FAIR-Calib: Frontier-Aware Instability-Reweighted Calibration for Post-Training Quantization of Diffusion Large Language Models
Organizations: National College for Excellent Engineers, Beihang University, Beijing, China · State Key Laboratory of Media Convergence and Communication, Communication University of China, Beijing, China · Independent researcher · School of Artificial Intelligence, Beihang University, Beijing, China · Ningbo Institute of Digital Twin, Eastern Institute of Technology, Ningbo, China · Huawei Noah’s Ark Lab, China · Artificial Intelligence Research Center, Lobachevsky State University, Nizhny Novgorod 603022, Russia
Abstract
Diffusion Large Language Models (dLLMs) refine tokens iteratively but commit them irreversibly, leading to a "stability lag" where early decisions remain fragile even after being written. We reveal that Post-Training Quantization (PTQ) error easily flips these borderline decisions at the write frontier, which are then permanently locked in and amplified. To address this, we propose Frontier-Aware Instability-Reweighted Calibration (FAIR-Calib), a two-stage PTQ framework for dLLMs. Stage I probes a full-precision teacher to estimate a position prior that combines frontier hits and masked-stage reliability. Stage II performs off-policy, layer-wise calibration by minimizing a reweighted hidden-state MSE, effectively prioritizing the protection of fragile frontier states without requiring expensive end-to-end diffusion rollouts. We further theoretically justify our weighted objective as a surrogate for output KL divergence. Empirically, FAIR-Calib consistently outperforms state-of-the-art baselines on LLaDA and Dream (W4A4), significantly reducing frontier decision flips and suppressing post-commit mismatches across diverse benchmarks.