Source code for torch.distributed.device_mesh
# Copyright (c) Meta Platforms, Inc. and affiliates
import logging
import math
from typing import Dict, List, Optional, Tuple, TYPE_CHECKING, Union
import torch
from torch.distributed import is_available
__all__ = ["init_device_mesh", "DeviceMesh"]
if not is_available():
import sys
# We need to create the stubs when distributed is not available.
# Otherwise, we would fail the doc tests (```./.ci/pytorch/docs-test.sh```),
# since it would try to import ``torch.distributed.device_mesh`` or
# ``torch.distributed.init_device_mesh`` but cannot find them.
class _DeviceMeshStub:
pass
def _init_device_mesh_stub():
pass
sys.modules["torch.distributed.device_mesh"].DeviceMesh = _DeviceMeshStub # type: ignore[attr-defined]
sys.modules[
"torch.distributed.device_mesh"
].init_device_mesh = _init_device_mesh_stub # type: ignore[attr-defined]
else:
from torch.distributed.distributed_c10d import (
_find_pg_by_ranks_and_tag,
_get_default_group,
_get_group_tag,
get_rank,
get_world_size,
init_process_group,
is_initialized,
new_group,
ProcessGroup,
)
logger = logging.getLogger(__name__)
# only import numpy typing when type checking
if TYPE_CHECKING:
try:
from numpy.typing import ArrayLike
except ImportError:
logger.warning(
"DeviceMesh requires numpy >= 1.21 to be installed for type checking"
)
class _MeshEnv:
def __init__(self) -> None:
self.mesh_stack: List[DeviceMesh] = []
self.child_to_parent_mapping: Dict[DeviceMesh, DeviceMesh] = {}
def get_current_mesh(self) -> "DeviceMesh":
if len(self.mesh_stack) == 0:
raise RuntimeError("No device mesh is currently active!")
return self.mesh_stack[-1]
def create_child_mesh(
self, device_mesh: "DeviceMesh", mesh_dim: int, mesh_dim_name: str
) -> "DeviceMesh":
# swap the current dim to the last dim then reshape to flatten out other
# dims, so we can just extract the list of ranks which contains cur_rank.
cur_rank = device_mesh.get_rank()
pg_ranks_by_dim = device_mesh.mesh.swapdims(-1, mesh_dim).reshape(
-1, device_mesh.mesh.size(mesh_dim)
)
for mesh_1d in pg_ranks_by_dim:
sub_mesh = DeviceMesh(
device_mesh.device_type,
mesh_1d,
mesh_dim_names=(mesh_dim_name,),
_init_process_groups=False,
)
if cur_rank in mesh_1d:
res_sub_mesh = sub_mesh
res_sub_mesh._dim_group_infos = [device_mesh._dim_group_infos[mesh_dim]]
# Assign the current DeviceMesh as the parent of the child DeviceMesh.
self.child_to_parent_mapping[res_sub_mesh] = device_mesh
return res_sub_mesh
def get_parent_mesh(self, device_mesh: "DeviceMesh") -> Optional["DeviceMesh"]:
return self.child_to_parent_mapping.get(device_mesh, None)
def get_parent_mesh_dim(self, device_mesh: "DeviceMesh") -> Optional[int]:
"""
Return the index of the mesh dim in the parent mesh.
The device_mesh passed in needs to be sliced out from a parent mesh.
"""
parent_mesh = self.get_parent_mesh(device_mesh)
child_mesh_dim_names = device_mesh.mesh_dim_names
if parent_mesh and child_mesh_dim_names:
assert (
len(child_mesh_dim_names) == 1
), "The child mesh can only be a 1D mesh."
child_mesh_dim_name = child_mesh_dim_names[0]
if parent_mesh.mesh_dim_names:
return parent_mesh._get_mesh_dim_by_name(child_mesh_dim_name)
return None
@staticmethod
def num_devices_per_host(device_type: str) -> int:
return _get_device_handle(device_type).device_count()
@staticmethod
def num_hosts(device_type: str) -> int:
# ProcessGroup can't tell us this info so we have to infer it, assume
# homogeneous hardware for now
return get_world_size() // _MeshEnv.num_devices_per_host(device_type)
_mesh_resources: _MeshEnv = _MeshEnv()
def _get_device_handle(device_type: str = "cuda"):
"""
Get the module corresponding to the device_type which is cuda or cuda-like device.
For example, when the device_type is cuda, the module `torch.cuda` is returned.
Return None when there is no corresponding module for device_type, otherwise
return the corresponding module.
"""
return getattr(torch, device_type, None)
[docs] class DeviceMesh:
"""
DeviceMesh represents a mesh of devices, where layout of devices could be
represented as a n-d dimension array, and each value of the n-d dimensional
array is the global id of the default process group ranks.
DeviceMesh could be used to describe the layout of devices across the cluster,
and serves as a proxy for communication among the device lists within the cluster.
DeviceMesh can be used as a context manager.
.. note::
DeviceMesh follows SPMD programming model, which means the same PyTorch Python program
is running on all processes/ranks in the cluster. Therefore, users need to make sure the
`mesh` array (which describes the layout of devices) should be identical across all ranks.
Inconsistent `mesh` will lead to silent hang.
Args:
device_type (str): The device type of the mesh. Currently supports: "cpu", "cuda/cuda-like".
mesh (ndarray): A multi-dimensional array or an integer tensor describing the layout
of devices, where the IDs are global IDs of the default process group.
Returns:
DeviceMesh: A :class:`DeviceMesh` object representing the device layout.
The following program runs on each process/rank in an SPMD manner. In this example, we have 2
hosts with 4 GPUs each.
A reduction over the first dimension of mesh will reduce across
columns (0, 4), .. and (3, 7), a reduction over the second dimension
of mesh reduces across rows (0, 1, 2, 3) and (4, 5, 6, 7).
Example::
>>> # xdoctest: +SKIP("no rank")
>>> from torch.distributed.device_mesh import DeviceMesh
>>>
>>> # Initialize device mesh as (2, 4) to represent the topology
>>> # of cross-host(dim 0), and within-host (dim 1).
>>> mesh = DeviceMesh(device_type="cuda", mesh=[[0, 1, 2, 3],[4, 5, 6, 7]])
"""
device_type: str
mesh: torch.Tensor
mesh_dim_names: Optional[Tuple[str, ...]]
def __init__(
self,
device_type: str,
mesh: Union[torch.Tensor, "ArrayLike"],
*,
mesh_dim_names: Optional[Tuple[str, ...]] = None,
_init_process_groups: bool = True,
) -> None:
self.device_type = device_type
self.mesh = (
mesh.detach()
if isinstance(mesh, torch.Tensor)
else torch.tensor(mesh, dtype=torch.int)
)
self.mesh_dim_names = mesh_dim_names
# private field to pre-generate DeviceMesh's hash
self._flatten_mesh_list = tuple(self.mesh.flatten().tolist())
self._hash = hash((self._flatten_mesh_list, self.mesh.shape, id(self)))
# Skip process group initialization if xla device.
# TODO(yeounoh) implement DeviceMesh backend and register XLA backend.
if device_type != "xla":
# always try to create default (world) pg, even if it is not initialized
# already. The world pg is used for device mesh identity (rank) on each
# process (we need to know if the current global rank is in the mesh or not).
self._get_or_create_default_group()
if _init_process_groups:
self._init_process_groups()
def _get_or_create_default_group(self):
default_initialized = is_initialized()
if not default_initialized:
init_process_group()
world_size = get_world_size()
if self.mesh.numel() > world_size:
raise RuntimeError(
f"Mesh should not be bigger than default world size, but found {self.mesh.numel()} ranks!"
)
device_handle = _get_device_handle(self.device_type)
# TODO: if user want to pass pg_options, offer a way to do it
if not default_initialized and device_handle:
# automatically set the current cuda/cuda-like device base on num of gpu devices available in each host
# NOTE: This device selection would only work for homogeneous hardware.
num_devices_per_host = device_handle.device_count()
if (
world_size > num_devices_per_host
and world_size % num_devices_per_host != 0
):
raise RuntimeError(
f"DeviceMesh only support homogeneous hardware, but found "
f"{world_size} ranks and {num_devices_per_host} {self.device_type} devices!"
)
device_handle.set_device(get_rank() % num_devices_per_host)
# calculate the coordinates of the current global rank on the mesh
rank_coords = (self.mesh == get_rank()).nonzero()
assert rank_coords.size(0) in (0, 1)
self._coordinate_on_dim: Optional[List[int]] = (
rank_coords[0].tolist() if rank_coords.size(0) > 0 else None
)
return _get_default_group()
def _init_process_groups(self):
# group tag/ranks associated with each mesh dimension, each mesh dimension should
# have one sub-group per rank
dim_group_infos: List[Tuple[str, List[int]]] = []
if self.mesh.ndim == 1 and self.mesh.numel() == get_world_size():
# if the mesh is the same as world_pg, we just append the default
# pg to the first dim groups, as new_group cannot have the exact
# same ranks as world
dim_group_infos.append(
(
_get_group_tag(_get_default_group()),
list(range(get_world_size())),
)
)
else:
# create sub pgs base on the mesh argument specified
for dim in range(self.mesh.ndim):
# swap the current dim to the last dim
# then reshape to flatten out other dims
pg_ranks_by_dim = self.mesh.swapdims(-1, dim).reshape(
-1, self.mesh.size(dim)
)
# multi-dim mesh, create subgroups by looping over the pg_ranks
# for each dim and append the groups
for dim_mesh in pg_ranks_by_dim:
subgroup_ranks = dim_mesh.tolist()
# call new_group regardless of the current rank in the
# pg or not, it's required that all ranks participate
# in subgroup construction
dim_group = new_group(ranks=subgroup_ranks)
# only add to dim_groups if the current rank in the subgroup
if self.get_rank() in subgroup_ranks:
if len(dim_group_infos) > dim:
raise RuntimeError(
f"Each device mesh dimension should get only one process group, but got {self.get_rank} "
f"in {subgroup_ranks}!"
)
dim_group_infos.append(
(_get_group_tag(dim_group), subgroup_ranks)
)
self._dim_group_infos = dim_group_infos
def __enter__(self) -> "DeviceMesh":
# set this mesh as the current mesh in mesh env
_mesh_resources.mesh_stack.append(self)
return self
# pyre-fixme[2]: Parameter must be annotated.
def __exit__(self, exc_type, exc_value, exc_traceback) -> None:
# pop this mesh from mesh env
_mesh_resources.mesh_stack.pop()
def __repr__(self) -> str:
return f"DeviceMesh({self.mesh.tolist()})"
def __hash__(self):
return self._hash
def __eq__(self, other: object) -> bool:
if not isinstance(other, DeviceMesh):
return False
if id(self.mesh) == id(other.mesh):
return True
return (
self.mesh.shape == other.mesh.shape
and self._flatten_mesh_list == other._flatten_mesh_list
)
def __getitem__(self, mesh_dim_name: str) -> "DeviceMesh":
"""
Slice the current DeviceMesh based on the mesh_dim_name given to create a child
DeviceMesh.
Args:
mesh_dim_name (str): the name of the mesh dimension of the parent DeviceMesh
to create a child DeviceMesh for.
Returns:
A :class:`DeviceMesh` object
The following program runs on each process/rank in an SPMD manner. In this example, we have 2
hosts with 4 GPUs each.
Calling mesh["tp"] on rank 0, 1, 2, 3 would return a 1D child DeviceMesh:([0, 1, 2, 3]).
Calling mesh["tp"] on rank 4, 5, 6, 7 would return a 1D child DeviceMesh:([4, 5, 6, 7]).
Calling mesh["dp"] on rank 0, 4 would return a 1D child DeviceMesh:([0, 4]).
Calling mesh["dp"] on rank 1, 5 would return a 1D child DeviceMesh:([1, 5]).
Calling mesh["dp"] on rank 2, 6 would return a 1D child DeviceMesh:([2, 6]).
Calling mesh["dp"] on rank 3, 7 would return a 1D child DeviceMesh:([3, 7]).
Example::
>>> # xdoctest: +SKIP("no rank")
>>> from torch.distributed.device_mesh import DeviceMesh
>>>
>>> # Initialize device mesh as (2, 4) to represent the topology
>>> # of cross-host(dim 0), and within-host (dim 1).
>>> mesh = DeviceMesh(device_type="cuda", mesh=[[0, 1, 2, 3],[4, 5, 6, 7]])
"""
if self.mesh.ndim <= 1:
raise RuntimeError(
f"Cannot slice a DeviceMesh with {self.mesh.ndim} dimension."
)
mesh_dim = self._get_mesh_dim_by_name(mesh_dim_name)
submesh = _mesh_resources.create_child_mesh(self, mesh_dim, mesh_dim_name)
return submesh
def get_group(
self, mesh_dim: Optional[Union[int, str]] = None
) -> Union[ProcessGroup, List[ProcessGroup]]:
"""
Returns a list of ProcessGroups corresponding to the mesh dimensions, or
returns a single ProcessGroup if mesh_dim is specified or the given mesh has
only one mesh dimension.
Args:
mesh_dim (str/int, optional): it can be the name of the mesh dimension or the index
of the mesh dimension. Default is None.
Returns:
A list of :class:`ProcessGroup` object when `mesh_dim` is not specified for
a DeviceMesh with more than 1 dimension; otherwise, returns a single
:class:`ProcessGroup` object.
"""
if not hasattr(self, "_dim_group_infos"):
raise RuntimeError("DeviceMesh process groups not initialized!")
if self.mesh.ndim == 1:
return _find_pg_by_ranks_and_tag(*self._dim_group_infos[0])
if mesh_dim is not None:
if isinstance(mesh_dim, str):
mesh_dim = self._get_mesh_dim_by_name(mesh_dim)
return _find_pg_by_ranks_and_tag(*self._dim_group_infos[mesh_dim])
else:
dim_groups = []
for ith_dim in range(self.mesh.ndim):
dim_groups.append(
_find_pg_by_ranks_and_tag(*self._dim_group_infos[ith_dim])
)
return dim_groups
def size(self, mesh_dim: Optional[int] = None) -> int:
return self.mesh.numel() if mesh_dim is None else self.mesh.size(mesh_dim)
@property
def ndim(self) -> int:
return self.mesh.ndim
@property
def shape(self) -> Tuple[int, ...]:
return tuple(self.mesh.shape)
def get_rank(self) -> int:
"""
Returns the current global rank.
"""
return get_rank()
def get_local_rank(self, mesh_dim: Optional[Union[int, str]] = None) -> int:
"""
Returns the local rank of the given mesh_dim of the DeviceMesh.
Args:
mesh_dim (str/int, optional): it can be the name of the mesh dimension or the index
of the mesh dimension. Default is None.
Returns:
An integer denotes the local rank.
The following program runs on each process/rank in an SPMD manner. In this example, we have 2
hosts with 4 GPUs each.
Calling mesh_2d.get_local_rank(mesh_dim=0) on rank 0, 1, 2, 3 would return 0.
Calling mesh_2d.get_local_rank(mesh_dim=0) on rank 4, 5, 6, 7 would return 1.
Calling mesh_2d.get_local_rank(mesh_dim=1) on rank 0, 4 would return 0.
Calling mesh_2d.get_local_rank(mesh_dim=1) on rank 1, 5 would return 1.
Calling mesh_2d.get_local_rank(mesh_dim=1) on rank 2, 6 would return 2.
Calling mesh_2d.get_local_rank(mesh_dim=1) on rank 3, 7 would return 3.
Example::
>>> # xdoctest: +SKIP("no rank")
>>> from torch.distributed.device_mesh import DeviceMesh
>>>
>>> # Initialize device mesh as (2, 4) to represent the topology
>>> # of cross-host(dim 0), and within-host (dim 1).
>>> mesh = DeviceMesh(device_type="cuda", mesh=[[0, 1, 2, 3],[4, 5, 6, 7]])
"""
if self.ndim > 1 and mesh_dim is None:
raise RuntimeError(
f"Found the DeviceMesh have {self.mesh.ndim} dimensions",
"Optional kwarg `mesh_dim` needs to be specified when device_mesh.ndim > 1.",
)
elif mesh_dim is None:
mesh_dim = 0
mesh_dim_group = self.get_group(mesh_dim) # type: ignore[arg-type]
assert isinstance(
mesh_dim_group, ProcessGroup
), "We expect ProcessGroup before calling `get_rank`!"
return get_rank(mesh_dim_group) # type: ignore[arg-type]
def get_coordinate(self) -> Optional[List[int]]:
"""
Return the relative indices of this rank relative to all
dimensions of the mesh. If this rank is not part of the mesh, return None.
"""
return self._coordinate_on_dim if self._coordinate_on_dim else None
def _get_mesh_dim_by_name(self, mesh_dim_name: str) -> int:
if self.mesh_dim_names is None or len(self.mesh_dim_names) == 0:
raise KeyError(
"No `mesh_dim_names` found.",
)
if mesh_dim_name not in self.mesh_dim_names:
raise KeyError(
f"Mesh dimension '{mesh_dim_name}' does not exist.",
f"Available mesh dimensions are: {self.mesh_dim_names}",
)
return self.mesh_dim_names.index(mesh_dim_name) # type: ignore[union-attr]
[docs] def init_device_mesh(
device_type: str,
mesh_shape: Tuple[int, ...],
*,
mesh_dim_names: Optional[Tuple[str, ...]] = None,
) -> DeviceMesh:
"""
Initializes a `DeviceMesh` based on `device_type`, `mesh_shape`, and `mesh_dim_names` parameters.
This creates a DeviceMesh with an n-dimensional array layout, where `n` is the length of `mesh_shape`.
If `mesh_dim_names` is provided, each dimension is labeled as `mesh_dim_names[i]`.
.. note::
`init_device_mesh` follows SPMD programming model, meaning the same PyTorch Python program
runs on all processes/ranks in the cluster. Ensure `mesh_shape` (the dimensions of the nD array
describing device layout) is identical across all ranks. Inconsistent `mesh_shape` may lead to hanging.
.. note::
If no process group is found, init_device_mesh will initialize distributed process group/groups
required for distributed communications behind the scene.
Args:
device_type (str): The device type of the mesh. Currently supports: "cpu", "cuda/cuda-like".
mesh_shape (Tuple[int]): A tuple defining the dimensions of the multi-dimensional array
describing the layout of devices.
mesh_dim_names (Tuple[str], optional): A tuple of mesh dimension names to assign to each dimension
of the multi-dimensional array describing the layout of devices. Its length must match the length
of `mesh_shape`. Each string in `mesh_dim_names` must be unique.
Returns:
DeviceMesh: A :class:`DeviceMesh` object representing the device layout.
Example::
>>> # xdoctest: +SKIP("no rank")
>>> from torch.distributed.device_mesh import init_device_mesh
>>>
>>> mesh_1d = init_device_mesh("cuda", mesh_shape=(8,))
>>> mesh_2d = init_device_mesh("cuda", mesh_shape=(2, 8), mesh_dim_names=("dp", "tp"))
"""
if mesh_dim_names is not None:
if len(set(mesh_dim_names)) != len(mesh_dim_names):
raise RuntimeError(
"Each mesh_dim_name must be unique.",
f"Found repeated mesh_dim_name in mesh_dim_names {mesh_dim_names}",
)
if len(mesh_shape) != len(mesh_dim_names):
raise RuntimeError(
"mesh_shape and mesh_dim_names should have same length!",
f"Found len(mesh_dim_names): {len(mesh_dim_names)} and len(mesh_shape):{len(mesh_shape)}.",
)
mesh = torch.arange(math.prod(mesh_shape)).view(mesh_shape)
device_mesh = DeviceMesh(
device_type=device_type,
mesh=mesh,
mesh_dim_names=mesh_dim_names,
)
return device_mesh
