Lightning-AI / Lightning-AI/pytorch-lightning
FSDP with HYBRID_SHARD loss doesn't improve with more nodes
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- Python
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Description
### Bug description
When using the FSDP strategy with HYBRID SHARD set, the loss behaves as if only one node is training. When it is set to FULL_SHARD/etc the loss drops as expected when more nodes are added and batch size is left constant. I have verified NCCL connections are working correctly as everything behaves as expected when using FULL_SHARD.
### What version are you seeing the problem on?
v2.4
### How to reproduce the bug
_No response_
### Error messages and logs
```
# Error messages and logs here please
```
### Environment
Current environment
```
#- PyTorch Lightning Version (e.g., 2.4.0):
#- PyTorch Version (e.g., 2.4):
#- Python version (e.g., 3.12):
#- OS (e.g., Linux):
#- CUDA/cuDNN version:
#- GPU models and configuration:
#- How you installed Lightning(`conda`, `pip`, source):
```
### More info
_No response_
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the reported behavior with FSDP HYBRID_SHARD and a fixed batch size while varying the node count, then compare it with FULL_SHARD. Check the FSDP strategy and distributed-training tests or entry points that cover shard strategies; done means loss improves with additional nodes under HYBRID_SHARD as expected.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python, pytorch
- Domain
- distributed-systems, machine-learning
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100