core.distributed: Communication operations");

  nb::class_<mx::distributed::Group>(
      m,
      "Group",
      R"pbcopy(
        An :class:`mlx.core.distributed.Group` represents a group of independent mlx
        processes that can communicate.
      )pbcopy")
      .def(
          "rank", &mx::distributed::Group::rank, "Get the rank of this process")
      .def("size", &mx::distributed::Group::size, "Get the size of the group")
      .def(
          "split",
          &mx::distributed::Group::split,
          "color"_a,
          "key"_a = -1,
          nb::sig("def split(self, color: int, key: int = -1) -> Group"),
          R"pbdoc(
            Split the group to subgroups based on the provided color.

            Processes that use the same color go to the same group. The ``key``
            argument defines the rank in the new group. The smaller the key the
            smaller the rank. If the key is negative then the rank in the
            current group is used.

            Args:
              color (int): A value to group processes into subgroups.
              key (int, optional): A key to optionally change the rank ordering
                of the processes.
          )pbdoc");

  m.def(
      "is_available",
      [](const std::string& backend) {
        return mx::distributed::is_available(backend);
      },
      "backend"_a = "any",
      nb::sig("def is_available(backend: str = 'any') -> bool"),
      R"pbdoc(
      Check if a communication backend is available.

      Note, this function returns whether MLX has the capability of
      instantiating that distributed backend not whether it is possible to
      create a communication group. For that purpose one should use
      ``init(strict=True)``.

      Args:
        backend (str, optional): The name of the backend to check for availability.
          It takes the same values as ``init()``. Default: ``any``.

      Returns:
        bool: Whether the distributed backend is available.
      )pbdoc");

  m.def(
      "init",
      &mx::distributed::init,
      "strict"_a = false,
      "backend"_a = "any",
      nb::sig("def init(strict: bool = False, backend: str = 'any') -> Group"),
      R"pbdoc(
        Initialize the communication backend and create the global communication group.

        Example:

          .. code:: python

            import mlx.core as mx

            group = mx.distributed.init(backend="ring")

        Args:
          strict (bool, optional): If set to False it returns a singleton group
            in case ``mx.distributed.is_available()`` returns False otherwise
            it throws a runtime error. Default: ``False``
          backend (str, optional): Which distributed backend to initialize.
            Possible values ``mpi``, ``ring``, ``nccl``, ``ibv``, ``any``. If
            set to ``any`` all available backends are tried and the first one
            that succeeds becomes the global group which will be returned in
            subsequent calls. Default: ``any``

        Returns:
          Group: The group representing all the launched processes.
      )pbdoc");

  m.def(
      "all_sum",
      [](const ScalarOrArray& x,
         std::optional<mx::distributed::Group> group,
         mx::StreamOrDevice s) {
        return mx::distributed::all_sum(to_array(x), group, s);
      },
      "x"_a,
      nb::kw_only(),
      "group"_a = nb::none(),
      "stream"_a = nb::none(),
      nb::sig(
          "def all_sum(x: array, *, group: Optional[Group] = None, stream: Union[None, Stream, Device] = None) -> array"),
      R"pbdoc(
        All reduce sum.

        Sum the ``x`` arrays from all processes in the group.

        Args:
          x (array): Input array.
          group (Group): The group of processes that will participate in the
            reduction. If set to ``None`` the global group is used. Default:
            ``None``.
          stream (Stream, optional): Stream or device. Defaults to ``None``
            in which case the default stream of the default device is used.

        Returns:
          array: The sum of all ``x`` arrays.
      )pbdoc");
  m.def(
      "all_max",
      [](const ScalarOrArray& x,
         std::optional<mx::distributed::Group> group,
         mx::StreamOrDevice s) {
        return mx::distributed::all_max(to_array(x), group, s);
      },
      "x"_a,
      nb::kw_only(),
      "group"_a = nb::none(),
      "stream"_a = nb::none(),
      nb::sig(
          "def all_max(x: array, *, group: Optional[Group] = None, stream: Union[None, Stream, Device] = None) -> array"),
      R"pbdoc(
        All reduce max.

        Find the maximum of the ``x`` arrays from all processes in the group.

        Args:
          x (array): Input array.
          group (Group): The group of processes that will participate in the
            reduction. If set to ``None`` the global group is used. Default:
            ``None``.
          stream (Stream, optional): Stream or device. Defaults to ``None``
            in which case the default stream of the default device is used.

        Returns:
          array: The maximum of all ``x`` arrays.
      )pbdoc");
  m.def(
      "all_min",
      [](const ScalarOrArray& x,
         std::optional<mx::distributed::Group> group,
         mx::StreamOrDevice s) {
        return mx::distributed::all_min(to_array(x), group, s);
      },
      "x"_a,
      nb::kw_only(),
      "group"_a = nb::none(),
      "stream"_a = nb::none(),
      nb::sig(
          "def all_min(x: array, *, group: Optional[Group] = None, stream: Union[None, Stream, Device] = None) -> array"),
      R"pbdoc(
      All reduce min.

      Find the minimum of the ``x`` arrays from all processes in the group.

      Args:
        x (array): Input array.
        group (Group): The group of processes that will participate in the
          reduction. If set to ``None`` the global group is used. Default:
          ``None``.
        stream (Stream, optional): Stream or device. Defaults to ``None``
          in which case the default stream of the default device is used.

      Returns:
        array: The minimum of all ``x`` arrays.
    )pbdoc");
  m.def(
      "all_gather",
      [](const ScalarOrArray& x,
         std::optional<mx::distributed::Group> group,
         mx::StreamOrDevice s) {
        return mx::distributed::all_gather(to_array(x), group, s);
      },
      "x"_a,
      nb::kw_only(),
      "group"_a = nb::none(),
      "stream"_a = nb::none(),
      nb::sig(
          "def all_gather(x: array, *, group: Optional[Group] = None, stream: Union[None, Stream, Device] = None) -> array"),
      R"pbdoc(
        Gather arrays from all processes.

        Gather the ``x`` arrays from all processes in the group and concatenate
        them along the first axis. The arrays should all have the same shape.

        Args:
          x (array): Input array.
          group (Group): The group of processes that will participate in the
            gather. If set to ``None`` the global group is used. Default:
            ``None``.
          stream (Stream, optional): Stream or device. Defaults to ``None``
            in which case the default stream of the default device is used.

        Returns:
          array: The concatenation of all ``x`` arrays.
      )pbdoc");

  m.def(
      "send",
      [](const ScalarOrArray& x,
         int dst,
         std::optional<mx::distributed::Group> group,
         mx::StreamOrDevice s) {
        return mx::distributed::send(to_array(x), dst, group, s);
      },
      "x"_a,
      "dst"_a,
      nb::kw_only(),
      "group"_a = nb::none(),
      "stream"_a = nb::none(),
      nb::sig(
          "def send(x: array, dst: int, *, group: Optional[Group] = None, stream: Union[None, Stream, Device] = None) -> array"),
      R"pbdoc(
        Send an array from the current process to the process that has rank
        ``dst`` in the group.

        Args:
          x (array): Input array.
          dst (int): Rank of the destination process in the group.
          group (Group): The group of processes that will participate in the
            send. If set to ``None`` the global group is used. Default:
            ``None``.
          stream (Stream, optional): Stream or device. Defaults to ``None``
            in which case the default stream of the default device is used.

        Returns:
          array: An array identical to ``x`` which when evaluated the send is performed.
      )pbdoc");

  m.def(
      "recv",
      &mx::distributed::recv,
      "shape"_a,
      "dtype"_a,
      "src"_a,
      nb::kw_only(),
      "group"_a = nb::none(),
      "stream"_a = nb::none(),
      nb::sig(
          "def recv(shape: Sequence[int], dtype: Dtype, src: int, *, group: Optional[Group] = None, stream: Union[None, Stream, Device] = None) -> array"),
      R"pbdoc(
        Recv an array with shape ``shape`` and dtype ``dtype`` from process
        with rank ``src``.

        Args:
          shape (Tuple[int]): The shape of the array we are receiving.
          dtype (Dtype): The data type of the array we are receiving.
          src (int): Rank of the source process in the group.
          group (Group): The group of processes that will participate in the
            recv. If set to ``None`` the global group is used. Default:
            ``None``.
          stream (Stream, optional): Stream or device. Defaults to ``None``
            in which case the default stream of the default device is used.

        Returns:
          array: The array that was received from ``src``.
      )pbdoc");

  m.def(
      "recv_like",
      [](const ScalarOrArray& x,
         int src,
         std::optional<mx::distributed::Group> group,
         mx::StreamOrDevice s) {
        return mx::distributed::recv_like(to_array(x), src, group, s);
      },
      "x"_a,
      "src"_a,
      nb::kw_only(),
      "group"_a = nb::none(),
      "stream"_a = nb::none(),
      nb::sig(
          "def recv_like(x: array, src: int, *, group: Optional[Group] = None, stream: Union[None, Stream, Device] = None) -> array"),
      R"pbdoc(
        Recv an array with shape and type like ``x`` from process with rank
        ``src``.

        It is equivalent to calling ``mx.distributed.recv(x.shape, x.dtype, src)``.

        Args:
          x (array): An array defining the shape and dtype of the array we are
            receiving.
          src (int): Rank of the source process in the group.
          group (Group): The group of processes that will participate in the
            recv. If set to ``None`` the global group is used. Default:
            ``None``.
          stream (Stream, optional): Stream or device. Defaults to ``None``
            in which case the default stream of the default device is used.

        Returns:
          array: The array that was received from ``src``.
      )pbdoc");

  m.def(
      "sum_scatter",
      [](const ScalarOrArray& x,
         std::optional<mx::distributed::Group> group,
         mx::StreamOrDevice s) {
        return mx::distributed::sum_scatter(to_array(x), group, s);
      },
      "x"_a,
      nb::kw_only(),
      "group"_a = nb::none(),
      "stream"_a = nb::none(),
      nb::sig(
          "def sum_scatter(x: array, *, group: Optional[Group] = None, stream: Union[None, Stream, Device] = None) -> array"),
      R"pbdoc(
      Sum ``x`` across all processes in the group and shard the result along the first axis across ranks.
      ``x.shape[0]`` must be divisible by the group size.

      The result is equivalent to ``all_sum(x)[rank*chunk_size:(rank+1)*chunk_size]``, where ``chunk_size = x.shape[0] // group.size()`` and ``rank`` is the rank of this process in the group.
      Note: ``all_sum`` is mentioned only for illustration; the actual implementation does not perform ``all_sum`` and uses a single reduce-scatter collective instead.
      Currently supported only for the NCCL backend.

      Args:
        x (array): Input array.
        group (Group): The group of processes that will participate in the
          sum scatter. If set to ``None`` the global group is used. Default:
          ``None``.
        stream (Stream, optional): Stream or device. Defaults to ``None``
          in which case the default stream of the default device is used.
      Returns:
        array: The output array with shape ``[x.shape[0] // group.size(), *x.shape[1:]]``.
    )pbdoc");
}