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Do layernorm before allgather for DP attention #8631
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Summary of Changes
Hello @trevor-m, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!
This pull request focuses on a critical performance optimization within the data parallel communication layer. By strategically moving the layer normalization step to occur before the all-gather operation, the system can perform computations on smaller, local data chunks, leading to reduced overhead and improved overall throughput. The changes are validated by significant benchmark improvements without any reported accuracy regressions.
Highlights
- Performance Optimization: Reordered the Layer Normalization (layernorm) operation to occur before the Data Parallel (DP) gather operation within the
_gather_hidden_states_and_residual
function inpython/sglang/srt/layers/communicator.py
. - Efficiency Improvement: This change allows
layernorm
to operate on a smaller, local subset of tokens (specifically,1/DP
th of the tokens), which significantly reduces the computational time required for the normalization step. - Code Refinement: Removed an unnecessary
dp_scatter
operation, simplifying the data flow within the communicator layer. - Benchmark Results: Achieved a notable 3.79% end-to-end speedup, with total token throughput increasing from approximately 27310 tokens/s to 28345 tokens/s, demonstrating the effectiveness of the optimization.
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Code Review
This pull request aims to improve performance by moving the Layer Normalization operation before the all-gather in data parallel settings. The benchmark results show a speedup.
My review identifies a critical correctness issue. The change, combined with existing code, leads to inconsistent tensors across tensor parallel ranks when tp_size
is not equal to dp_size
. This will cause silent numerical errors in such configurations.
LGTM! Thx. |
@ch-wan could you please take a look? tnx |
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LGTM
Motivation
By doing layernorm before all-gather, we operate on
1/DP
th of the tokens reducing the computation time.Modifications
Perform layernorm before DP gather in layer communicator.
Currently only enabled when DP==TP.
Accuracy Test
Benchmark & Profiling
Speedup: 3.79% end to end
BEFORE
AFTER
Checklist