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Updated FSDP -> FSDPModule in doc #538

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Aug 20, 2024
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Updated FSDP -> FSDPModule in doc #538

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2 changes: 1 addition & 1 deletion docs/fsdp.md
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Expand Up @@ -49,7 +49,7 @@ def fully_shard(
- `fully_shard(module)` is similar to `FullyShardedDataParallel(module)`, constructing one communication bucket from `module.parameters()` except those already assigned to a nested `fully_shard`/`FullyShardedDataParallel` call.
- `fully_shard(module)` adds an `FSDPState` object on `module`, accessible via `fully_shard.state(module)`, instead of being an `nn.Module` wrapper. This is done via the `@contract` decorator.
- Calling `model.named_parameters()` for a `model` with FSDP2 applied returns unchanged parameter names and `DTensor` sharded parameters. This means that the optimizer and gradient norm clipping see `DTensor`s.
- `fully_shard(module)` performs a dynamic class swap on `module`. E.g., if `type(module) is Transformer`, then FSDP2 constructs a new class `FSDPTransformer` that inherits from a class `FSDP` and `Transformer` and sets `module.__class__` to be `FSDPTransformer`. This allows us to add new methods and override methods via `FSDP` without constructing an `nn.Module` wrapper.
- `fully_shard(module)` performs a dynamic class swap on `module`. E.g., if `type(module) is Transformer`, then FSDP2 constructs a new class `FSDPTransformer` that inherits from a class `FSDPModule` and `Transformer` and sets `module.__class__` to be `FSDPTransformer`. This allows us to add new methods and override methods via `FSDPModule` without constructing an `nn.Module` wrapper.
- FSDP1's `sharding_strategy` and `process_group`/`device_mesh` maps to FSDP2's `mesh` and `reshard_after_forward`.
- `mesh` should be 1D for FSDP and 2D for HSDP. For HSDP, we assume replication on the 0th mesh dim and sharding on the 1st mesh dim. If `mesh is None`, then FSDP2 initializes a 1D global mesh over the default process group.
- `reshard_after_forward=True` or `False` determines whether parameters are resharded (freed) after forward. If `True`, then they are re-all-gathered in backward. This trades off saving memory at the cost of extra communication.
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