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## Zero Bubble Schedules
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The key of achieving zero bubble is to breaking a backward pass into a B pass and W pass. B on one stage will only depend on the B on its next stage, compared to depending on both B and W of in 1F1B.
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, where white text colors represent the first chunk and black text colors represent the second chunk. The sequence of dependencies among model chunks follows a ”V” shape pattern for both the forward and backward passes.
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### Comparision of Schedules
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* 1F1B
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* ZB1P
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* ZB2P
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* ZBV - Each device is assigned to exactly 2 chunks (virtual stages), where white text colors represent the first chunk and black text colors represent the second chunk. The sequence of dependencies among model chunks follows a ”V” shape pattern for both the forward and backward passes.
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In most practices of PP there's an all-reduce cross all pipeline stages for numerical robustness, e.g. global gradient norm for gradient clipping. INF/NAN check for mixed precision training, etc. This all-reduce breaks parallelogram and makes zero bubble impossible.
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Under the observation that during a stable training both the gradient clipping and INF/NAN rarely triggers, we replace the before-hand synchronizations with a post update validation.
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We eagerly step the optimizers assuming the grad cliping, INF/NAN conditions are not triggered. In case an amendment to the gradient is required, a rollback will be issued and then we redo the optimizer step based on the fully reduced global state.
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