diff --git "a/venv/lib/python3.10/site-packages/torch/distributed/fsdp/_flat_param.py" "b/venv/lib/python3.10/site-packages/torch/distributed/fsdp/_flat_param.py" new file mode 100644--- /dev/null +++ "b/venv/lib/python3.10/site-packages/torch/distributed/fsdp/_flat_param.py" @@ -0,0 +1,2723 @@ +import contextlib +import functools +import logging +import os +import warnings +from enum import auto, Enum +from itertools import accumulate, chain +from typing import ( + Any, + Callable, + cast, + Dict, + Generator, + Iterator, + List, + NamedTuple, + no_type_check, + Optional, + Sequence, + Set, + Tuple, + Union, +) + +import torch +import torch.distributed as dist +import torch.nn as nn +import torch.nn.functional as F +from torch import Tensor +from torch.distributed.fsdp._common_utils import ( + _FSDPDeviceHandle, + _named_parameters_with_duplicates, + _no_dispatch_record_stream, + _set_fsdp_flattened, + HandleTrainingState, +) +from torch.distributed.utils import ( + _alloc_storage, + _data_ptr_allocated, + _free_storage, + _p_assert, +) +from torch.nn.parameter import _ParameterMeta # type: ignore[attr-defined] +from torch.testing._internal.distributed.fake_pg import FakeProcessGroup + +from ._fsdp_extensions import ( + _ext_post_unflatten_transform, + _ext_pre_flatten_transform, + FSDPExtensions, +) + +__all__ = [ + "FlatParameter", + "FlatParamHandle", + "FlatParamShardMetadata", + "ParamInfo", + "SharedParamInfo", + "HandleShardingStrategy", +] + +log = logging.getLogger(__name__) + + +""" +[Note: Fully Sharded Module] +We define the "fully sharded module" to be the original ``nn.Module`` that owns +a ``FlatParamHandle``. It is the *single* module logically responsible for the +*single* unshard/reshard pair for the handle's ``FlatParameter`` for a given +forward or backward pass. The fully sharded module should be passed to the +``FlatParamHandle`` constructor. + +For the wrapper code path: +- The ``FullyShardedDataParallel`` module wrapping the fully sharded module +runs the unshard/reshard on behalf of the fully sharded module by overriding +``nn.Module.forward``. +- The fully sharded module is exactly the module passed to the +``FullyShardedDataParallel`` constructor's ``module`` argument. + +For the non-wrapper code path: +- Hooks registered on the fully sharded module run the unshard/reshard. +- The fully sharded module may either be the direct argument to ``fully_shard`` +or a submodule chosen by the provided wrapping policy. +""" + +# Environment variable toggling whether to use unsafe `setattr()` for view +# setting in `_use_sharded_views()` and `_use_unsharded_views()` +# We should use 'safe' by default since it respects method overrides, but for +# special cases such as for high CPU overhead or for intentionally bypassing +# checks in the overrides, we may use 'unsafe'. +_FSDP_USE_UNSAFE_SETATTR = "FSDP_USE_UNSAFE_SETATTR" + +# Environment variable toggling whether to check for parameter/gradient +# writeback in case their storages change after FSDP initialization +# We should check by default since it prevents silent correctness errors, but +# since such changes are atypical, we may want to skip the check to save CPU +# overhead, especially since the check happens in the pre-forward and +# pre-backward each iteration. +_FSDP_SKIP_WRITEBACK_CHECK = "FSDP_SKIP_WRITEBACK_CHECK" + +# Env var toggling whether when model is in .eval() mode, should we run in fp32 +# or the reduced precision. +_FSDP_USE_FULL_PREC_IN_EVAL = "FSDP_USE_FULL_PREC_IN_EVAL" + +# Some value to set padding in tensors to for debuggability +_FLAT_PARAM_PADDING_VALUE = 42 + +# Environment variables for disabling the all-gather and reduce-scatter +# communication ops for ablation studies. Note that without these communication +# ops the training won't converge, and you probably need to disable correctness +# checks in your model. +_FSDP_USE_FAKE_ALL_GATHER = "FSDP_USE_FAKE_ALL_GATHER" +_FSDP_USE_FAKE_REDUCE = "FSDP_USE_FAKE_REDUCE" + + +# TODO: Define this for now to avoid circular imports. See if we can remove. +class HandleShardingStrategy(Enum): + FULL_SHARD = auto() + SHARD_GRAD_OP = auto() + NO_SHARD = auto() + HYBRID_SHARD = auto() + _HYBRID_SHARD_ZERO2 = auto() + + +RESHARD_AFTER_FORWARD_HANDLE_STRATEGIES = ( + HandleShardingStrategy.FULL_SHARD, + HandleShardingStrategy.HYBRID_SHARD, +) +NO_RESHARD_AFTER_FORWARD_HANDLE_STRATEGIES = ( + HandleShardingStrategy.SHARD_GRAD_OP, + HandleShardingStrategy._HYBRID_SHARD_ZERO2, +) + + +class ParamInfo(NamedTuple): + """Information for an original parameter.""" + + param_name: str # unprefixed + module: nn.Module + module_name: str + + +class SharedParamInfo(NamedTuple): + """ + Additional information for a shared parameter. + + For each shared parameter, we designate one module and its parameter + variable to be the primary owner, determined as the first one encountered + in the parameter walk. These are prefixed with "prim". The primary module + and parameter do not have their own :class:`SharedParamInfo` instance. + """ + + param_name: str # unprefixed + module: nn.Module + module_name: str + prim_param_name: str # unprefixed + prim_module: nn.Module + prim_module_name: str + + +class _ShardParamInfo(NamedTuple): + """Shard-related information for an original parameter.""" + + in_shard: bool + # Use to index into the sharded flat parameter, e.g. + # `flat_param[offset_in_shard : offset_in_shard + numel_in_shard]` + offset_in_shard: Optional[int] + numel_in_shard: Optional[int] + # Use to get part of the parameter in the local shard from a flattened + # version of the unsharded parameter, e.g. + # `param.flatten()[intra_param_start_idx : intra_param_end_idx + 1]` + intra_param_start_idx: Optional[int] + intra_param_end_idx: Optional[int] # inclusive + + +class FlatParamShardMetadata(NamedTuple): + """ + This holds metadata specific to this rank's shard of the flat parameter. + + Attributes: + param_names (Tuple[str, ...]): Prefixed parameter names of this rank's + shard of the parameters; see :class:`FlatParameter`. + param_shapes (Tuple[torch.Size, ...]): Parameter shapes of this rank's + shard of the parameters; see :class:`FlatParameter`. + param_numels (Tuple[int, ...]): Parameter numels of this rank's shard + of the parameters; see :class:`FlatParameter`. + param_offsets (Tuple[Tuple[int, int], ...]): [start, end] offsets (in + units of numels) giving this rank's part of each flattened + original parameter. + """ + + param_names: Tuple[str, ...] + param_shapes: Tuple[torch.Size, ...] + param_numels: Tuple[int, ...] + param_offsets: Tuple[Tuple[int, int], ...] + + +class _FlatParameterMeta(_ParameterMeta): + # Make `isinstance(t, FlatParameter)` return True for custom tensor + # instances that have the _is_flat_param flag for BC + def __instancecheck__(self, instance): + # NB: do NOT test the super implementation + return isinstance(instance, torch.Tensor) and getattr( + instance, "_is_flat_param", False + ) + + +class FlatParameter(nn.Parameter, metaclass=_FlatParameterMeta): + """ + This is the flat parameter used by :class:`FullyShardedDataParallel`. + + It is comprised of one or more original parameters, which are flattened and + concatenated to construct the flat parameter. + + Under the current design, this parameter logically represents both the + unsharded and sharded flat parameter, and its data changes storages + dynamically. + - In the :class:`FullyShardedDataParallel` constructor, the parameter + is initialized as unsharded and then sharded in-place. + - At runtime, the parameter is lazily (re)-initialized. The sharded + parameter data is saved in ``self._local_shard``, and a new ``Tensor`` + ``self._full_param_padded`` is created, which is the all-gather + destination and owns the unsharded parameter storage thereafter. (See + :meth:`FlatParamHandle.init_flat_param_attributes`.) + - Throughout runtime, the parameter data changes storages as needed, + e.g. to the sharded flat parameter, low precision sharded flat + parameter, or the unsharded flat parameter. + + NOTE: Since ``use_orig_params=True`` supports intra-``FlatParameter`` + padding, we have two versions of the per-parameter numels, one that + includes the padding (``_numels_with_padding``) and one that does not + (``_numels``). The former may have length longer than the other data + structures, while the latter has the same length as the number of actual + original parameters like the other per-parameter data structures. + + NOTE: This is not a real class; instead, you will always get a Parameter + back out if you try to create one of these. This is similar to the trick + we implemented for Parameter to get it to work with subclasses; this + is primarily so that FlatParameter supports combination with FakeTensor. + + Attributes: + _unpadded_unsharded_size (torch.Size): Unsharded flat parameter's size + without right-hand-side padding for divisibility by the world size. + For ``use_orig_params=True``, this includes alignment padding. + _padded_unsharded_size (torch.Size): Unsharded flat parameter's size + with right-hand-side padding for divisibility by the world size. + For ``use_orig_params=True``, this includes alignment padding. This + is only set for sharded strategies since they require padding for + the all-gather. + _sharded_size (torch.Size): Sharded flat parameter's size with padding. + This is also set for ``NO_SHARD``, in which case it is the same as + the unsharded sizes. (We omit "padded" because there is no + analogous unpadded one.) + + _num_params (int): Number of original parameters flattened into this + flat parameter. This is the length of the per-parameter data + structures. + _param_infos (Tuple[ParamInfo, ...]): Each parameter's parameter info + entry; see :class:`ParamInfo` for details. + _shapes (Tuple[torch.Size, ...]): Each parameter's original shape. + _fqns (Tuple[str, ...]): Each parameter's fully-qualified name (FQN) + prefixed from the ``_fully_sharded_module``. The names are + guaranteed to be unique in the subtree rooted at that module. + _param_extensions (Tuple[Optional[Any], ...]): Each parameter's + extension (i.e. some per-parameter state) used to customize + pre-flatten and post-unflatten behavior or ``None``. This is + experimental, and users should not depend on its existence in the + future. + _numels_with_padding (Tuple[int, ...]): Each parameter's numel + including entries for the padding. This is used to construct views + into the flat parameter via ``torch.split()``. This may have length + longer than ``_num_params``. + _numels (Tuple[int, ...]): Each parameter's numel excluding entries for + padding. This has length equal to ``_num_params``. + _shard_param_infos (Tuple[_ShardParamInfo, ...]): Each parameter's + shard parameter info; see :class:`_ShardParamInfo` for details. + _shared_param_infos (Tuple[SharedParamInfo, ...]): Shared parameter + info entries; see :class:`SharedParamInfo` for details. + _modules (Set[nn.Module]): Modules that contain some original parameter + that is flattened into the flat parameter. + + _shard_numel_padded (int): Numel padded for this rank's sharded flat + parameter. + _local_shard (Tensor): Sharded flat parameter with padding if using a + sharded strategy. If using ``NO_SHARD``, then this is the unpadded + unsharded flat parameter, and there is no notion of a sharded flat + parameter or padded unsharded flat parameter. + _full_param_padded (Tensor): Unsharded flat parameter with padding. + This is not defined for ``NO_SHARD``. When using mixed precision + for parameters, this has the low precision. + _full_prec_full_param_padded (Tensor): Full precision unsharded flat + parameter with padding. This is used for unsharding outside of + computation when using mixed precision for parameters. This is + never defined for ``NO_SHARD``. + _post_backward_hook_handle (RemovableHandle): + Flat parameter's post-backward hook handle. (Compile only) + _post_backward_hook_state (Tuple[AccumulateGrad, RemovableHandle]): + Flat parameter's :class:`AccumulateGrad` object and post-backward + hook handle. (Eager only) + _mp_shard (Tensor): Low precision sharded flat parameter with padding. + This is only defined when parameter mixed precision is enabled. For + ``NO_SHARD``, this is used for computation. + _cpu_grad (Tensor): Sharded gradient with padding stored on CPU. + This is only defined when offloading parameters is enabled. + _saved_grad_shard (Tensor): Sharded gradient with padding from previous + iterations for gradient accumulation without :meth:`no_sync`. + + _params (Optional[List[nn.Parameter]]): If ``use_orig_params=True``, + then each original parameter variable; otherwise, ``None``. This + does not include any padding tensors. + _shared_params (Optional[List[nn.Parameter]]): The original shared + parameter variables if ``use_orig_params=True`` and ``None`` + otherwise. + _tensors (Optional[List[Optional[Tensor]]]): This saves the ``Tensor`` + views created in the forward and tracked by autograd when + ``use_orig_params=True`` and is ``None`` otherwise. This is to + preserve those ``Tensor`` variables for the backward to ensure that + the ``FlatParameter`` 's ``AccumulateGrad`` object does not change + in which case the post-backward hook does not run. This is relevant + for cases like reentrant activation checkpointing. + _is_grad_none_mask (Optional[List[bool]]): If ``use_orig_params=True``, + a mask over the original parameters' gradients indicating if it is + logically ``None`` or not; otherwise, ``None``. This does not + include entries for padding. This mask is needed because only some + of the parameters may have ``None`` gradient, in which case the + flat gradient must be non-``None`` and must use zeros to + approximate those original ``None`` gradients. This mask informs + FSDP to set the original parameter gradients to ``None`` (instead + of zeros) as needed. + """ + + _unpadded_unsharded_size: torch.Size + _padded_unsharded_size: torch.Size + _sharded_size: torch.Size + _num_params: int + _param_infos: Tuple[ParamInfo, ...] + _shapes: Tuple[torch.Size, ...] + _fqns: Tuple[str, ...] + _param_extensions: Tuple[Optional[Any], ...] + _numels_with_padding: Tuple[int, ...] + _numels: Tuple[int, ...] + _shard_param_infos: Tuple[_ShardParamInfo, ...] + _shared_param_infos: Tuple[SharedParamInfo, ...] + _modules: Set[nn.Module] + _shard_numel_padded: int + _local_shard: Tensor + _full_param_padded: Tensor + _full_prec_full_param_padded: Tensor + # Eager only + _post_backward_hook_state: Tuple[Any, Any] + # Compile only + _post_backward_hook_handle: Any + _mp_shard: Tensor + _cpu_grad: Tensor + _saved_grad_shard: Tensor + _params: Optional[List[nn.Parameter]] + _shared_params: Optional[List[nn.Parameter]] + _tensors: Optional[List[Optional[Tensor]]] + _is_grad_none_mask: Optional[List[bool]] + + _is_padding_mask: List[bool] + + def __new__(cls, data=None, requires_grad=True): + assert cls is FlatParameter, "subclasses FlatParameter not supported" + r = nn.Parameter.__new__(nn.Parameter, data, requires_grad) # type: ignore[call-arg] + r._is_flat_param = True # type: ignore[attr-defined] + return r + + # NB: This is not a regular method, because FlatParameters are not actually + # instances of this class (see __new__ above). So you must indirectly + # call this directly through the classmethod. + @classmethod + def _init_metadata( + cls, + self, + param_infos: List[ParamInfo], + numels: List[int], + shapes: List[torch.Size], + fqns: List[str], + shared_param_infos: List[SharedParamInfo], + param_extensions: List[Optional[Any]], + params: Optional[List[nn.Parameter]], + shared_params: Optional[List[nn.Parameter]], + is_padding_mask: List[bool], + ) -> None: + """ + Initialize attributes holding metadata about the original parameters comprising the flat parameter. + + We expose this method separate from the constructor to keep the + constructor only responsible for the flat parameter's tensor data. This + method should only be called once per model, while the constructor may + be called multiple times, e.g. when reloading from a checkpoint, in + which case only the tensor data needs to be passed to the constructor. + Since :meth:`load_state_dict` is implemented via :meth:`copy_`, the + metadata is correctly assumed to be unchanged. + + Args: + See the Attributes in the class docstring. + """ + assert len(param_infos) == len(shapes) + assert len(param_infos) == len(fqns) + assert len(param_infos) == len(param_extensions) + self._num_params = len(param_infos) + self._param_infos = param_infos + self._shapes = shapes + self._fqns = fqns + self._param_extensions = param_extensions + self._is_padding_mask = is_padding_mask + + numels_without_padding: List[int] = [] + for numel, is_padding in zip(numels, is_padding_mask): + if not is_padding: + numels_without_padding.append(numel) + self._numels = tuple(numels_without_padding) + self._numels_with_padding = tuple(numels) + assert len(self._numels) == self._num_params + + self._shared_param_infos = tuple(shared_param_infos) + self._modules = {pi.module for pi in self._param_infos}.union( + {spi.module for spi in self._shared_param_infos} + ) + assert (params is None) == (shared_params is None) + if params is not None: + assert shared_params is not None and len(shared_params) == len( + shared_param_infos + ) + self._params = [] + for param, is_padding in zip(params, is_padding_mask): + if not is_padding: + self._params.append(param) + self._shared_params = shared_params + # Mark the original parameters to avoid flattening them into + # another `FlatParameter` during recursive construction + for param in chain(self._params, self._shared_params): + _set_fsdp_flattened(param) + self._is_grad_none_mask = [False for _ in range(self._num_params)] + self._tensors = [None for _ in range(self._num_params)] + else: + self._params = None + self._shared_params = None + self._is_grad_none_mask = None + self._tensors = None + self._unpadded_unsharded_size = self.size() + _set_fsdp_flattened(self) + # Tracks whether the `FlatParameter`'s post-backward hook has been + # called to modify the behavior of the post-backward callback + self._post_backward_called = False + + +class FlatParamHandle: + """ + A handle that manages a flat parameter (:class:`FlatParameter`). + + This includes sharding and view management. + + Args: + params (Sequence[nn.Parameter]): The parameters to flatten into the + flat parameter. + fully_sharded_module (nn.Module): See [Note: Fully Sharded Module]. + device (torch.device): The compute and communication device, which + should be a non-CPU device. We refer to it as the compute device. + sharding_strategy (ShardingStrategy): Sharding strategy to apply to + this handle's ``FlatParameter``. + offload_params (bool): Whether to offload the handle's + ``FlatParameter`` to CPU. + mp_param_dtype (Optional[torch.dtype]): Parameter mixed precision + setting passed to the FSDP constructor. + mp_reduce_dtype (Optional[torch.dtype]): Gradient reduction mixed + precision setting passed to the FSDP constructor. + keep_low_precision_grads (bool): Whether to keep gradients in low + precision. + use_orig_params (bool): If ``True``, then FSDP preserves the original + parameter variables and returns them from ``named_parameters()`` + (e.g. to support different optimizer hyperparameters within one + :class:`FlatParameter`). If ``False``, then FSDP reconstructs the + parameters every iteration and returns the :class:`FlatParameter` s + from ``named_parameters()``. + """ + + ################## + # INITIALIZATION # + ################## + def __init__( + self, + params: Sequence[Union[nn.Parameter, Tensor]], + fully_sharded_module: nn.Module, + device: torch.device, + sharding_strategy: HandleShardingStrategy, + offload_params: bool, + mp_param_dtype: Optional[torch.dtype], + mp_reduce_dtype: Optional[torch.dtype], + keep_low_precision_grads: bool, + process_group: dist.ProcessGroup, + use_orig_params: bool, + *, + fsdp_extension: Optional[FSDPExtensions] = None, + ): + super().__init__() + params = list(params) + if len(params) == 0: + raise ValueError( + f"Cannot construct a {self.__class__.__name__} with an empty parameter list" + ) + self._init_setattr_fns() + self._skip_writeback_check = ( + os.environ.get(_FSDP_SKIP_WRITEBACK_CHECK, "") == "1" + ) + self._use_full_prec_in_eval = ( + os.environ.get(_FSDP_USE_FULL_PREC_IN_EVAL, "") == "1" + ) + self._use_fake_all_gather = os.environ.get(_FSDP_USE_FAKE_ALL_GATHER, "") == "1" + self._use_fake_reduce = os.environ.get(_FSDP_USE_FAKE_REDUCE, "") == "1" + if self._skip_writeback_check: + _warn_skip_writeback_check( + log, + f"Since {_FSDP_SKIP_WRITEBACK_CHECK}=1, FSDP will not check " + "for parameter or gradient writeback. Changing parameter or " + "gradient storages may lead to silent correctness errors.", + ) + if self._use_fake_all_gather: + _warn_use_fake_all_gather( + log, + f"Since {_FSDP_USE_FAKE_ALL_GATHER}=1, FSDP will not execute " + "all-gather ops. Your training will be incorrect, but " + "can reveal how much time spent on all-gather ops.", + ) + if self._use_fake_reduce: + _warn_use_fake_reduce( + log, + f"Since {_FSDP_USE_FAKE_REDUCE}=1, FSDP will not execute " + "reduce-scatter ops. Your training will be incorrect, but " + "can reveal how much time spent on reduce-scatter ops.", + ) + # Only align addresses for `use_orig_params=True` (for now) + align_addresses = use_orig_params + self._init_get_unflat_views_fn(align_addresses) + self.device = device + self._device_handle = _FSDPDeviceHandle.from_device(self.device) + self.process_group = process_group + if self._use_fake_all_gather or self._use_fake_reduce: + self._fake_process_group = FakeProcessGroup( + rank=process_group.rank(), world_size=process_group.size() + ) + self.rank = process_group.rank() + self.world_size = process_group.size() + self._sharding_strategy = sharding_strategy + self._offload_params = offload_params + self._use_orig_params = use_orig_params + self._keep_low_precision_grads = keep_low_precision_grads + self._training_state = HandleTrainingState.IDLE + self._debug_level = dist.get_debug_level() + self._fully_sharded_module = fully_sharded_module + # For strategies that do not free after forward, we skip using sharded + # views after forward since the unsharded data exists. We still switch + # `self.flat_param` to point to the sharded flat parameter since what + # it points to parameterizes behavior. We use the following attribute + # to track which tensor data the parameters are unsharded views into. + self._unsharded_flat_param_for_skipped_views: Optional[Tensor] = None + # The index in the state's `all_handles`, which must be the + # same across ranks for the execution order validation to work + self._handle_index: Optional[int] = None + # Index in handles_to_pre_forward_order + self._pre_forward_order_index: Optional[int] = None + # Index in `handles_post_forward_order` + self._post_forward_index: Optional[int] = None + # Used for guarding against mistargeted forward prefetches + self._needs_pre_forward_unshard = False + # Used for guarding against mistargeted backward prefetches + self._needs_pre_backward_unshard = False + # Was the handle prefetched? Set on successful _prefetch_handle and unshard + self._prefetched = False + # Optimistically assume a valid input `params` and set dtype attributes + # before `_init_flat_param()`, which performs the actual validation + self._orig_param_dtype = params[0].dtype + self._init_param_reduce_dtypes(mp_param_dtype, mp_reduce_dtype) + assert self._fwd_bwd_param_dtype is not None # mypy + self._aligned_numel = ( + _get_aligned_numel(unsharded_dtype=self._fwd_bwd_param_dtype) + if align_addresses + else 0 + ) + self._fsdp_extension = fsdp_extension + self._init_flat_param_and_metadata( + params, fully_sharded_module, self._aligned_numel, use_orig_params # type: ignore[arg-type] + ) + self._use_unsharded_views(as_params=False) + + def _init_setattr_fns(self): + use_unsafe_setattr = os.environ.get(_FSDP_USE_UNSAFE_SETATTR, "") == "1" + self._setattr_tensor: Callable[[nn.Module, str, Tensor], None] + self._setattr_param: Callable[[nn.Module, str, nn.Parameter], None] + if use_unsafe_setattr: + self._setattr_tensor = _unsafe_setattr_tensor + self._setattr_param = _unsafe_setattr_param + else: + self._setattr_tensor = _safe_setattr_tensor_or_param + self._setattr_param = _safe_setattr_tensor_or_param + + def _init_get_unflat_views_fn(self, align_addresses: bool): + self._get_unflat_views = ( + self._get_unflat_views_aligned + if align_addresses + else self._get_unflat_views_unaligned + ) + + def _init_flat_param_and_metadata( + self, + params: List[Union[Tensor, nn.Parameter]], + module: nn.Module, + aligned_numel: int, + use_orig_params: bool, + ) -> None: + """ + Initialize the ``FlatParameter`` and its metadata. + + NOTE: This should only be called once at construction time, after which + the ``FlatParameter`` metadata is assumed to be static. + + NOTE: The elements of ``params`` should only be ``Tensor`` s when + composing with ``DTensor`` -based tensor parallelism, in which case the + elements may be ``DTensor`` local shards. + """ + if len(params) == 0: + raise ValueError("Expects non-empty `params`") + if aligned_numel < 0: + raise ValueError( + f"Expects non-negative `aligned_numel` but got {aligned_numel}" + ) + ( + dtype, + flat_param_requires_grad, + device, + ) = self._validate_tensors_to_flatten(params) + params_set = set(params) + # For alignment padding, only `numels` gets strictly non-`None` + # elements, and all other lists get `None` elements for padding. + param_infos: List[ParamInfo] = [] + numels: List[int] = [] + shapes: List[torch.Size] = [] + fqns: List[str] = [] + shared_param_infos: List[SharedParamInfo] = [] + shared_param_memo: Dict[ + Union[Tensor, nn.Parameter], Tuple[nn.Module, str, str] + ] = {} + params_to_flatten: List[Union[Tensor, nn.Parameter]] = [] + shared_params: List[Union[Tensor, nn.Parameter]] = [] + param_extensions: List[Any] = [] + is_padding_mask: List[bool] = [] + total_numel = total_numel_without_padding = 0 + for submodule_name, submodule in module.named_modules(remove_duplicate=False): + for param_name, param in _named_parameters_with_duplicates( + submodule, recurse=False + ): + if param not in params_set: + continue + if param in shared_param_memo: # shared reference + prim_module, prim_module_name, prim_param_name = shared_param_memo[ + param + ] + shared_params.append(param) + shared_param_infos.append( + SharedParamInfo( + param_name, + submodule, + submodule_name, + prim_param_name, + prim_module, + prim_module_name, + ) + ) + else: + if aligned_numel > 0: + numel_to_pad = aligned_numel - (total_numel % aligned_numel) + if numel_to_pad > 0 and numel_to_pad < aligned_numel: + padding_tensor = _construct_padding_tensor( + numel_to_pad, dtype, False, device + ) + params_to_flatten.append(padding_tensor) + is_padding_mask.append(True) + numels.append(numel_to_pad) + total_numel += numel_to_pad + transform_t, extension = _ext_pre_flatten_transform( + param, + self._fsdp_extension, + ) + param = cast(nn.Parameter, transform_t) + param_extensions.append(extension) + shared_param_memo[param] = (submodule, submodule_name, param_name) + params_to_flatten.append(param) + is_padding_mask.append(False) + param_infos.append(ParamInfo(param_name, submodule, submodule_name)) + numels.append(param.numel()) + shapes.append(param.shape) + fqn = ( + submodule_name + "." + param_name + if submodule_name + else param_name + ) + fqns.append(fqn) + total_numel += param.numel() + total_numel_without_padding += param.numel() + if len(params_to_flatten) == 0: + raise ValueError( + f"`params` were not found in `module`'s tree" + f"params: {params}\nmodule: {module}" + ) + if ( + self.rank == 0 + and aligned_numel > 0 + and total_numel != total_numel_without_padding + ): + log.info( + "FSDP FlatParameter address alignment created " + "%s numel of padding (%s vs. %s)", + total_numel - total_numel_without_padding, + total_numel, + total_numel_without_padding, + ) + if aligned_numel > 0: + # Pad to be divisible by world size to avoid a copy for the + # post-backward reduce-scatter + numel_to_pad = self.world_size - (total_numel % self.world_size) + if numel_to_pad > 0 and numel_to_pad < self.world_size: + if self.rank == 0: + log.info( + "FSDP FlatParameter world size divisibility created " + "%s numel of padding", + numel_to_pad, + ) + padding_tensor = _construct_padding_tensor( + numel_to_pad, dtype, False, device + ) + params_to_flatten.append(padding_tensor) + is_padding_mask.append(True) + numels.append(numel_to_pad) + total_numel += numel_to_pad + # Pass `aligned_numel=0` since we already included padding tensors + self.flat_param: FlatParameter = self.flatten_tensors_into_flat_param( + params_to_flatten, + aligned_numel=0, + requires_grad=flat_param_requires_grad, + ) + FlatParameter._init_metadata( + self.flat_param, + param_infos, + numels, + shapes, + fqns, + shared_param_infos, + param_extensions, + _convert_to_params(params_to_flatten) if use_orig_params else None, + _convert_to_params(shared_params) if use_orig_params else None, + is_padding_mask, + ) + + def _validate_tensors_to_flatten( + self, tensors: List[Union[Tensor, nn.Parameter]] + ) -> Tuple: + """Validate the tensors to flatten and returns any necessary metadata.""" + dtype: Optional[torch.dtype] = None + # Return as the logical OR over each tensor's value + flat_param_requires_grad: Optional[bool] = None + device: Optional[torch.device] = None + # For `use_orig_params=True`, permit non-uniform `requires_grad` + for tensor in tensors: + if isinstance(tensor, FlatParameter): + raise ValueError("Cannot flatten a `FlatParameter`") + if dtype is None and not tensor.is_floating_point(): + raise ValueError("Cannot flatten integer dtype tensors") + if dtype is not None and tensor.dtype != dtype: + raise ValueError( + f"Must flatten tensors with uniform dtype but got {dtype} " + f"and {tensor.dtype}" + ) + if ( + not self._use_orig_params + and flat_param_requires_grad is not None + and tensor.requires_grad != flat_param_requires_grad + ): + raise ValueError( + "Must flatten tensors with uniform `requires_grad` when " + "`use_orig_params=False`" + ) + if device is not None and tensor.device != device: + raise ValueError( + "Must flatten tensors on the same device but got both " + f"{device} and {tensor.device}" + ) + dtype = tensor.dtype + flat_param_requires_grad = flat_param_requires_grad or tensor.requires_grad + device = tensor.device + assert flat_param_requires_grad is not None, "Requires non-empty `tensors` list" + return dtype, flat_param_requires_grad, device + + def flatten_tensors( + self, + tensors: List[Tensor], + aligned_numel: int, + ) -> Tensor: + """ + Flatten ``tensors`` into a single flat tensor. + + The flattening optionally includes + padding if ``aligned_numel`` is greater than 0, where ``aligned_numel`` + gives the numel required to have address alignment. + + NOTE: The padding alignment algorithm must be kept in sync with + :meth:`_init_flat_param_metadata`. We separate the two methods because + the initialization happens once, whereas this method may be called + multiple times throughout training (e.g. for checkpointing). + """ + if len(tensors) == 0: + raise ValueError("Expects non-empty `tensors`") + if aligned_numel < 0: + raise ValueError( + f"Expects non-negative `aligned_numel` but got {aligned_numel}" + ) + dtype, _, device = self._validate_tensors_to_flatten(tensors) + flat_tensors: List[Tensor] = [] + if aligned_numel > 0: + total_numel = 0 + for tensor in tensors: + numel_to_pad = aligned_numel - (total_numel % aligned_numel) + if numel_to_pad > 0 and numel_to_pad < aligned_numel: + padding_tensor = _construct_padding_tensor( + numel_to_pad, dtype, False, device + ) + flat_tensors.append(padding_tensor) + total_numel += numel_to_pad + flat_tensors.append(torch.flatten(_detach_if_needed(tensor))) + total_numel += tensor.numel() + numel_to_pad = self.world_size - (total_numel % self.world_size) + if numel_to_pad > 0 and numel_to_pad < self.world_size: + padding_tensor = _construct_padding_tensor( + numel_to_pad, dtype, False, device + ) + flat_tensors.append(padding_tensor) + total_numel += numel_to_pad + else: + flat_tensors = [ + torch.flatten(_detach_if_needed(tensor)) for tensor in tensors + ] + return torch.cat(flat_tensors, dim=0) + + def flatten_tensors_into_flat_param( + self, + tensors: List[Tensor], + aligned_numel: int, + requires_grad: bool, + ) -> FlatParameter: + flat_param_data = self.flatten_tensors(tensors, aligned_numel) + return FlatParameter(flat_param_data, requires_grad=requires_grad) + + def _init_param_reduce_dtypes( + self, + mp_param_dtype: Optional[torch.dtype], + mp_reduce_dtype: Optional[torch.dtype], + ) -> None: + """ + Initialize param and reduce dtypes. + + Precondition: ``self.flat_param`` is set. This ensures that this + handle's parameters have a single dtype. + + Postcondition: This sets ``self._fwd_bwd_param_dtype`` and + ``self._reduce_dtype``. If ``mp_param_dtype`` or ``mp_reduce_dtype`` + is ``None``, then we assume the original parameter dtype. One special + case is if ``mp_param_dtype`` is not ``None`` and ``mp_reduce_dtype`` + is ``None``, in which case we assume the gradient reduction dtype + matches the forward/backward parameter dtype. + """ + # Save whether these dtypes were specified so that we permit the + # parameter dtype to change up until the lazy initialization + self._low_prec_param_dtype_specified = mp_param_dtype is not None + self._low_prec_reduce_dtype_specified = mp_reduce_dtype is not None + if ( + self._low_prec_param_dtype_specified + and not self._low_prec_reduce_dtype_specified + ): + # Special case: infer gradient reduction mixed precision + self._fwd_bwd_param_dtype = mp_param_dtype + self._reduce_dtype = self._fwd_bwd_param_dtype + else: + self._fwd_bwd_param_dtype = mp_param_dtype or self._orig_param_dtype + self._reduce_dtype = mp_reduce_dtype or self._orig_param_dtype + assert self._fwd_bwd_param_dtype is not None + assert self._reduce_dtype is not None + + ################################### + # SHARD INITIALIZATION & METADATA # + ################################### + @torch.no_grad() + def shard(self): + """ + Shard the handle's ``FlatParameter``. + + This allocates new memory for + the sharded flat parameter and frees the unsharded flat parameter's + storage. + + Postcondition: ``self.flat_param`` is the sharded flat parameter. Shard + metadata attributes are set for all sharding strategies. + """ + flat_param = self.flat_param + if not self.uses_sharded_strategy: + self._init_shard_metadata(0, 0, flat_param.numel() - 1) + else: + _p_assert( + flat_param.storage_offset() == 0, + "The `FlatParameter` is not the sole occupant of its storage", + ) + sharded_flat_param, numel_padded = FlatParamHandle._get_shard( + flat_param, self.rank, self.world_size + ) + if not torch.distributed._functional_collectives.is_torchdynamo_compiling(): + allocated = flat_param._typed_storage()._size() > 0 + if allocated: + flat_param._typed_storage()._resize_(0) + flat_param.set_(sharded_flat_param) # type: ignore[call-overload] + start_idx = sharded_flat_param.numel() * self.rank + end_idx = sharded_flat_param.numel() * (self.rank + 1) - 1 # inclusive + self._init_shard_metadata(numel_padded, start_idx, end_idx) + if self._use_orig_params: + self._use_sharded_views() + + def _init_shard_metadata( + self, + numel_padded: int, + unsharded_start_idx: int, + unsharded_end_idx: int, + ) -> None: + """ + Initialize shard-related metadata for this rank's shard of the flat parameter. + + This includes ``_sharded_size``, ``_shard_param_infos``, and ``_shard_numel_padded``. + + Args: + numel_padded (int): Numel padded for this rank's sharded flat + parameter. + unsharded_start_idx (int): Start index in the unsharded flat + parameter assigned to this rank. + unsharded_end_idx (int): End index (inclusive) in the unsharded + flat parameter assigned to this rank. + + Precondition: ``self.flat_param`` 's data is the sharded flat + parameter. + """ + flat_param = self.flat_param + flat_param._sharded_size = flat_param.size() # type: ignore[attr-defined] + sharded_flat_param_numel = flat_param.numel() # includes `numel_padded` + _p_assert( + unsharded_start_idx >= 0 and unsharded_start_idx <= unsharded_end_idx, + f"unsharded_start_idx: {unsharded_start_idx} unsharded_end_idx: {unsharded_end_idx}", + ) + _p_assert( + numel_padded <= sharded_flat_param_numel, + f"numel_padded: {numel_padded} " + f"sharded_flat_param_numel: {sharded_flat_param_numel}", + ) + shard_param_infos = self._get_shard_metadata( + unsharded_start_idx, unsharded_end_idx + ) + assert ( + len(shard_param_infos) == flat_param._num_params + ), f"Expects length {flat_param._num_params} but got {len(shard_param_infos)}" + flat_param._shard_param_infos = shard_param_infos # type: ignore[attr-defined] + flat_param._shard_numel_padded = numel_padded # type: ignore[attr-defined] + + def _get_shard_metadata( + self, + unsharded_start_idx: int, + unsharded_end_idx: int, + ) -> Tuple[_ShardParamInfo, ...]: + """ + Compute the shard metadata based on ``unsharded_start_idx`` and ``unsharded_end_idx`` (inclusive). + + ``unsharded_start_idx`` and ``unsharded_end_idx`` give the interval of the + unsharded flat parameter specifying the shard. + """ + flat_param_offsets = self._get_flat_param_offsets() + assert len(flat_param_offsets) == len( + self.flat_param._numels_with_padding + ), f"Expected {len(self.flat_param._numels_with_padding)} but got {len(flat_param_offsets)}" + shard_param_infos: List[_ShardParamInfo] = [] + sharded_flat_param_numel = unsharded_end_idx - unsharded_start_idx + 1 + # `unsharded_param_start_idx` and `unsharded_param_end_idx` are indices + # into the unsharded flat parameter (inclusive) of the given parameter + for i, ( + (unsharded_param_start_idx, unsharded_param_end_idx), + is_padding, + ) in enumerate(zip(flat_param_offsets, self.flat_param._is_padding_mask)): + if is_padding: + continue + in_sharded_flat_param = ( + unsharded_start_idx <= unsharded_param_end_idx + and unsharded_end_idx >= unsharded_param_start_idx + ) + if not in_sharded_flat_param: + shard_param_info = _ShardParamInfo(False, None, None, None, None) + else: + if unsharded_start_idx <= unsharded_param_start_idx: + # This branch can only happen once since the rank's + # unsharded start index can only intersect one parameter + intra_param_start_idx = 0 + offset_in_shard = unsharded_param_start_idx - unsharded_start_idx + else: + intra_param_start_idx = ( + unsharded_start_idx - unsharded_param_start_idx + ) + offset_in_shard = 0 + assert ( + offset_in_shard >= 0 and offset_in_shard < sharded_flat_param_numel + ), ( + f"Invalid `offset_in_shard` of {offset_in_shard} for " + f"sharded flat parameter with {sharded_flat_param_numel} numel" + ) + intra_param_end_idx = ( + min(unsharded_param_end_idx, unsharded_end_idx) + - unsharded_param_start_idx + ) + numel_in_shard = intra_param_end_idx - intra_param_start_idx + 1 + shard_param_info = _ShardParamInfo( + True, + offset_in_shard, + numel_in_shard, + intra_param_start_idx, + intra_param_end_idx, + ) + shard_param_infos.append(shard_param_info) + return tuple(shard_param_infos) + + @staticmethod + def _get_unpadded_shard( + tensor: Tensor, + rank: int, + world_size: int, + ) -> Tuple[Tensor, int]: + """ + Return the unpadded shard of ``tensor`` for the given ``rank`` and ``world_size``. + + The returned value is a tuple of the shard of ``tensor`` without any + padding and the numel to pad for that shard. + + If ``tensor`` is already flattened or may be viewed in the flattened + shape (which is true in the expected usage), then this method does not + allocate any new tensor memory. + """ + chunks = torch.flatten(tensor).chunk(world_size) + if len(chunks) < (rank + 1): + # This rank gets an empty chunk fully padded with zeros since there + # are not enough chunks across ranks + chunk = chunks[0].new_empty(0) + else: + chunk = chunks[rank] + numel_to_pad = chunks[0].numel() - chunk.numel() + assert ( + numel_to_pad >= 0 + ), "Chunk's size should be at most the first chunk's size" + return chunk, numel_to_pad + + @staticmethod + def _get_shard( + tensor: Tensor, + rank: int, + world_size: int, + ) -> Tuple[Tensor, int]: + """ + Return the shard of ``tensor`` with padding for the given ``rank`` and ``world_size`` and the numel padded for that shard. + + This method allocates new memory (via :meth:`clone`) since the + unsharded ``tensor`` may be deallocated after this method returns. + """ + chunk, numel_to_pad = FlatParamHandle._get_unpadded_shard( + tensor, rank, world_size + ) + shard = chunk.clone() + if numel_to_pad > 0: + shard = F.pad(shard, [0, numel_to_pad]) + return shard, numel_to_pad + + @staticmethod + def _get_sharded_size(tensor: Tensor, rank: int, world_size: int) -> torch.Size: + """ + Return the shape of ``tensor`` after sharding including padding. + + This requires ``tensor`` to have 1D shape and ensures that the returned + shape is 1D. + """ + assert len(tensor.shape) == 1, f"{tensor.shape}" + unpadded_sharded_tensor, numel_to_pad = FlatParamHandle._get_unpadded_shard( + tensor, rank, world_size + ) + unpadded_sharded_size = unpadded_sharded_tensor.size() + assert len(unpadded_sharded_size) == 1, f"{unpadded_sharded_size}" + return torch.Size([unpadded_sharded_size[0] + numel_to_pad]) + + def _get_flat_param_offsets(self) -> List[Tuple[int, int]]: + """ + Return [start, end] offsets of each original parameter's flattened data in the unsharded flat parameter (without padding). + + NOTE: The returned list includes elements for alignment padding. + """ + cumulative_sum = list(accumulate(self.flat_param._numels_with_padding)) + starts = [0] + cumulative_sum[:-1] + ends = [end - 1 for end in cumulative_sum] # inclusive + param_offsets = list(zip(starts, ends)) + return param_offsets + + @no_type_check + def shard_metadata( + self, + ) -> FlatParamShardMetadata: + """ + Return the shard-related metadata specific to this rank's shard of the flat parameter. + + NOTE: The returned tuple does not include elements for alignment + padding but does account for the padding. + """ + fqns_list = [] + shapes_list = [] + numels_list = [] + shard_param_offsets = [] + for fqn, shape, numel, shard_param_info in zip( + self.flat_param._fqns, + self.flat_param._shapes, + self.flat_param._numels, + self.flat_param._shard_param_infos, + ): + if not shard_param_info.in_shard: + continue + fqns_list.append(fqn) + shapes_list.append(shape) + numels_list.append(numel) + shard_param_offsets.append( + ( + shard_param_info.intra_param_start_idx, + shard_param_info.intra_param_end_idx, + ) + ) + return FlatParamShardMetadata( + tuple(fqns_list), + tuple(shapes_list), + tuple(numels_list), + shard_param_offsets, + ) + + @no_type_check + @torch.no_grad() + def init_flat_param_attributes(self) -> None: + """ + This initializes some attributes on the handle's ``FlatParameter``. + This should be called during lazy initialization since it requires the + parameter to be on the compute device if not offloading to CPU and we + want to give users the chance to move the parameter appropriately after + the FSDP constructor. + + For each tensor attribute on the ``FlatParameter``, see the unshard and + reshard methods in this class for the allocation and free pattern. + """ + flat_param = self.flat_param + if flat_param.dtype != self._orig_param_dtype: + # Entering this branch means that the user changed the parameter + # dtype after FSDP initialization, in which case we may need to + # refresh some saved dtype attributes (dtypes specified as a part + # of mixed precision take precedence). + if not self._low_prec_param_dtype_specified: + self._fwd_bwd_param_dtype = flat_param.dtype + # For `reduce_dtype`, require `param_dtype` was not specified since + # then we infer the `reduce_dtype` from the specified `param_dtype` + if ( + not self._low_prec_reduce_dtype_specified + and not self._low_prec_param_dtype_specified + ): + self._reduce_dtype = flat_param.dtype + self._orig_param_dtype = flat_param.dtype + cpu_device = torch.device("cpu") + if self._offload_params: + _p_assert( + flat_param.device == cpu_device, + f"Expects the `FlatParameter` to be on CPU when parameter CPU " + f"offloading is enabled, not {flat_param.device}", + ) + else: + self._check_on_compute_device(self.flat_param) + flat_param._local_shard = flat_param.data + if self._offload_params: + # Pin the memory for faster H2D transfer + flat_param._local_shard = flat_param._local_shard.pin_memory() + # Pre-allocate the sharded gradient on CPU to enable non-blocking + # D2H transfer during the backward pass + flat_param._cpu_grad = torch.zeros_like( + flat_param._local_shard, device=cpu_device + ).pin_memory() + if self._uses_param_mixed_precision: + # For parameter mixed precision, we maintain a low precision + # sharded tensor on the compute device to be all-gathered (for + # sharded strategies) or directly used (for `NO_SHARD`) for + # computation. + flat_param._mp_shard = torch.empty_like( + flat_param._local_shard, + device=self.device, + dtype=self._fwd_bwd_param_dtype, + ) + _free_storage(flat_param._mp_shard) + if self.uses_sharded_strategy: + # We maintain a padded unsharded tensor that serves as the + # all-gather destination and owns the original parameter storages. + unsharded_param_dtype = ( + self._fwd_bwd_param_dtype + if self._uses_param_mixed_precision + else flat_param.dtype + ) # use low precision if parameter mixed precision is enabled + padded_unsharded_numel = flat_param.numel() * self.world_size + flat_param._full_param_padded = torch.empty( + padded_unsharded_numel, + device=self.device, + dtype=unsharded_param_dtype, + ) + flat_param._padded_unsharded_size = flat_param._full_param_padded.size() + _free_storage(flat_param._full_param_padded) + + if self._uses_param_mixed_precision: + # For parameter mixed precision, we maintain a full precision + # padded unsharded tensor for when we force full precision. + flat_param._full_prec_full_param_padded = torch.empty( + padded_unsharded_numel, + device=self.device, + dtype=flat_param.dtype, # full precision + ) + _free_storage(flat_param._full_prec_full_param_padded) + + ################### + # UNSHARD/RESHARD # + ################### + def pre_unshard(self) -> bool: + """ + Return ``False`` if this is a no-op and ``True`` otherwise. + + Postcondition: ``self.flat_param`` 's data is on the device for + communication and is what should be all-gathered. This means that it + matches the dtype of the expected unsharded parameter. + """ + if ( + self._training_state == HandleTrainingState.SUMMON_FULL_PARAMS + and self._skipped_use_sharded_views + ): + # Since this path imposes special semantics for the unsharded flat + # parameter (e.g. forcing full precision), use sharded views to + # reuse the existing logic for that special handling + self._use_sharded_views() + ret = False + if self._use_orig_params and not self._skip_writeback_check: + ret = self._writeback_orig_params() + if ( + self.uses_sharded_strategy + and not self._offload_params + and not self.needs_unshard() + ): + pass # no-op + elif self._uses_param_mixed_precision and not self._force_full_precision: + self._use_low_precision_shard() + ret = True + elif self._offload_params and self.flat_param.device != self.device: + # NOTE: This creates a new tensor distinct from any attributes. + self.flat_param_to(self.device, non_blocking=True) + ret = True + self._check_on_compute_device(self.flat_param) + return ret + + def _use_low_precision_shard(self): + """Allocate on the compute device and switch to using the low precision sharded flat parameter.""" + self._check_low_precision_shard() + flat_param = self.flat_param + _alloc_storage( + flat_param._mp_shard, flat_param._local_shard.size() # type: ignore[attr-defined] + ) + # `copy_()` implicitly casts to the low precision + flat_param._mp_shard.copy_( # type: ignore[attr-defined] + flat_param._local_shard.to( # type: ignore[attr-defined] + self.device, non_blocking=True + ) + ) + # Invariant: `_mp_shard` is always on the compute device. + flat_param.data = flat_param._mp_shard # type: ignore[attr-defined] + + def unshard(self): + """ + Run the unshard logic. + + This includes all-gathering the flat parameter + and switching to using the unsharded flat parameter. If the handle does + not need unsharding, then this only switches to using the unsharded + flat parameter. For ``NO_SHARD``, this is a no-op. + + If FSDP is in :meth:`summon_full_params` and the handle uses parameter + mixed precision, then the parameter is forced to full precision. + """ + if not self.needs_unshard(): + # Even when not needing an unshard, we should switch to using + # the unsharded flat parameter + unsharded_flat_param = ( + self._get_padded_unsharded_flat_param() + if self.uses_sharded_strategy + else self.flat_param + ) + self._use_unsharded_flat_param(unsharded_flat_param) + return + unsharded_flat_param = self._alloc_padded_unsharded_flat_param() + padded_unsharded_flat_param = self._all_gather_flat_param(unsharded_flat_param) + self._use_unsharded_flat_param(padded_unsharded_flat_param) + + def needs_unshard(self) -> bool: + """Return if the handle's flat parameter needs to be unsharded.""" + if not self.uses_sharded_strategy: + return False + unsharded_flat_param = self._get_padded_unsharded_flat_param() + already_unsharded = _same_storage_size( + unsharded_flat_param, unsharded_flat_param.numel() + ) + return not already_unsharded + + def _alloc_padded_unsharded_flat_param(self): + """ + Allocate the *padded* unsharded flat parameter. + + The unpadded unsharded + flat parameter is always a view into the padded one. This padded + parameter is saved to a different attribute on the ``FlatParameter`` + depending on if we force full precision. + """ + self._check_sharded_strategy() + flat_param = self.flat_param + unsharded_flat_param = self._get_padded_unsharded_flat_param() + self._check_storage_freed(unsharded_flat_param) + _alloc_storage(unsharded_flat_param, flat_param._padded_unsharded_size) # type: ignore[attr-defined] + return unsharded_flat_param + + def _get_padded_unsharded_flat_param(self) -> torch.Tensor: + """ + Return a reference to the padded unsharded flat parameter depending on the calling context. + + This should only be called if using a sharded strategy. + """ + self._check_sharded_strategy() + flat_param = self.flat_param + if self._force_full_precision and self._uses_param_mixed_precision: + # When parameter mixed precision is enabled, we use a different + # tensor as the all-gather destination to preserve the invariant + # that `_full_param_padded` is in the low precision + unsharded_flat_param = flat_param._full_prec_full_param_padded # type: ignore[attr-defined] + _p_assert( + unsharded_flat_param.dtype != self._fwd_bwd_param_dtype, + f"Expects full precision but got {self._fwd_bwd_param_dtype}", + ) + # For no-reshard-after-forward strategies, `_full_param_padded` may + # still be allocated from a previous forward. As we are forcing + # full precision here, the full-precision unsharded copy may be + # modified, invalidating the existing low-precision unsharded copy, + # so we should free it here to ensure a new all-gather for the next + # forward/backward computation to persist the modifications. + if flat_param._full_param_padded.untyped_storage().size() > 0: + _free_storage(flat_param._full_param_padded) + else: + unsharded_flat_param = flat_param._full_param_padded # type: ignore[attr-defined] + return unsharded_flat_param + + def _all_gather_flat_param( + self, + padded_unsharded_flat_param: Tensor, + ) -> Tensor: + """ + All-gather the handle's flat parameter to the destination ``padded_unsharded_flat_param``. + + Then switch to use the all-gathered tensor. + """ + _p_assert( + hasattr(self, "process_group") and hasattr(self, "world_size"), + "Expects a process group and world size to have been set via `shard()`", + ) + sharded_flat_param = self.flat_param.data + expected_numel = sharded_flat_param.numel() * self.world_size + _p_assert( + padded_unsharded_flat_param.numel() == expected_numel, + f"Expects {expected_numel} numel but got {padded_unsharded_flat_param.numel()}", + ) + + pg = ( + self._fake_process_group + if self._use_fake_all_gather + else self.process_group + ) + + # HACK this should be handled by C10D + if sharded_flat_param.is_cpu: # type: ignore[attr-defined] + tensor_list = list( + torch.chunk(padded_unsharded_flat_param, dist.get_world_size(pg)) + ) + work = dist.all_gather(tensor_list, sharded_flat_param, group=pg) + else: + dist.all_gather_into_tensor( + padded_unsharded_flat_param, + sharded_flat_param, + pg, + ) + + if self._offload_params: + # In case of offloading, `flat_param.data` (i.e. sharded param) is + # created on the pre-unshard stream. We need to hand it over to the + # unshard stream for all-gather + _no_dispatch_record_stream( + sharded_flat_param, + self._device_handle.current_stream(), # unshard_stream + ) + return padded_unsharded_flat_param + + def _use_unsharded_flat_param( + self, + padded_unsharded_flat_param: torch.Tensor, + ) -> None: + """ + Switch to use the *unpadded* unsharded flat parameter. + + This is a view into the *padded* unsharded flat parameter. + """ + unsharded_size = self.flat_param._unpadded_unsharded_size + flat_param_part = padded_unsharded_flat_param[: unsharded_size.numel()] + # slicing [:] is not visible to autograd because of .data + self.flat_param.data = flat_param_part + in_forward = self._training_state == HandleTrainingState.FORWARD + in_pre_backward = self._training_state == HandleTrainingState.BACKWARD_PRE + if self._use_orig_params: + if self._skipped_use_sharded_views and in_pre_backward: + # This call corresponds to the complementary pre-backward + # `_use_unsharded_views()` to the skipped pre-forward + # `_use_sharded_views()`, so we should skip this one too. + return + # We use `Tensor` views in the forward so that they are tracked by + # autograd. We use them in the pre-backward as well to support + # reentrant activation checkpointing, which needs the views to be + # tracked by autograd in the backward pass's recomputed forward. + self._use_unsharded_views( + as_params=(not in_forward and not in_pre_backward) + ) + elif in_forward: + self._use_unsharded_views(as_params=False) + + def post_unshard(self): + """ + Run the post-unshard logic. + + This includes freeing the low precision shard if needed. + """ + if self._uses_param_mixed_precision and self.uses_sharded_strategy: + self._free_low_precision_sharded_param() + self._check_on_compute_device(self.flat_param) + + def _free_low_precision_sharded_param(self): + """Frees the low precision sharded flat parameter.""" + self._check_low_precision_shard() + # `_mp_shard` is allocated in the pre-unshard stream, consumed in the + # unshard stream for sharded strategies, and consumed in both the + # unshard and default streams for `NO_SHARD`. For sharded strategies, + # the current stream here is the unshard stream, and for `NO_SHARD`, + # it is the default stream. For `NO_SHARD`, only recording for the + # default stream suffices since the default stream waits for the + # unshard stream. + _no_dispatch_record_stream( + self.flat_param._mp_shard, self._device_handle.current_stream() # type: ignore[attr-defined] + ) + _free_storage(self.flat_param._mp_shard) # type: ignore[attr-defined] + + @torch.no_grad() + def unshard_grad(self): + """ + Unshard the handle's ``FlatParameter``'s gradient. + + If all ranks have + ``None`` gradient, then all original parameters will as well. This + method performs an all-reduce and an all-gather. The additional + all-reduce is tolerable since this method is not meant to be used on + the computation critical path. + + Postcondition: ``_saved_grad_shard`` is defined and contains the value + to set ``flat_param.grad`` after gradients are resharded. + """ + if not self.uses_sharded_strategy: + self._use_unsharded_grad_views() + return + flat_param = self.flat_param + self._check_unsharded(flat_param) + + # Check if all ranks have a `None` gradient + num_grad_none = torch.zeros(1, dtype=torch.int32, device=self.device) + num_grad_none[0] = flat_param.grad is None + dist.all_reduce(num_grad_none, group=self.process_group) + if num_grad_none[0] == self.world_size: + flat_param._saved_grad_shard = None # type: ignore[assignment] + self._use_unsharded_grad_views() + return + + if flat_param.grad is None: + # In the case that only some ranks have `None` gradient, we use + # zeros to approximate as a best effort attempt + if self._debug_level == dist.DebugLevel.INFO: + warnings.warn( + f"[Rank {self.rank}] Only some but not all ranks have a " + "`None` `FlatParameter` gradient, so FSDP is using zeros to " + "approximate those ranks' sharded gradients being `None`" + ) + flat_param._saved_grad_shard = None # type: ignore[assignment] + sharded_grad = torch.zeros(flat_param._sharded_size, device=self.device) # type: ignore[attr-defined] + else: + self._check_sharded(flat_param.grad) + flat_param._saved_grad_shard = flat_param.grad # type: ignore[attr-defined] + sharded_grad = flat_param._saved_grad_shard # type: ignore[attr-defined] + padded_unsharded_grad = torch.empty( + flat_param._padded_unsharded_size, # type: ignore[attr-defined] + device=self.device, + dtype=sharded_grad.dtype, + ) + dist.all_gather_into_tensor( + padded_unsharded_grad, sharded_grad, self.process_group + ) + unsharded_size = self.flat_param._unpadded_unsharded_size + flat_param.grad = padded_unsharded_grad[: unsharded_size.numel()].view( + unsharded_size + ) + self._use_unsharded_grad_views() + + def reshard_grad(self): + if self._use_orig_params: + self._use_sharded_grad_views() + if not self.uses_sharded_strategy: + return + self.flat_param.grad = self.flat_param._saved_grad_shard # type: ignore[attr-defined] + delattr(self.flat_param, "_saved_grad_shard") + + def prepare_gradient_for_backward(self): + """ + Prepare the gradient for the backward computation. + + This is done by saving and clearing any existing sharded gradient + in ``.grad`` to enable computing a new unsharded gradient. + """ + _p_assert( + self._training_state + in (HandleTrainingState.BACKWARD_PRE, HandleTrainingState.IDLE), + "Expects to be in `BACKWARD_PRE` or `IDLE` (if prefetching)", + ) + flat_param = self.flat_param + if flat_param.grad is not None and ( + flat_param.grad.size() != flat_param._unpadded_unsharded_size + or flat_param.grad.device != flat_param.device # grad on CPU + ): + self._check_on_compute_device(self.flat_param) + grad_offloaded = flat_param.grad.device != self.device + _p_assert( + not grad_offloaded or self._offload_params, + f"Expects the sharded gradient to be on {self.device} " + f"but got {flat_param.grad.device}", + ) + prev_iter_synced_gradients = ( + flat_param.grad.size() + == flat_param._local_shard.size() # type: ignore[attr-defined] + ) + if prev_iter_synced_gradients: + # TODO (awgu): Gradient accumulation outside `no_sync()` + # does not work with CPU offloading. The issue should be + # that, in the post-backward hook, we cannot do an addition + # between a CPU tensor (the existing sharded gradient) and + # a GPU tensor (the new sharded gradient). + if not grad_offloaded: + flat_param._saved_grad_shard = flat_param.grad.data # type: ignore[attr-defined] + sharded_grad = flat_param._saved_grad_shard # type: ignore[attr-defined] + else: + _p_assert( + hasattr(flat_param, "_cpu_grad"), + "`_cpu_grad` should be defined if the gradient is on CPU", + ) + sharded_grad = flat_param._cpu_grad # type: ignore[attr-defined] + # If user specified to keep the gradient in low precision, then + # the gradient may still be of the low precision dtype if the + # user did not set the gradient to `None` after the previous + # backward, in which case FSDP should cast back to the full + # precision dtype so that FSDP can accumulate in that dtype in + # the post-backward hook and assign to `.grad` in that dtype in + # the post-backward callback. + local_shard_dtype = flat_param._local_shard.dtype # type: ignore[attr-defined] + if ( + self._keep_low_precision_grads + and sharded_grad.dtype != local_shard_dtype + ): + sharded_grad.data = sharded_grad.to(local_shard_dtype) + else: + padded_unsharded_size = flat_param._padded_unsharded_size # type: ignore[attr-defined] + _p_assert( + flat_param.grad.size() == padded_unsharded_size, + "Expects `.grad` to be the unsharded gradient in " + f"`no_sync()` with size {padded_unsharded_size} " + f"but got size {flat_param.grad.size()}", + ) + flat_param.grad = None + + def prepare_gradient_for_optim(self): + """Prepare the gradient for optimizer computation by moving the sharded gradient to the ``.grad`` attribute.""" + + def cast_grad_to_param_dtype_if_needed(flat_param): + # TODO (rohan-varma): test for full precision with keep_low_precision_grads + if not self._force_full_precision and self._keep_low_precision_grads: + _p_assert(flat_param.grad is not None, "Unexpected None grad!") + if flat_param.grad.dtype != self._fwd_bwd_param_dtype: + flat_param.grad.data = flat_param.grad.to(self._fwd_bwd_param_dtype) + if self._use_orig_params: + self._use_sharded_grad_views() + + flat_param = self.flat_param + # TODO (awgu): We should replace these conditional checks to encode + # the logical intention more directly. + if hasattr(flat_param, "_cpu_grad"): + # NOTE: This branch includes `NO_SHARD`. + self._check_sharded(flat_param) + self._check_on_cpu(flat_param) + flat_param.grad = flat_param._cpu_grad # type: ignore[attr-defined] + cast_grad_to_param_dtype_if_needed(flat_param) + elif hasattr(flat_param, "_saved_grad_shard"): + self._check_sharded(flat_param) + self._check_on_compute_device(flat_param) + if flat_param._saved_grad_shard is not None: + self._check_on_compute_device(flat_param._saved_grad_shard) # type: ignore[attr-defined] + # If no sharded gradient was computed this iteration, then there is + # no need to forward `_saved_grad_shard` to `grad` + if flat_param._post_backward_called: # type: ignore[attr-defined] + flat_param.grad = flat_param._saved_grad_shard # type: ignore[attr-defined] + if flat_param.grad is not None: + cast_grad_to_param_dtype_if_needed(flat_param) + else: + _p_assert( + not self.uses_sharded_strategy + or not flat_param._post_backward_called, # type: ignore[attr-defined] + "All sharded parameters that received a gradient in the " + "post-backward should use `_saved_grad_shard`", + ) + # Delete `_saved_grad_shard` since its existence indicates a previous + # gradient to accumulate with in the post-backward hook + if hasattr(flat_param, "_saved_grad_shard"): + delattr(flat_param, "_saved_grad_shard") + + @contextlib.contextmanager + def to_cpu(self): + """ + Move the unpadded unsharded flat parameter to CPU while in the context and moves it back to the previous device upon exit. + + For now, this assumes the ``FlatParameter`` is the unpadded unsharded flat parameter + since (1) there is no reason to include the padding in the copy and (2) + there is no use case for the sharded flat parameter. + + Precondition: ``self.flat_param`` 's data is the unpadded unsharded + flat parameter on the compute device, and the handle uses a sharded + strategy. + Postcondition: Same as the precondition. + """ + self._check_sharded_strategy() + _p_assert( + self.flat_param.size() == self.flat_param._unpadded_unsharded_size, + f"Expects size {self.flat_param._unpadded_unsharded_size} but got {self.flat_param.size()}", + ) + self._check_on_compute_device(self.flat_param) + # Check that the unpadded unsharded flat parameter is a view into the + # padded unsharded flat parameter as expected + # NOTE: This check is not strictly needed for correctness but is a + # useful sanity check since the tensor should only be used internally. + _p_assert( + _same_storage(self.flat_param, self._get_padded_unsharded_flat_param()), + "Expects the unpadded parameter to be a view into the padded parameter", + ) + self.flat_param_to(torch.device("cpu")) + self._free_unsharded_flat_param() + try: + yield + finally: + _p_assert( + self.flat_param.size() == self.flat_param._unpadded_unsharded_size, + f"Expects size {self.flat_param._unpadded_unsharded_size} but got {self.flat_param.size()}", + ) + padded_unsharded_flat_param = self._alloc_padded_unsharded_flat_param() + # Copy from CPU to the compute device + padded_unsharded_flat_param[: self.flat_param.numel()].copy_( + self.flat_param + ) + self._use_unsharded_flat_param(padded_unsharded_flat_param) + + def reshard(self, free_unsharded_flat_param: bool): + """ + Run the reshard logic. + + This includes freeing the unsharded flat + parameter if ``free_unsharded_flat_param`` and switching to using the + sharded flat parameter. Note that this also implicitly offloads + the sharded flat parameter (if CPU offload is enabled) by pointing + it to the ``_local_shard`` attribute which resides on CPU. + """ + # Switch to the sharded `FlatParameter` before freeing to prevent + # "use-after-free"-type bugs with external profiling tools, where for + # `use_orig_params=True`, the `param` does not point to valid memory + # when setting `param.data = ...` in `_use_sharded_views()`. + self._use_sharded_flat_param() + if free_unsharded_flat_param: + self._free_unsharded_flat_param() + + def post_reshard(self): + """ + Run the post-reshard logic. + + This includes freeing any memory that + can now be freed given that the ``FlatParameter`` points to the full + precision sharded flat parameter. + + Precondition: ``self.flat_param`` 's data points to the full precision + sharded flat parameter. + """ + # For `NO_SHARD`, `_mp_shard` is not freed in the post-unshard since it + # is also the low precision *unsharded* flat parameter. Hence, we delay + # the free until the reshard. + if ( + self._uses_param_mixed_precision + and not self.uses_sharded_strategy + and not self._force_full_precision # did not use the low precision shard + ): + self._free_low_precision_sharded_param() + + def _free_unsharded_flat_param(self): + """ + Free the padded unsharded flat parameter. We allow this + function to be called even when storage is not allocated + + The tensor to free depends + on the calling context since the unshard may have forced full + precision, in which case a different tensor is used. + """ + self._check_sharded_strategy() + unsharded_flat_param = self._get_padded_unsharded_flat_param() + self._check_on_compute_device(unsharded_flat_param) + # Do not free the memory until all ops in the current stream finish + _no_dispatch_record_stream( + unsharded_flat_param, self._device_handle.current_stream() + ) + _free_storage(unsharded_flat_param) + + def _use_sharded_flat_param(self) -> None: + """Switches to using the sharded flat parameter.""" + flat_param = self.flat_param + if self._use_orig_params: + in_forward = self._training_state == HandleTrainingState.FORWARD + skip_use_sharded_views = ( + torch.is_grad_enabled() + and in_forward + and self._sharding_strategy + in NO_RESHARD_AFTER_FORWARD_HANDLE_STRATEGIES + ) + # Only incur the extra `.data` call if needed + if skip_use_sharded_views: + unsharded_flat_param = flat_param.data + if self._offload_params: + device = flat_param._local_shard.device # type: ignore[attr-defined] + _p_assert( + device == torch.device("cpu"), + f"Expects the local shard to be on CPU but got {device}", + ) + flat_param.data = flat_param._local_shard # type: ignore[attr-defined] + if self._use_orig_params: + if skip_use_sharded_views: # type: ignore[possibly-undefined] + self._unsharded_flat_param_for_skipped_views = unsharded_flat_param # type: ignore[possibly-undefined] + else: + self._use_sharded_views() + # For the post-forward reshard, we may try to use sharded gradient + # views (or unsharded gradient views if a gradient was accumulated + # in `no_sync()`), but for the post-backward reshard, we delay the + # call to after the reduce-scatter. + if ( + in_forward # type: ignore[possibly-undefined] + # Skip using gradient views if skipped using sharded views + # since exposing unsharded parameters with sharded gradients + # may be confusing to the user + and not self._skipped_use_sharded_views + ): + # TODO: Change `_unpadded_unsharded_size` if we change the + # gradient to be computed directly with padding. + accumulated_grad_in_no_sync = ( + flat_param.grad is not None + and self.uses_sharded_strategy + and flat_param.grad.shape == flat_param._unpadded_unsharded_size + ) + if accumulated_grad_in_no_sync: + self._use_unsharded_grad_views() + else: + self._use_sharded_grad_views() + + ######### + # VIEWS # + ######### + @no_type_check + def _get_unflat_views_unaligned( + self, + tensor: Optional[torch.Tensor] = None, + ) -> Iterator[Tensor]: + """ + Return unflattened ``Tensor`` views into ``tensor``. + + If `tensor`` is ``None``, ``flat_param`` is used. The unflattening is based + on ``flat_param`` 's metadata. + + Examples for ``tensor`` include ``flat_param.grad`` or unsharded + tensor optimizer state. + """ + flat_param = self.flat_param + if tensor is None: + tensor = flat_param + views = ( + _ext_post_unflatten_transform( + subtensor.view(shape), + param_extension, + self._fsdp_extension, + ) + for (subtensor, shape, param_extension) in zip( + torch.split(tensor, flat_param._numels, dim=0), + flat_param._shapes, + flat_param._param_extensions, + ) + ) + return views + + @no_type_check + def _get_unflat_views_aligned( + self, + tensor: Optional[Tensor] = None, + ) -> List[Tensor]: + """ + Return unflattened ``Tensor`` views into ``tensor`` with handling for padding. + + This method has the same contract as :meth:`_get_unflat_views_unaligned` + except it checks for ``None`` placeholders representing padding for + alignment, which may incur slightly more CPU overhead. + """ + flat_param = self.flat_param + if tensor is None: + tensor = flat_param + splits: List[Tensor] = torch.split( + tensor, flat_param._numels_with_padding, dim=0 + ) + idx = 0 + views: List[Tensor] = [] + for split, is_padding in zip(splits, flat_param._is_padding_mask): + if is_padding: + continue + views.append( + _ext_post_unflatten_transform( + split.view(flat_param._shapes[idx]), + flat_param._param_extensions[idx], + self._fsdp_extension, + ) + ) + idx += 1 + return views + + @no_type_check + @torch.enable_grad() + def _use_unsharded_views(self, as_params: bool) -> None: + """ + Unflatten the unsharded flat parameter by setting the original parameter variables to be views into it. + + Args: + as_params (bool): If ``True``, then registers the original + parameters as ``nn.Parameter`` s; if ``False``, then registers + the original parameters only as ``Tensor`` s. ``False`` should + be used during forward/backward computation and when hiding the + original parameters from :meth:`nn.Module.named_parameters`. + + Note: + when prefetching for next forward, current forward may be + annotated with `@torch.no_grad()` + `@torch.enable_grad()` ensures non-empty `view.grad_fn` + otherwise `_post_backward_hook` will not get called + """ + flat_param = self.flat_param + self._check_unsharded(flat_param) + views = self._get_unflat_views() + from torch.distributed._tensor import DTensor + + for i, (view, (param_name, module, _)) in enumerate( + zip(views, flat_param._param_infos) + ): + if self._use_orig_params and as_params: + if type(view) is DTensor: + # A `DTensor` `view` is not compatible with assigning + # `param.data = view`, so we cannot preserve the parameter + # variable. + self._setattr_param( + module, + param_name, + nn.Parameter(view, requires_grad=flat_param.requires_grad), + ) + continue + param = self.flat_param._params[i] + self._setattr_param(module, param_name, param) + param.data = view + elif as_params: + self._setattr_param( + module, + param_name, + nn.Parameter(view, requires_grad=flat_param.requires_grad), + ) + else: # `as_params=False` + param_var: Tensor = view + if self._use_orig_params: + if self._training_state == HandleTrainingState.FORWARD: + # Save the `Tensor` for the pre-backward + self.flat_param._tensors[i] = view # save for pre-backward + elif self._training_state == HandleTrainingState.BACKWARD_PRE: + # Use the saved `Tensor` variable from the forward to + # preserve the autograd graph so that the post-backward + # hook fires (e.g. for reentrant AC) + tensor = self.flat_param._tensors[i] + tensor.data = view + param_var = tensor + self._setattr_tensor(module, param_name, param_var) + if ( + self._use_orig_params + and self._training_state == HandleTrainingState.FORWARD + ): + module._parameters[param_name] = param_var + for i, ( + param_name, + module, + _, + prim_param_name, + prim_module, + _, + ) in enumerate(self.flat_param._shared_param_infos): + prim_param: Union[Tensor, nn.Parameter] = getattr( + prim_module, prim_param_name + ) + _p_assert( + not as_params or isinstance(prim_param, nn.Parameter), + f"as_params={as_params} type(prim_param)={type(prim_param)}", + ) + if self._use_orig_params and as_params: + shared_param = self.flat_param._shared_params[i] + self._setattr_param(module, param_name, shared_param) + shared_param.data = prim_param + elif as_params: + self._setattr_param(module, param_name, prim_param) + else: + self._setattr_tensor(module, param_name, prim_param) + if ( + self._use_orig_params + and self._training_state == HandleTrainingState.FORWARD + ): + module._parameters[param_name] = prim_param + + @no_type_check + def _use_unsharded_grad_views(self) -> None: + """ + Unflatten the unsharded flat parameter's gradient. + + The original parameter variables' gradients are set to be views into + the unsharded flat parameter's gradient. + """ + # Expects the gradient to be in `flat_param.grad` + if self.flat_param.grad is None: + for param in chain(self.flat_param._params, self.flat_param._shared_params): + param.grad = None + return + self._check_unsharded(self.flat_param.grad) + views = self._get_unflat_views(self.flat_param.grad) + for i, (view, (param_name, module, _)) in enumerate( + zip(views, self.flat_param._param_infos) + ): + _p_assert( + hasattr(module, param_name), + f"{self.flat_param._fqns[i]} is missing", + ) + param = getattr(module, param_name) + if ( + param.shape != view.shape + or param.dtype != view.dtype + or param.device != view.device + ): + # NOTE: This is a hack using `.data` to side step the check + # that parameter/gradient sizes/dtypes/devices match. From + # calling `reshard()`, `param` has the sharded size, has the + # full precision dtype, and if CPU offloading is enabled, is on + # CPU. Thus, one or more of the following cases can hold when + # in `no_sync()`, where `view` is the original parameter's + # gradient: + # 1. `view` can have the unsharded size. + # 2. `view` can have the parameter low precision dtype. + # 3. `view` can be on GPU. + if param.grad is None: + param.grad = torch.empty_like(param) + param.grad.data = view + else: + param.grad = view + for i, ( + param_name, + module, + module_name, + prim_param_name, + prim_module, + _, + ) in enumerate(self.flat_param._shared_param_infos): + _p_assert( + hasattr(module, param_name), + f"{module_name + '.' + param_name if module_name else param_name} is missing", + ) # did not save FQN info in `_shared_param_infos` + param = getattr(module, param_name) + prim_param = getattr(prim_module, prim_param_name) + if ( + param.shape != prim_param.grad.shape + or param.dtype != prim_param.grad.dtype + or param.device != prim_param.grad.device + ): + # NOTE: This is the same hack to use `.data` to side step the + # size check. + if param.grad is None: + param.grad = torch.empty_like(param) + param.grad.data = prim_param.grad + else: + param.grad = prim_param.grad + + @contextlib.contextmanager + def unflatten_as_params(self) -> Generator: + """ + Unflatten the original parameters. + + The function assumes that the flat parameter is unsharded. When in the context, + unflattens the original parameters as ``nn.Parameter`` views into the + flat parameter, and after the context, restores the original parameters + as ``Tensor`` views into the flat parameter. + """ + self._use_unsharded_views(as_params=True) + try: + yield + finally: + self._use_unsharded_views(as_params=False) + + @no_type_check + @torch.no_grad() + def _use_sharded_views(self) -> None: + """ + Set the original parameter variables' data to be flattened views into the sharded flat parameter. + + The views are kept as flattened to simplify the case where a parameter + is sharded across ranks. Parameters whose data is not present in the + sharded flat parameter have their data set to a size-0 empty tensor. We + do not delete them to ensure to preserve expected behaviors like model + printability. Parameters whose data is present must preserve their + variables to be passable to an optimizer. + """ + self._unsharded_flat_param_for_skipped_views = None + if not self.uses_sharded_strategy: + # For `NO_SHARD`, use the *unflattened* unsharded views since we + # have the unsharded parameter + self._use_unsharded_views(as_params=True) + return + flat_param = self.flat_param + self._check_sharded(flat_param) + # Construct once and reuse for all parameters not in the local shard + size_0_empty_tensor = torch.empty( + 0, + dtype=self.flat_param.dtype, # in case `flat_param` changed dtype + device=self.flat_param.device, + requires_grad=False, + ) + for param, shard_param_info, (param_name, module, _) in zip( + flat_param._params, flat_param._shard_param_infos, flat_param._param_infos + ): + self._setattr_param(module, param_name, param) + if not shard_param_info.in_shard: + # Allow the original data to be freed via garbage collection + param.data = size_0_empty_tensor + else: + offset = shard_param_info.offset_in_shard + numel_in_shard = shard_param_info.numel_in_shard + param.data = flat_param[offset : offset + numel_in_shard] + assert self.flat_param._shared_params is not None + for i, ( + param, + (param_name, module, _, prim_param_name, prim_module, _), + ) in enumerate( + zip(self.flat_param._shared_params, self.flat_param._shared_param_infos) + ): + self._setattr_param(module, param_name, param) + prim_param = getattr(prim_module, prim_param_name) + param.data = prim_param # could be both empty and non-empty + if self._training_state == HandleTrainingState.BACKWARD_POST: + # Clear the saved `Tensor`s since they are unneeded now + for i in range(len(self.flat_param._tensors)): + self.flat_param._tensors[i] = None + + @no_type_check + @torch.no_grad() + def _use_sharded_grad_views(self) -> None: + """ + Set the original parameter variables' gradients to be flattened views into the sharded flat parameter's gradient. + + This is a no-op if there is no gradient. + + Parameters whose data is not present in the sharded flat parameter and + parameters with ``requires_grad=False`` have their gradients set to + ``None``. Since the gradient variables do not need to be preserved, + this method does not manipulate existing ``Tensor`` data directly and + creates new ``Tensor`` variables instead. + """ + flat_param = self.flat_param + self._check_sharded(flat_param) + grad = self.sharded_grad + if grad is None: + for param in chain(flat_param._params, flat_param._shared_params): + param.grad = None + return + self._check_sharded(grad) + for param, shard_param_info, is_grad_none in zip( + flat_param._params, + flat_param._shard_param_infos, + flat_param._is_grad_none_mask, + ): + if not shard_param_info.in_shard: + param.grad = None + else: + numel_in_shard = shard_param_info.numel_in_shard + if param.requires_grad and not is_grad_none: + offset = shard_param_info.offset_in_shard + if self._keep_low_precision_grads or param.dtype != grad.dtype: + # NOTE: This is a hack using `.data` to side step the + # check that parameter/gradient dtypes match. Here, + # `param` has full precision; `grad` has low precision. + if param.grad is None: + # `.grad` must have the same shape as `param` + param.grad = torch.empty_like(param) + param.grad.data = grad[ + offset : offset + numel_in_shard + ].reshape(param.shape) + else: + param.grad = grad[offset : offset + numel_in_shard].reshape( + param.shape + ) + else: + param.grad = None + assert flat_param._shared_params is not None + for i, (param, (_, _, _, prim_param_name, prim_module, _)) in enumerate( + zip(flat_param._shared_params, flat_param._shared_param_infos) + ): + in_sharded_flat_param = hasattr(prim_module, prim_param_name) + if in_sharded_flat_param and param.requires_grad: + prim_param = getattr(prim_module, prim_param_name) + param.grad = prim_param.grad # share the same reference + else: + param.grad = None + + @no_type_check + @torch.no_grad() + def _writeback_orig_params(self) -> bool: + """ + Write back any parameters that changed storage to the handle's ``FlatParameter``. + + Iterates over the original parameters and writes back any parameters + that changed storages (due to a non-inplace operator) to the handle's + ``FlatParameter``. This method preserves the ``FlatParameter` 's + device even if an original parameter's device changes. + + Raises: + RuntimeError: If an original parameter or gradient changes storages + but no longer has the expected flattened shape. + Returns: ``True`` if some writeback happened, and ``False`` otherwise. + """ + if ( + self.uses_sharded_strategy + and not self.is_sharded(self.flat_param) + and not self._skipped_use_sharded_views + ): + # For `NO_SHARD`, we may still need to writeback + return False + flat_param = self.flat_param + wroteback = False + if self._skipped_use_sharded_views and self.uses_sharded_strategy: + # NOTE: We must use the unsharded flat parameter from which the + # unsharded views were computed, not the one from the current + # calling context (`_get_padded_unsharded_flat_param()`) since that + # may be different (e.g. the model changed from train to eval). + flat_param_tensor = self._unsharded_flat_param_for_skipped_views + _p_assert( + _data_ptr_allocated(flat_param_tensor), + "If skipped using sharded views, the unsharded flat parameter " + "should be allocated", + ) + else: + flat_param_tensor = flat_param + # NOTE: Since this method is called in the pre-unshard, which is only + # called during computation in the pre-forward or pre-backward, the + # sharded gradient should be guaranteed to be in `.grad`, not in + # `._saved_grad_shard`. + flat_param_grad = ( + flat_param.grad + if self.uses_sharded_strategy or not self._offload_params + else flat_param._cpu_grad + ) + for i, ( + param, + (in_shard, offset_in_shard, numel_in_shard, _, _), + (param_name, module, _), + ) in enumerate( + zip( + flat_param._params, + flat_param._shard_param_infos, + flat_param._param_infos, + ) + ): + if not in_shard: + continue + if not hasattr(module, param_name): + # Do not writeback if original parameters are deregistered + # (e.g. during model checkpointing) + continue + + # Check for parameter writeback + if self._skipped_use_sharded_views: + param = flat_param._tensors[i] + _p_assert( + param is not None, + f"Expects to have saved tensor for {flat_param._fqns[i]}", + ) + param_changed = getattr(module, param_name) is not param + needs_param_writeback = ( + param_changed # changed parameter variable itself + or not _same_storage(param, flat_param_tensor) + ) + if self._skipped_use_sharded_views and ( + param_changed or needs_param_writeback + ): + raise AssertionError( + "FSDP does not support changing the parameters between " + f"forward and backward for {self._sharding_strategy}" + ) + if param_changed: + # NOTE: The gradient is not preserved after a parameter change. + param = getattr(module, param_name) + flat_param._params[i] = param + if needs_param_writeback: + expected_shape = torch.Size([numel_in_shard]) + self._writeback_tensor( + param, flat_param, i, expected_shape, offset_in_shard, True + ) + wroteback = True + + # Check for gradient writeback + if self._skipped_use_sharded_views: + # Skip the writeback check because we do not expose gradients + # when we skipped using sharded views + continue + if param.grad is None and flat_param.grad is not None: + expected_shape = torch.Size([numel_in_shard]) + self._writeback_tensor( + None, flat_param.grad, i, expected_shape, offset_in_shard, False + ) + elif param.grad is not None: + # For `NO_SHARD` + CPU offloading, `_cpu_grad` is always in + # memory and owns the gradient storage, so it will never + # require gradient writeback. + if not self.uses_sharded_strategy and self._offload_params: + # Explicitly continue to handle the case of `no_sync()`, + # where `param.grad` is a view into the GPU gradient + # referenced by `flat_param.grad`, while `flat_param_grad` + # is `flat_param._cpu_grad`, which is on CPU + continue + + needs_grad_writeback = flat_param_grad is None or not _same_storage( + param.grad, flat_param_grad + ) + if needs_grad_writeback: + if flat_param_grad is None: + flat_param_grad = torch.zeros_like(flat_param) + expected_shape = torch.Size([numel_in_shard]) + self._writeback_tensor( + param.grad, + flat_param_grad, + i, + expected_shape, + offset_in_shard, + False, + ) + flat_param.grad = flat_param_grad + flat_param_grad = flat_param.grad + + # TODO: If we want to handle shared parameters, we need to re-generate + # the shared parameter data structures in case sharedness changed. + for i, ( + param_name, + module, + _, + prim_param_name, + prim_module, + _, + ) in enumerate(flat_param._shared_param_infos): + if getattr(module, param_name) is not getattr(prim_module, prim_param_name): + raise NotImplementedError( + "Changing shared parameters is not supported yet" + ) + return wroteback + + def _writeback_tensor( + self, + src_tensor: Optional[Tensor], + dst_tensor: Tensor, + tensor_index: int, + expected_shape: torch.Size, + offset: int, + is_param: bool, # else gradient + ) -> None: + """ + Write back ``src_tensor`` to ``dst_tensor`` at offset ``offset``, where ``src_tensor`` should have shape ``expected_shape``. + + ``is_param`` indicates if the tensor is the parameter (if ``True``) or gradient (if + ``False``). If ``src_tensor`` is ``None``, then the effect is zeroing + instead of copying. ``tensor_index`` gives the index of ``src_tensor`` + in the metadata structures. + + Raises: + RuntimeError: If the ``src_tensor`` does not have the expected + shape. + """ + _p_assert( + len(expected_shape) == 1, + f"Expects a 1D expected shape but got {expected_shape}", + ) + if self._debug_level == dist.DebugLevel.INFO: + rank = self.rank if hasattr(self, "rank") else dist.get_rank() + src_shape = src_tensor.shape if src_tensor is not None else None + src_device = src_tensor.device if src_tensor is not None else None + warnings.warn( + f"[Rank {rank}] {'Parameter' if is_param else 'Gradient'} needs " + f"writeback in {self._training_state}\n" + f"expected shape={expected_shape} shape={src_shape} " + f"expected device={dst_tensor.device} device={src_device}" + ) + if src_tensor is not None and src_tensor.shape != expected_shape: + # NOTE: Gradient shape mismatch is not possible in practice since + # the gradient shape is enforced to match that of the parameter and + # we already check for parameter shape mismatch. + raise RuntimeError( + f"Cannot writeback when the {'parameter' if is_param else 'gradient'} " + f"shape changes\nExpects {expected_shape} but got {src_tensor.shape}" + ) + if src_tensor is not None: + dst_tensor[offset : offset + expected_shape.numel()].copy_(src_tensor) + else: + dst_tensor[offset : offset + expected_shape.numel()].zero_() + assert self.flat_param._is_grad_none_mask is not None + self.flat_param._is_grad_none_mask[tensor_index] = True + + def _reset_flat_param_grad_info_if_needed(self): + """ + Reset ``flat_param.grad`` if needed. + + When ``use_orig_params=True``: + (1) sets the underlying ``flat_param.grad`` to ``None`` if *all* of the + original parameters' ``.grad`` are ``None``, and + (2) sets ``flat_param.requires_grad=False`` if *none* of the original + parameters require gradient. + For (1), this is targeting ``optim.zero_grad(set_to_none=True)``, in + which case we want to free the gradients as soon after the + ``zero_grad()`` call as possible. + """ + if not self._use_orig_params: + return + flat_param = self.flat_param + assert flat_param._params is not None # mypy + all_grad_none = True + requires_grad = False + for param in flat_param._params: + all_grad_none &= param.grad is None + requires_grad |= param.requires_grad + if all_grad_none: + flat_param.grad = None + # As long as one parameter requires gradient, then the flat parameter + # must require gradient + flat_param.requires_grad = requires_grad + + def _deregister_orig_params(self): + for param_info in self.flat_param._param_infos: + param_name, module, _ = param_info + if hasattr(module, param_name): + delattr(module, param_name) + for param_name, module, _, _, _, _ in self.flat_param._shared_param_infos: + if hasattr(module, param_name): + delattr(module, param_name) + + ########### + # HELPERS # + ########### + def flat_param_to(self, *args, **kwargs): + """Wrap an in-place call to ``.to()`` for ``self.flat_param``.""" + self.flat_param.data = self.flat_param.to(*args, **kwargs) + if self._use_orig_params: + # Refresh the views because their storage may have changed + if self.is_sharded(self.flat_param): + self._use_sharded_views() + else: + self._use_unsharded_views(as_params=True) + + def _get_modules(self) -> Set[nn.Module]: + """Return a :class:`set` of the modules whose parameters are included in this handle's flat parameter.""" + return {pi.module for pi in self.flat_param._param_infos}.union( + {spi.module for spi in self.flat_param._shared_param_infos} + ) + + def is_sharded(self, tensor: Tensor) -> bool: + """ + Return whether ``tensor`` is *currently* sharded. + + For ``NO_SHARD``, we choose to have this always return ``False`` for clarity. + """ + if ( + not hasattr(self.flat_param, "_sharded_size") + or not self.uses_sharded_strategy + ): + # `_sharded_size` is defined iff `handle.shard()` has been called + return False + sharded_size = self.flat_param._sharded_size # type: ignore[attr-defined] + return tensor.size() == sharded_size + + def param_module_names(self) -> Iterator[Tuple[str, str]]: + shared_param_infos = [ + ParamInfo(param_name, module, module_name) + for ( + param_name, + module, + module_name, + _, + _, + _, + ) in self.flat_param._shared_param_infos + ] + for param_info in chain(self.flat_param._param_infos, shared_param_infos): + param_name, _, module_name = param_info # type: ignore[misc] + yield (param_name, module_name) + + def shared_param_module_names(self) -> Iterator[Tuple[str, str]]: + for param_name, _, module_name in [ + ParamInfo(param_name, module, module_name) + for ( + param_name, + module, + module_name, + _, + _, + _, + ) in self.flat_param._shared_param_infos + ]: + yield (param_name, module_name) + + @property + def _fqns_in_shard(self) -> List[str]: + """Return the FQNs of the parameters present in this rank's shard.""" + fqns_in_shard: List[str] = [] + for fqn, shard_param_info in zip( + self.flat_param._fqns, self.flat_param._shard_param_infos # type: ignore[attr-defined] + ): + if shard_param_info.in_shard: + fqns_in_shard.append(fqn) + return fqns_in_shard + + @property + def sharded_grad(self) -> Optional[Tensor]: + """Return the handle's sharded gradient.""" + flat_param = self.flat_param + # Priority for non-`None`: `_cpu_grad` > `_saved_grad_shard` > `grad` + # - CPU offloading: `_cpu_grad` + # - No CPU offloading + sharded strategies: `_saved_grad_shard` + # - No CPU offloading + `NO_SHARD`: `grad` + grad: Optional[Tensor] + if hasattr(flat_param, "_cpu_grad"): + grad = flat_param._cpu_grad # type: ignore[attr-defined] + elif hasattr(flat_param, "_saved_grad_shard"): + # In the post-backward hook, the sharded gradient is still in + # `_saved_grad_shard`. + grad = flat_param._saved_grad_shard # type: ignore[attr-defined] + else: + # If in IDLE or in FORWARD states, then there may be an + # (accumulated) gradient. If accessed in IDLE, then this should + # be due to re-registering the original parameters (e.g. in state + # dict load). + _p_assert( + flat_param.grad is None + or not self.uses_sharded_strategy + or self._training_state + in (HandleTrainingState.FORWARD, HandleTrainingState.IDLE), + "Sharded strategies should use `_cpu_grad` or `_saved_grad_shard` " + "unless in IDLE or FORWARD", + ) + grad = flat_param.grad + return grad + + def _reset_is_grad_none(self) -> None: + """ + Reset ``_is_grad_none_mask`` as needed. + + This method should only be + called in the post-backward after gradient computation, in which case + if a parameter requires gradient, then it will surely receive a + gradient and we may reset its mask entry to ``False``. + """ + if not self._use_orig_params: + return + _p_assert( + self._training_state == HandleTrainingState.BACKWARD_POST, + "Expects to only be called in the post-backward after gradient computation", + ) + flat_param = self.flat_param + assert flat_param._params is not None # mypy + for i, param in enumerate(flat_param._params): # type: ignore[arg-type] + # As long as the parameter requires gradient, it should receive a + # meaningful gradient (even if the gradient happens to be zeros) + if param.requires_grad: + assert flat_param._is_grad_none_mask is not None # mypy + flat_param._is_grad_none_mask[i] = False + + ####################### + # CHECKS & INVARIANTS # + ####################### + def _check_sharded_strategy(self): + _p_assert(self.uses_sharded_strategy, "Expects sharded strategy") + + def _check_on_compute_device(self, tensor: Tensor): + _p_assert( + tensor.device == self.device, + f"Expects tensor to be on the compute device {self.device}, was on {tensor.device}", + ) + + def _check_on_cpu(self, tensor: Tensor): + _p_assert( + tensor.device == torch.device("cpu"), + f"Expects tensor to be on CPU but got {tensor.device}", + ) + + @staticmethod + def _check_storage_freed(tensor: Tensor): + # Compile does not resize during trace + if not torch.distributed._functional_collectives.is_torchdynamo_compiling(): + _p_assert( + _same_storage_size(tensor, 0), + "Expects storage to be freed but got storage with size > 0", + ) + + @staticmethod + def _check_storage_allocated(tensor: Tensor): + _p_assert(_storage_size_allocated(tensor), "Expects storage to be allocated") + + def _check_low_precision_shard(self): + _p_assert( + self._uses_param_mixed_precision, + "Not using low precision for parameters", + ) + _p_assert( + getattr(self.flat_param, "_mp_shard", None) is not None, + "Expects `_mp_shard` to exist", + ) + device = self.flat_param._mp_shard.device # type: ignore[attr-defined] + _p_assert( + device == self.device, + f"Expects the low precision shard to be on {self.device} but got {device}", + ) + + def _check_unsharded(self, tensor: Tensor): + msg_prefix = "Expects tensor to be unsharded " + _p_assert(tensor is not None, msg_prefix + "but got `None`") + unsharded_size = self.flat_param._unpadded_unsharded_size + _p_assert( + tensor.size() == unsharded_size, + msg_prefix + f"with size {unsharded_size} but got {tensor.size()}", + ) + + def _check_sharded(self, tensor: Tensor): + msg_prefix = "Expects tensor to be sharded " + _p_assert(tensor is not None, msg_prefix + "but got `None`") + sharded_size = self.flat_param._sharded_size # type: ignore[attr-defined] + _p_assert( + tensor.size() == sharded_size, + msg_prefix + f"with size {sharded_size} but got {tensor.size()}", + ) + + ############## + # PROPERTIES # + ############## + @property + def uses_sharded_strategy(self) -> bool: + return self._sharding_strategy != HandleShardingStrategy.NO_SHARD + + @property + def _uses_param_mixed_precision(self) -> bool: + return self._fwd_bwd_param_dtype != self._orig_param_dtype + + @property + def _uses_reduce_mixed_precision(self) -> bool: + return self._reduce_dtype != self._orig_param_dtype + + @property + def _force_full_precision(self) -> bool: + return ( + self._uses_param_mixed_precision or self._uses_reduce_mixed_precision + ) and ( + self._training_state == HandleTrainingState.SUMMON_FULL_PARAMS + or + # Also disable mixed precision in model eval mode, if configured + (not self._fully_sharded_module.training and self._use_full_prec_in_eval) + ) + + @property + def _skipped_use_sharded_views(self) -> bool: + """ + This property is used for sharding strategies that do not free after forward with ``use_orig_params=True``. + + This returns if this handle is + currently in a state where it has skipped using sharded views, in which + case it can restore view invariants via ``_use_sharded_views()``. + """ + return self._unsharded_flat_param_for_skipped_views is not None + + +# NOTE: These are hacks to bypass `nn.Module.__setattr__` checks. +def _unsafe_setattr_param( + module: nn.Module, param_name: str, param: nn.Parameter +) -> None: + module._parameters[param_name] = param + # This bypasses any overrides in case `module` is an instance of an + # `nn.Module` subclass + super(nn.Module, module).__setattr__(param_name, param) + + +def _unsafe_setattr_tensor(module: nn.Module, param_name: str, tensor: Tensor) -> None: + module._parameters.pop(param_name, None) + # This bypasses any overrides in case `module` is an instance of an + # `nn.Module` subclass + super(nn.Module, module).__setattr__(param_name, tensor) + + +def _safe_setattr_tensor_or_param( + module: nn.Module, param_name: str, tensor_or_param: Union[Tensor, nn.Parameter] +): + # Call `delattr()` and `setattr()` to go through `nn.Module` checks + if hasattr(module, param_name): + delattr(module, param_name) + setattr(module, param_name, tensor_or_param) + + +def _convert_to_params( + tensors: List[Union[torch.Tensor, nn.Parameter]] +) -> List[nn.Parameter]: + return [t if isinstance(t, nn.Parameter) else nn.Parameter(t) for t in tensors] + + +def _detach_if_needed(param_or_tensor: Union[nn.Parameter, Tensor]) -> Tensor: + return ( + param_or_tensor.detach() + if isinstance(param_or_tensor, nn.Parameter) + else param_or_tensor + ) + + +def _get_aligned_numel(unsharded_dtype: torch.dtype): + # NOTE: This alignment constraint comes from TorchInductor. + ALIGNMENT = 16 # bytes + unsharded_dtype_size = _get_dtype_size(unsharded_dtype) + aligned_numel = ALIGNMENT // unsharded_dtype_size + return aligned_numel + + +@functools.lru_cache(8) +def _get_dtype_size(dtype): + return torch.empty((), dtype=dtype).element_size() + + +def _construct_padding_tensor( + padding_numel: int, dtype: torch.dtype, requires_grad: bool, device: torch.device +): + # NOTE: Set the padding value as a magic number for debuggability. The + # value itself should never be used in any user-facing computation. + return ( + torch.ones( + (padding_numel,), dtype=dtype, requires_grad=requires_grad, device=device + ) + * _FLAT_PARAM_PADDING_VALUE + ) + + +# Use `lru_cache(1)` to only log the warning once (assuming the fixed warning +# messasge is passed in) +@functools.lru_cache(1) +def _warn_skip_writeback_check(log: logging.Logger, warning: str): + log.warning(warning) + + +# Use `lru_cache(1)` to only log the warning once +@functools.lru_cache(1) +def _warn_use_fake_all_gather(log: logging.Logger, warning: str): + log.warning(warning) + + +# Use `lru_cache(1)` to only log the warning once +@functools.lru_cache(1) +def _warn_use_fake_reduce(log: logging.Logger, warning: str): + log.warning(warning) + + +def _same_storage(a, b): + return a.untyped_storage().data_ptr() == b.untyped_storage().data_ptr() + + +def _same_storage_size(a: torch.Tensor, b: int): + return a.untyped_storage().size() // a.element_size() == b + + +def _storage_size_allocated(tensor: Tensor): + storage_size: int = tensor.untyped_storage().size() + return storage_size > 0