Layernorm bwd OPT #1880
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Layernorm bwd OPT #1880
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Maybe we can follow recent cuda change here pytorch/pytorch@73b4938 |
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toyxu
approved these changes
Aug 8, 2025
liangan1
approved these changes
Aug 19, 2025
Co-authored-by: Copilot <[email protected]>
Co-authored-by: Copilot <[email protected]>
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Pull Request Overview
This PR optimizes the backward pass for LayerNorm by implementing a two-stage column reduction for gamma and beta gradients when dealing with cases where M (rows) >> N (columns). The optimization addresses performance bottlenecks where the original implementation would only launch a single workgroup for column reduction, resulting in poor GPU utilization.
- Implements a new
GammaBetaReduceFunctorfor optimized two-stage reduction - Adds intelligent heuristics to determine when to use the optimized path vs. the simple kernel
- Provides significant performance improvements (from 9ms to 0.15ms on PVC, 8.5ms to 0.27ms on BMG)
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I noticed layer norm backward on gamma and beta is very slow when column is much longer. i.e. [M,N] column reduction and M>>N.
For example, in timm tnt_s_patch16_224 training, layernorm bwd shape [25088,16,24], normalized shape [24]. it will only launch one workgroup. I use a two staged column reduction to increase parallelism. GammaBetaBackwardSimpleKernelFunctor takes 9 ms on PVC, 8.5ms on BMG. After opt, we use GammaBetaReduceFunctor and two sum to do column reduction, they will take 0.09ms + 0.06ms x2 on PVC and 0.19ms + 0.04ms x 2 on BMG