Skip expanding scales for rowwise fp8 quantize#2950
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🔗 Helpful Links🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/2950
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drisspg
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Sep 6, 2025
drisspg
reviewed
Sep 6, 2025
| return scale | ||
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| # For rowwise quantization, just return the scale as is | ||
| if scale.shape[:-1] == target_shape[:-1] and scale.shape[-1] == 1: |
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you could probably do something fun, like
def is_trivial_expandable(scale, target_shape):
return all(a == b or a == 1 for a, b in zip(scale.shape, target_shape))**Summary:** #2253 added a step in `quantize_affine_float8` to expand the scales for blockwise quantization. The purpose of this step is to make the scales always broadcastable with the input tensor. However, this is unnecessary for rowwise quantization, which already has broadcastable shapes, e.g. ``` scale = [32, 1] input = [32, 16] ``` Today, we will `repeat_interleave` the above scales to pad the scale tensor until it reaches `[32, 16]`, which adds non-trivial memory and latency overhead. This commit adds a fast path to skip this expanding step if we detect rowwise quantization. **Test Plan:** ``` python test/quantization/test_quant_primitives.py -k test_maybe_expand_scale_to_tensor_shape ``` Also compared fine-tuning Qwen3-1.7B with fp8-fp8 QAT using batch size 32 on a single H100 GPU: - Before: 25.34 GB peak memory, 3047.25 tok/s - After: 22.53 GB peak memory, 3358.49 tok/s - This PR uses 11.1% less memory and is 10.2% faster
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drisspg
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Summary: #2253 added a step in
quantize_affine_float8to expand the scales for blockwise quantization. The purpose of this step is to make the scales always broadcastable with the input tensor. However, this is unnecessary for rowwise quantization, which already has broadcastable shapes, e.g.Today, we will
repeat_interleavethe above scales to pad the scale tensor until it reaches[32, 16], which adds non-trivial memory and latency overhead. This commit adds a fast path to skip this expanding step if we detect rowwise quantization.Test Plan:
Also compared fine-tuning Qwen3-1.7B with fp8-fp8 QAT using batch size 32 on a single H100 GPU: