Source code for torch.distributed.optim.zero_redundancy_optimizer

# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
# This source code is licensed under the BSD license found in the
# LICENSE file in the root directory of this source tree.

import collections
import copy
import io
from collections import OrderedDict
from itertools import chain
from typing import Any, Callable, Dict, List, Optional, Type

import torch
import torch.distributed as dist
from torch.nn import Parameter
from torch.optim import Optimizer
import logging

__all__ = ["ZeroRedundancyOptimizer"]

# Credits:  classy_vision/generic/
def _recursive_copy_to_device(value: Any, non_blocking: bool, device: torch.device) -> Any:
    Recursively searches lists, tuples, dicts and copies tensors to device if
    possible. Non-tensor values are passed as-is in the result.

    .. note:  These are all copies, so if there are two objects that reference
    the same object, then after this call, there will be two different objects
    referenced on the device.

    if isinstance(value, torch.Tensor):
        return, non_blocking=non_blocking)

    if isinstance(value, (list, tuple)):
        values = [_recursive_copy_to_device(val, non_blocking=non_blocking, device=device) for val in value]
        return values if isinstance(value, list) else tuple(values)

    if isinstance(value,
        return {
            key: _recursive_copy_to_device(val, non_blocking=non_blocking, device=device) for key, val in value.items()

    return value

def _is_trainable(param: torch.Tensor) -> bool:
    return param.requires_grad

def _broadcast_object(
    obj: Any,
    src_rank: int,
    group: object =,
    dist_device: torch.device = torch.device("cpu"),
) -> Any:
    Either broadcast from master to the fleet (default),
    or use the src setting as the original rank.

    if dist.get_rank() == src_rank:
        # Emit data
        buffer = io.BytesIO(), buffer)
        data = bytearray(buffer.getbuffer())
        length_tensor = torch.LongTensor([len(data)]).to(dist_device)
        data_send_tensor = torch.ByteTensor(data).to(dist_device)
        dist.broadcast(length_tensor, src=src_rank, group=group, async_op=False)
        dist.broadcast(data_send_tensor, src=src_rank, group=group, async_op=False)
        # Fetch from the source
        length_tensor = torch.LongTensor([0]).to(dist_device)
        dist.broadcast(length_tensor, src=src_rank, group=group, async_op=False)
        data_recv_tensor = torch.empty([int(length_tensor.item())], dtype=torch.uint8, device=dist_device)
        dist.broadcast(data_recv_tensor, src=src_rank, group=group, async_op=False)
        buffer = io.BytesIO(data_recv_tensor.cpu().numpy())
        obj = torch.load(buffer, map_location=dist_device)
    return obj

def _get_global_rank(group: Any, rank: int) -> int:
    return rank if group is else dist.distributed_c10d._get_global_rank(group, rank)

[docs]class ZeroRedundancyOptimizer(Optimizer): r""" This class wraps an arbitrary :class:`optim.Optimizer <torch.optim.Optimizer>` and shards its states across ranks in the group as described by ZeRO_. The optimizer instance in each rank is only responsible for updating ``1 / world_size`` parameters and hence only needs to keep ``1 / world_size`` optimizer states. After parameters are updated locally, each rank will broadcast its parameters to all other peers to keep all model replicas in the same state. ``ZeroRedundancyOptimizer`` can be used in conjunction with :class:`torch.nn.parallel.DistributedDataparallel` to reduce per-rank peak memory consumption. ``ZeroRedundancyOptimizer`` use a greedy algorithm to pack a number of parameters at each rank. Each parameter belongs to a single rank and is not divided among ranks. The partition is arbitrary and might not match the the parameter registration or usage order. Arguments: params (``Iterable``): an ``Iterable`` of :class:`torch.Tensor` s Keyword Args: optimizer_class (:class:`torch.nn.Optimizer`): the class of the local optimizer. group (``ProcessGroup``, optional): ``torch.distributed`` ``ProcessGroup`` (default: ``group.WORLD`` initialized by :meth:`torch.distributed.init_process_group`). parameters_as_bucket_views (bool): when enabled, parameters will be packed into larger buckets to speed up communication and ```` fields will point to bucket views at different offsets. When disabled, each individual parameter will be communicated separately, but ```` will stay intact. **default: all trailing arguments will be forwarded to the given optimizer. Example:: >>> import torch.nn as nn >>> from torch.distributed.optim import ZeroRedundancyOptimizer >>> from torch.nn.parallel import DistributedDataParallel as DDP >>> model = nn.Sequential(*[nn.Linear(2000, 2000).to(rank) for _ in range(20)]) >>> ddp = DDP(model, device_ids=[rank]) >>> opt = ZeroRedundancyOptimizer( >>> ddp.parameters(), >>> optimizer_class=torch.optim.Adam, >>> lr=0.01 >>> ) >>> ddp(inputs).sum().backward() >>> opt.step() .. warning: ZeroRedundancyOptimizer is experimental and subject to change. .. _ZeRO: """ def __init__( self, params, optimizer_class: Type[Optimizer], group: Optional[Any] = None, parameters_as_bucket_view: bool = False, **default: Any, ): # Hold all the model params in the root .param_groups # NOTE: the default constructor uses `add_param_group` which is partially overloaded here # we introduce the `initialized` flag for be able to dissociate the behaviour of # `add_param_group` in between super() and ZeroRedundancyOptimizer self.initialized = False super().__init__(params, default) # Partition information. lazy evaluation, computed if requested self._per_device_params_cache: "OrderedDict[torch.device, List[List[Parameter]]]" = ( OrderedDict() ) # device, rank, params self._param_rank_cache: Dict[torch.Tensor, int] = {} self._param_to_index_cache: Dict[int, int] = {} self._partition_parameters_cache: List[List[Dict]] = [] self._index_to_param_cache: Dict[int, torch.Tensor] = {} self._all_params = params self._reference_is_trainable_mask = list(map(_is_trainable, self._all_params)) # Build the wrapped optimizer, responsible for a shard of the params = group if group is not None else self.world_size = dist.get_world_size( self.rank = dist.get_rank( self.global_rank = _get_global_rank(, self.rank) self.parameters_as_bucket_view = parameters_as_bucket_view self._optim_defaults = default self._optim_constructor = optimizer_class # Optional consolidated optimizer state self._all_states: List[Dict[str, Any]] = [] # Current default device is set by the parameters allocated to this rank self._device = list(self._per_device_params.keys())[0] self.buckets: Dict[torch.device, List[torch.Tensor]] = {} self._update_trainable() self.initialized = True def _clear_cache(self) -> None: self._partition_parameters_cache.clear() self._per_device_params_cache.clear() self._param_rank_cache.clear() self._index_to_param_cache.clear() self._param_to_index_cache.clear()
[docs] def add_param_group(self, param_group: dict) -> None: r""" Add a param group to the :class:`Optimizer` s ``param_groups``. This can be useful when fine tuning a pre-trained network, as frozen layers can be made trainable and added to the :class:`Optimizer` as training progresses. Arguments: param_group (dict): Specifies what Tensors should be optimized along with group specific optimization options. .. warning: This method handles updating the shards on all partitions, but needs to be called on all ranks. Calling this on a subset of the ranks will cause the training to hang, because communication primitives are called depending on the managed parameters, and expect all the ranks to participate on the sane set of parameters. """ super().add_param_group(param_group) if self.initialized: # Force a re-partitioning self._clear_cache() param_groups = self.partition_parameters()[self.rank] if len(param_groups) == len(self.optim.param_groups) + 1: self.optim.add_param_group(param_groups[-1]) # Update the bucketing strategy accordingly if self.parameters_as_bucket_view: self._setup_flat_buffers()
[docs] def consolidate_state_dict(self, to: int = 0) -> None: r""" Update the consolidated state_dict list, one per rank. Arguments: to (int): the rank that receives the global states. (default: 0) .. warning: This needs to be called on all replicas """ # Sync lr and other attributes in case its been updated self._sync_param_groups(self.param_groups, self.optim.param_groups) empty_messenger = torch.tensor([0], dtype=torch.uint8, device=self._device) # Pull the sharded state from all the other replicas # Store all the states in order, rank by rank # NOTE: In practice, `broadcast` is used, which is wasteful (gather would have been appropriate) # compatibility issues with some backends make the use of broadcast mandatory for now. # a possible follow up would be to move all sharded state management to RPC RRef self._all_states = [] for rank in range(self.world_size): global_rank = _get_global_rank(, rank) # This rank collects the whole state if self.rank == to: if rank == self.rank: self._all_states.append( _recursive_copy_to_device( self.local_state_dict(), non_blocking=True, device=torch.device("cpu"), ) ) else: # Fetch the optim state from the other replicas replica_state = _broadcast_object( empty_messenger, src_rank=global_rank,, dist_device=self._device, ) self._all_states.append( _recursive_copy_to_device(replica_state, non_blocking=True, device=torch.device("cpu")) ) else: # Acknowledge broadcasts, and send this rank's shard when needed # Default to CPU space to gain some memory headroom if rank == self.rank: # Send the state to the reference replica _ = _broadcast_object( self.local_state_dict(), src_rank=self.global_rank,, dist_device=self._device, ) elif rank != to: # Discard this tensor/rank, broadcast was being use for compatibility reasons _ = _broadcast_object( empty_messenger, src_rank=global_rank,, dist_device=self._device, )
[docs] def partition_parameters(self) -> List[List[Dict]]: r""" Partitions parameters across distributed data parallel ranks. Returns: a list of ``param_groups`` (which is a list of dict) where each element of the list contains the param_groups for a rank. Element 0 corresponds to rank 0, etc. We need all the ranks for the broadcast inside ``step()``. """ if len(self._partition_parameters_cache) == 0: self._partition_parameters_cache = [list() for _ in range(self.world_size)] sizes = [0] * self.world_size for param_group in self.param_groups: param_lists: List[List] = [list() for _ in range(self.world_size)] for param in param_group["params"]: # Add this param to rank with smallest size. rank = sizes.index(min(sizes)) param_lists[rank].append(param) sizes[rank] += param.numel() for rank, params in enumerate(param_lists): param_group_rank = copy.copy(param_group) param_group_rank["params"] = params self._partition_parameters_cache[rank].append(param_group_rank) return self._partition_parameters_cache
@property def _per_device_params(self) -> Dict[torch.device, List[List[Parameter]]]: r""" Sorted list of all the params, first per device then per rank. Within a list params are sorted per number of elements to allow for an easy bucketing. """ if len(self._per_device_params_cache) == 0: # Go through all params, log them per device # The ordering is important here, needs to be the same on all ranks # So that ulterior broadcast calls are matching for param_group in self.param_groups: for param in param_group["params"]: device = param.device if self._per_device_params_cache.get(device) is None: self._per_device_params_cache[device] = [[] for _ in range(self.world_size)] self._per_device_params_cache[device][self._param_to_rank[param]] += [param] # Sort param_lists by size for k in self._per_device_params_cache.keys(): for r in self._per_device_params_cache[k]: r.sort(key=lambda x: x.numel()) return self._per_device_params_cache @property def _param_to_rank(self) -> Dict[torch.Tensor, int]: r"""Look up table to match a given param with a data parallel rank""" if len(self._param_rank_cache) == 0: for rank, param_groups in enumerate(self.partition_parameters()): for param_group in param_groups: for param in param_group["params"]: self._param_rank_cache[param] = rank return self._param_rank_cache @property def _param_to_index(self) -> Dict[int, int]: r""" Hash table in between parameter indices in the global optimizer scheme, and the actual params. """ if len(self._param_to_index_cache) == 0: self._param_to_index_cache = { id(p): i for i, p in enumerate(chain(*(g["params"] for g in self.param_groups))) } return self._param_to_index_cache @property def _index_to_param(self) -> Dict[int, torch.Tensor]: r""" Hash table in between parameter indices in the global optimizer scheme, and the actual params. """ if len(self._index_to_param_cache) == 0: self._index_to_param_cache = {i: p for i, p in enumerate(chain(*(g["params"] for g in self.param_groups)))} return self._index_to_param_cache
[docs] def step(self, closure: Optional[Callable[[], float]] = None, **kwargs: Any) -> Optional[float]: r""" Performs a single optimization step (parameter update). Arguments: closure (callable): A closure that reevaluates the model and returns the loss. Optional for most optimizers. Returns: optional loss, depends on the underlying optimizer .. note: Any extra parameter is passed to the base optimizer as-is """ # Check whether the model trainability graph changed trainable_mask = list(map(_is_trainable, self._all_params)) if trainable_mask != self._reference_is_trainable_mask: logging.warning( "ZeroRedundancyOptimizer detected that the trainable params changed, updating the partitioning" ) self._update_trainable() self._reference_is_trainable_mask = trainable_mask # Sync oss param_groups attributes in case they've been updated by a scheduler. self._sync_param_groups(self.param_groups, self.optim.param_groups) # Run the optimizer step on this shard only: if closure is not None: loss = self.optim.step(closure=closure, **kwargs) # type: ignore[call-arg] else: loss = self.optim.step(**kwargs) # Sync all the updated shards in between the ranks handles = [] if self.parameters_as_bucket_view: for device in self.buckets.keys(): for src_rank, bucket in enumerate(self.buckets[device]): global_src_rank = _get_global_rank(, src_rank) handles.append(dist.broadcast(tensor=bucket, src=global_src_rank,, async_op=True)) else: for device, per_rank_params in self._per_device_params.items(): for dst_rank, params in enumerate(per_rank_params): global_dst_rank = _get_global_rank(, dst_rank) for param in params: handles.append( dist.broadcast(, src=global_dst_rank,, async_op=True) ) _ = list(map(lambda x: x.wait(), handles)) # Sync hypothethical new results from the wrapped optimizer to the exposed param_groups self._sync_param_groups(self.optim.param_groups, self.param_groups) return loss
[docs] def load_state_dict(self, state_dict: Dict[str, Any]) -> None: r""" Restore the global parameter groups as well as the shard. Arguments: state_dict (dict): optimizer state. Should be an object returned from a call to :meth:`state_dict` """ for key, value in state_dict["state"].items(): param = self._index_to_param[key] # Populate the sharded optimizer state on the fly if self._param_to_rank[param] != self.rank: state_dict["state"][key] = None else: self.optim.state[param] = _recursive_copy_to_device(value, non_blocking=True, device=param.device) super().load_state_dict(state_dict) # Sync with the optimizer param groups ZeroRedundancyOptimizer._sync_param_groups(state_dict["param_groups"], self.param_groups) ZeroRedundancyOptimizer._sync_param_groups(self.param_groups, self.optim.param_groups)
[docs] def local_state_dict(self) -> Dict: r""" Gets this rank's ``state_dict``. Returns: The state of the optimizer as a :class:`dict`. It contains two entries: * state - a dict holding current optimization state. Its content differs between optimizer classes. * param_groups - a dict containing all parameter groups """ return self.optim.state_dict()
[docs] def state_dict(self) -> Dict[str, Any]: r""" Returns: the last known global optimizer state, which consist of a list of the shards. .. warning: If the state has not been consolidated, this returns a shard's worth, not the global state. .. warning: Returning the global state is limited to the replica which was responsible for the consolidation. The state may also not be up to date, depending on when :meth:`consolidate_state_dict` was last called. """ if len(self._all_states) == 0: raise RuntimeError( "Optimizer state has not been consolidated on this rank. \ Please call `consolidate_state_dict()` on all ranks beforehand if you meant to save the global state" ) # Unify the shard states and the state that pytorch would expect, given the model. # Indexation needs several redirections, since each shard only knows a limited scope of the model # - get the pytorch compliant parameter indexing state_dict = super().state_dict() # - go through the per-shard states, which are all indexed locally for rank, s in enumerate(self._all_states): # -- match the local indexing and the global partition, update the corresponding saved state globally for local_pg, global_pg in zip(s["param_groups"], self.partition_parameters()[rank]): local_index_to_param_id = { i_param: id(global_pg["params"][i]) for i, i_param in enumerate(local_pg["params"]) } for local_param_index in local_pg["params"]: # Update the state, if any if local_param_index in s["state"].keys(): global_id = self._param_to_index[local_index_to_param_id[local_param_index]] state_dict["state"][global_id] = s["state"][local_param_index] # Make sure that the parameters are sorted in the state, as expected state_dict["state"] = dict(sorted(state_dict["state"].items())) return state_dict
[docs] @staticmethod def rank_local_state_dict(rank: int, state_dict: dict) -> dict: r""" Returns the local_state_dict for a given rank. Arguments: rank (int): rank to get ``local_state_dict`` for state_dict (dict): global ``state_dict`` """ param_groups = state_dict["param_groups"][state_dict["partition"][rank][0] : state_dict["partition"][rank][1]] return {"state": state_dict["state"][rank], "param_groups": param_groups}
@staticmethod def _sync_param_groups(source: List[Dict[Any, Any]], destination: List[Dict[Any, Any]]) -> None: r"""Sync learning rate and other optimizer attributes (needed to support schedulers).""" for source_group, destination_group in zip(source, destination): # Sync everything but the parameters for k in filter(lambda x: x != "params", source_group.keys()): destination_group[k] = source_group[k] def _setup_flat_buffers(self) -> None: r""" Make all params which are on the same device and tied to the same rank views of a single buffer. This is used at construction time, and anytime parameter trainability is changed (frozen or unfrozen) and ``_update_trainable`` is called. """ for device, per_rank_params in self._per_device_params.items(): # Only wipe the existing buckets if there are none # (could be that this is called twice, when trainability changes) if device not in self.buckets.keys(): self.buckets[device] = [] # Make parameters a view of the bucket for dst_rank, params in enumerate(per_rank_params): if len(params) > 0: # Clone the non-trainable params, if in a bucket it will get destroyed for param in filter(lambda x: not x.requires_grad, params): = # Merge all the trainable params in a single bucket trainable_params = list(filter(_is_trainable, params)) buffer_size = sum(map(lambda x: x.numel(), trainable_params)) bucket = torch.empty(buffer_size, dtype=params[0].dtype, device=device) offset = 0 for param in trainable_params: offset_next = offset + param.numel() bucket[offset:offset_next].copy_( = bucket[offset:offset_next].view_as( offset = offset_next # Either replace the existing bucket, or create it if len(self.buckets[device]) == dst_rank: self.buckets[device].append(bucket) else: self.buckets[device][dst_rank] = bucket else: self.buckets[device].append(torch.zeros(1, device=device)) def _update_trainable(self) -> None: r""" Updates the partitioning and communication patterns if the trainability (``requires_grad``) of some parameters changed. """ # Create the optim which will work on the param shard if not hasattr(self, "optim"): self._clear_cache() self._default_device = list(self._per_device_params.keys())[0] self.optim = self._optim_constructor(self.partition_parameters()[self.rank], **self._optim_defaults) self._sync_param_groups(self.optim.param_groups, self.param_groups) if self.parameters_as_bucket_view: self._setup_flat_buffers()


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