arXiv AI By Lars Krupp, Daniel Gei{\ss}ler, Francisco M. Calatrava-Nicolas, Stefan Gerd Fritsch, Paul Lukowicz, Jakob Karolus

The Real Cost of Context: Predicting Compute Time and Energy Consumption in LLM Prefill and Decode Phases

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The paper investigates how token length affects compute time and energy usage in Large Language Models, focusing on both prefill (input) and decode (output) phases. It finds that decode energy scales linearly with output length, while prefill energy shows both linear and quadratic scaling with input length, highlighting the importance of large input contexts in agentic workflows. The study also demonstrates that compute time can reliably proxy energy consumption when direct measurements are unavailable, providing a framework for more sustainable AI deployment.

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