OWL: Overcoming Window Length-Dependence in Speculative Decoding for Long-Context Inputs

2025-10-08

AI Summary

Existing speculative decoding methods suffer from sharply declining acceptance rates and diminished inference acceleration under long-context inputs. This paper introduces OWL, an efficient speculative decoding framework tailored for long-text generation. Its core contributions are: (1) a lightweight LSTM-based draft model that operates solely on the final token’s hidden state, eliminating dependence on fixed-length context windows; (2) a verifier augmented with a dedicated [SPEC] token to enhance long-range semantic modeling; and (3) a hybrid verification strategy integrating tree-structured and sequential decoding to improve both acceptance length and robustness. Evaluated on LongSpecBench—a newly constructed long-context benchmark—we find that OWL increases average acceptance length by approximately 5× over EAGLE3 and achieves up to 1.23× end-to-end speedup. The code and dataset are publicly released.

Technology Category

• Natural Language Processing: Generation
• Machine Learning: Large Multimodal Models (LMMs)
• Planning, Routing, and Scheduling: Planning with Language Models

Application Category

• Economics, Online Markets and Human Computation: Architectures and workflows that use LLMs for crowd work
• Search and Retrieval-Augmented AI: Web learning to rank, online learning, and counterfactual learning for ranking
• Semantics and Knowledge: Methods to enhance, augment, integrate or synergize semantic models such as knowledge graphs and LLMs

Abstract

Speculative decoding promises faster inference for large language models (LLMs), yet existing methods fail to generalize to real-world settings. Benchmarks typically assume short contexts (e.g., 2K tokens), whereas practical workloads involve long contexts. We find current approaches degrade severely with long contexts; for instance, EAGLE3 even slows down the generation speed by 0.81x. We address these limitations by releasing a new long-context benchmark (LongSpecBench) and introducing a novel model (OWL). OWL achieves about 5x higher acceptance length than EAGLE3 on long-context inputs through three innovations: (1) an LSTM-based drafter conditioned only on the last-token state, making it generalize to various lengths, (2) a special token [SPEC] in the verifier that produces richer representation for drafter, and (3) a hybrid algorithm combining both tree and non-tree decoding methods. We release all code and datasets to advance future research.

Problem

• Overcoming performance degradation in speculative decoding with long contexts
• Addressing window length-dependence in speculative decoding methods
• Improving acceptance length for long-context inputs in LLMs

Innovation

• LSTM-based drafter conditioned on last-token state
• Special token [SPEC] produces richer drafter representation
• Hybrid algorithm combining tree and non-tree decoding