arXiv AI By Bole Ma, Ayesha Afzal, Jan Eitzinger, Harald Koestler, Gerhard Wellein

Only Accepted Tokens Count: An Energy Accounting of Speculative Decoding Under Latency Targets

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The paper investigates how speculative decoding—where a draft model proposes multiple future tokens that are then verified by the target model—affects energy consumption in large language model serving under latency constraints. It finds that energy savings depend heavily on the acceptance rate of drafted tokens, which varies by model architecture, drafting scheme, and the type of text (e.g., conversation vs. code, English vs. Romansh). The study shows that while speculation can reduce energy per token at small batch sizes, at larger batches the verification cost dominates, potentially turning speculation into an energy loss, especially when acceptance rates are low.

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arXiv Machine Learning
Oct 3

CAST: Cost-Aware Speculative Trees from One-Pass Block Drafters

CAST (Cost‑Aware Speculative Trees) is a method that improves speculative decoding for large language models by packing multiple drafted token candidates into a tree and verifying the entire tree in a single target‑model pass, rather than only the top‑scoring chain. The tree width is adaptively chosen based on a latency measurement, ensuring that each added candidate’s expected gain outweighs its verification cost. Experiments across five domains, three GPU generations, and two model families show that CAST can be up to 43 % faster than the standard chain, while preserving the target model’s output distribution under both greedy and sampled decoding.

By Jungseob Lee, Sugyeong Eo