Speculative decoding can cost more energy than it saves at high batch sizes, because verification rather than drafting dominates the bill
AspireAI community · arXiv
Speculative decoding is usually sold on speed, so this paper measures it on power instead, under production serving conditions with SGLang and CUDA graphs on H100 and H200 GPUs at batch sizes up to 64 across five corpora. On dense models verification, not drafting, dominates the energy cost, and the saving shrinks as the batch grows because batching has already shared the weight read that speculation would divide among accepted tokens. At large batch sizes block drafters push verification past the arithmetic-to-memory balance point into the compute-bound regime, where the saving becomes a loss; the authors report one drafter stopping saving energy between batches of 16 and 32 on the H100, and a speculative run serving 32 requests beating a non-speculative baseline even at a batch of 64 on five of eleven models. Two measurement caveats are worth keeping: every drafter accepted fewer tokens on conversation than on code and fewer on Romansh than on English, and the paper notes prior work reported savings without recording the condition, including a CUDA-graph bias of up to 39 percentage points on mixture-of-experts models.
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