PrefixQuant: Static Quantization Beats Dynamic through Prefixed Outliers
  in LLMs
Authors: name: Mengzhao Chen; name: Yi Liu; name: Jiahao Wang; name: Yi Bin; name: Wenqi Shao; name: Ping Luo
Published: 2024-10-07 17:59:35

Abstract: Quantization is essential for deploying Large Language Models (LLMs) by
enhancing memory efficiency and inference speed. Existing methods for
activation quantization mainly address channel-wise outliers, often neglecting
token-wise outliers, leading to reliance on costly per-token dynamic
quantization. To address this, we introduce PrefixQuant, a novel technique that
isolates outlier tokens offline without re-training. Specifically, PrefixQuant
identifies high-frequency outlier tokens and prefixes them in the KV cache,
preventing the generation of outlier tokens during inference and simplifying
quantization. To our knowledge, PrefixQuant is the first to enable efficient
per-tensor static quantization to outperform expensive per-token dynamic
quantization. For instance, in W4A4KV4 (4- bit weight, 4-bit activation, and
4-bit KV cache) Llama-3-8B, PrefixQuant with per-tensor static quantization
achieves a 7.43 WikiText2 perplexity and 71.08% average accuracy on 5
common-sense reasoning tasks, outperforming previous per-token dynamic
quantization methods like QuaRot with 0.98 perplexity improvement and +5.98
points accuracy. Additionally, the inference speed of W4A4 quantized models
using PrefixQuant is 1.60x to 2.81x faster than FP16 models and exceeds QuaRot
models by 1.2x to 1.3x. Our code is available at
https://github.com/ChenMnZ/PrefixQuant.