LLM-FP4: 4-Bit Floating-Point Quantized Transformers
Shih-yang Liu Zechun Liu Xijie Huang Pingcheng Dong Kwang-Ting Cheng
We propose LLM-FP4 for quantizing both weights and activations in large
language models (LLMs) down to 4-bit floating-point values, in a post-training
manner. Existing post-training quantization (PTQ) solutions are primarily
integer-based and struggle with bit widths below 8 bits. Compared to integer
quantization, floating-point (FP) quantization is more flexible and can better
handle long-tail or bell-shaped distributions, and it has emerged as a default
choice in many hardware platforms. One characteristic of FP quantization is
that its performance largely depends on the choice of exponent bits and
clipping range. In this regard, we construct a strong FP-PTQ baseline by
searching for the optimal quantization parameters. Furthermore, we observe a
high inter-channel variance and low intra-channel variance pattern in
activation distributions, which adds activation quantization difficulty. We
recognize this pattern to be consistent across a spectrum of transformer models
designed for diverse tasks, such as LLMs, BERT, and Vision Transformer models.
To tackle this, we propose per-channel activation quantization and show that
these additional scaling factors can be reparameterized as exponential biases
of weights, incurring a negligible cost. Our method, for the first time, can
quantize both weights and activations in the LLaMA-13B to only 4-bit and
achieves an average score of 63.1 on the common sense zero-shot reasoning
tasks, which is only 5.8 lower than the full-precision model, significantly
outperforming the previous state-of-the-art by 12.7 points. Code is available
at: https://github.com/nbasyl/LLM-FP4.