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Power capping barely touches LLM decode: the GPU idles waiting on memory and draws under half its 700W rating

AspireAI community · arXiv (PPAM 2026)
Power capping is a standard GPU energy lever and it appears to work, because the cap is set, the GPU reports drawing less, and the energy budget is met. This measurement shows the cap does little for the phase that dominates LLM serving, which is autoregressive decode. Across four attention paradigms, GQA, MLA, Gated DeltaNet and Mamba2, on an NVIDIA H200 under vLLM eager execution, decode kernels wait on HBM and the GPU idles between kernel launches, so decode draws less than half the GPU's 700W rating and throughput stays within a few percent under every cap tested. The cap only engages when a large batch runs a long generation. Locking the SM clock instead lowers power without slowing memory-bound decode and recovers up to 32 percent of decode energy at minimal throughput loss, and the authors also show recurrent architectures pay no prefill penalty once prefix caching is excluded.
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