Fast-dLLM: Training-free Acceleration of Diffusion LLM by Enabling KV
  Cache and Parallel Decoding
Authors: name: Chengyue Wu; name: Hao Zhang; name: Shuchen Xue; name: Zhijian Liu; name: Shizhe Diao; name: Ligeng Zhu; name: Ping Luo; name: Song Han; name: Enze Xie
Published: 2025-05-28 17:39:15

Abstract: Diffusion-based large language models (Diffusion LLMs) have shown promise for
non-autoregressive text generation with parallel decoding capabilities.
However, the practical inference speed of open-sourced Diffusion LLMs often
lags behind autoregressive models due to the lack of Key-Value (KV) Cache and
quality degradation when decoding multiple tokens simultaneously. To bridge
this gap, we introduce a novel block-wise approximate KV Cache mechanism
tailored for bidirectional diffusion models, enabling cache reuse with
negligible performance drop. Additionally, we identify the root cause of
generation quality degradation in parallel decoding as the disruption of token
dependencies under the conditional independence assumption. To address this, we
propose a confidence-aware parallel decoding strategy that selectively decodes
tokens exceeding a confidence threshold, mitigating dependency violations and
maintaining generation quality. Experimental results on LLaDA and Dream models
across multiple LLM benchmarks demonstrate up to 27.6times
throughput improvement with minimal accuracy loss, closing the performance gap
with autoregressive models and paving the way for practical deployment of
Diffusion LLMs.