Support overlapped grad sync with Megatron pipeline parallelism #1475
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Each grad bucket independently keeps track of grads that have been generated. Add helper function to create callback functions. Change default param arg in grad norm functions to None. Perform communication for checkpointing in main stream to avoid memory pool overheads.
Enables async grad reduction in first pipeline stage during last backward pass, and disables async grad reduction in all other pipeline stages.
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crcrpar
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Sep 8, 2022
apex/transformer/pipeline_parallel/schedules/fwd_bwd_pipelining_without_interleaving.py
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Add unit test for pipeline parallelism with custom sync context. Style tweaks.
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hubertlu-tw
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Dec 29, 2022
…atron pipeline parallelism (NVIDIA#1475) * Refactor how dist Adam handles overlapped grad sync Each grad bucket independently keeps track of grads that have been generated. Add helper function to create callback functions. Change default param arg in grad norm functions to None. Perform communication for checkpointing in main stream to avoid memory pool overheads. * Support Megatron pipeline parallelism with async grad reduction Enables async grad reduction in first pipeline stage during last backward pass, and disables async grad reduction in all other pipeline stages. * Review suggestions from crcrpar Add unit test for pipeline parallelism with custom sync context. Style tweaks. * Use unittest assert functions in pipeline parallelism test Review suggestion from crcrpar
yuanzhedong
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Jul 14, 2023
…atron pipeline parallelism (NVIDIA#1475) * Refactor how dist Adam handles overlapped grad sync Each grad bucket independently keeps track of grads that have been generated. Add helper function to create callback functions. Change default param arg in grad norm functions to None. Perform communication for checkpointing in main stream to avoid memory pool overheads. * Support Megatron pipeline parallelism with async grad reduction Enables async grad reduction in first pipeline stage during last backward pass, and disables async grad reduction in all other pipeline stages. * Review suggestions from crcrpar Add unit test for pipeline parallelism with custom sync context. Style tweaks. * Use unittest assert functions in pipeline parallelism test Review suggestion from crcrpar
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This PR adds functionality so that the distributed Adam optimizer can overlap grad reduce-scatters with backward compute in the first pipeline stage. Async grad reductions are disabled in the other pipeline stages since it slows down the backward pass, so the reduction should be done externally while waiting for the first stage to finish. Note that this is not compatible with
DistributedDataParallelsince I am not aware of an option to manually trigger a reduction after usingno_sync.I've also done some refactoring of distributed Adam to support NeMo-Megatron integration, mostly so that I can selectively disable async grad reductions for model-parallel operations:
[]toNone. This allows us to pass in empty iterators