Routing an LLM conversation to a greener region can cost more carbon than it saves, because the history has to be recomputed
AspireAI community · arXiv
Carbon-aware LLM routers pick a cleaner region or a smaller model per query, but a conversation is stateful: each turn reuses the KV cache of its history, which lives where earlier turns ran, so moving a turn means recomputing that history and paying its carbon cost. CarbonRouter jointly selects the model, GPU type and region per query with a cache-aware cost model that charges history recomputation to each candidate deployment using cached-token counts from the serving backends, plus a gate that keeps a session on its cached deployment when it would otherwise move back and rebuild again. In trace-driven experiments on two real conversation datasets with five LLMs, three GPU types and four regions, it produced the lowest carbon of all policies tested, cutting CO2 by 16.1% on average against FairCarbon, a state-of-the-art router with the same action space, and by 23.1% for sessions longer than 30 turns.
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