How to Enable Native Fully Sharded Data Parallel in PyTorch
Blog post from Lightning AI
PyTorch’s native Fully Sharded Data Parallel (FSDP) strategy, supported in Lightning starting with version 1.7.0 through the `fsdp_native` Trainer strategy, helps train large models despite limited hardware by distributing optimizer states and model weights across multiple GPUs. Unlike standard Distributed Data Parallel, where every GPU stores a full model copy, FSDP shards model parameters across devices and uses efficient communication to reduce per-device memory requirements, providing an approach similar to ZeRO Stage 3 while aiming to retain DDP-level performance. Users can customize the implementation through `DDPFullyShardedNativeStrategy`, and the native PyTorch version introduces streamlined APIs and performance improvements over FairScale’s earlier FSDP implementation, including faster model initialization when CPU offloading is enabled. Because PyTorch classifies native FSDP as beta, Lightning’s corresponding strategy is also subject to interface changes, while FairScale is expected to remain focused on research as broadly adopted functionality is incorporated into PyTorch.
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