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arXiv · 2604.12782

OSC: Hardware Efficient W4A4 Quantization via Outlier Separation in Channel Dimension

Zhiyuan Zhang·Yanzhao Li·Zhiqiang Zou·Bai Du·Yupeng Sun·Hui Dong·Hui Wang
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Abstract

While 4-bit quantization is essential for high-throughput deployment of Large Language Models, activation outliers often lead to significant accuracy degradation due to the restricted dynamic range of low-bit formats. In this paper, we systematically investigate the spatial distribution of outliers and demonstrate a token-persistent structural clustering effect, where high-magnitude outliers consistently occupy fixed channels across tokens. Building on this insight, we propose OSC, a hardware-efficient framework for outlier suppression. During inference, OSC executes a dual-path computation consisting of a low-precision 4-bit General Matrix Multiplication (GEMM) path and a high-precision 16-bit branch GEMM path. Specifically, OSC uses an offline group-wise strategy to identify the channels where outliers are located and then performs structured sub-tensor extraction to coalesce these scattered activation channels into a compact dense tensor online. This mechanism implements outlier protection through regularized and high-throughput GEMM operations, achieving a seamless fit with modern 4-bit micro-scaling hardware. Furthermore, for the inputs of W2 where outlier clustering is less pronounced, we integrate a fallback strategy to FP8. Evaluation on Qwen3-8B and Qwen3-30B restricts the average accuracy drop to 2.19 and 1.12 points, respectively. Notably, OSC is highly hardware-friendly, achieving a peak speedup of 1.78x over the W8A8 GEMM baseline on a modern AI accelerator.

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Zhiyuan Zhang, Yanzhao Li, Zhiqiang Zou, Bai Du, Yupeng Sun, Hui Dong, Hui Wang. 2026-04-14. OSC: Hardware Efficient W4A4 Quantization via Outlier Separation in Channel Dimension. https://arxiv.org/abs/2604.12782

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