To Keep or Not to Keep: Learning KV Cache Retention in Disaggregated LLM Serving Systems
Disaggregated LLM serving separates prefill and decode into distinct node pools, interposing a network fabric between the moment a key-value (KV) cache is computed and the moment it is consumed. This architectural shift invalidates a core assumption of classical cache policies: that the cost of a miss is simply recomputation on the same device. In disaggregated systems, a miss triggers both recomputation on a prefill node and a network transfer of the resulting KV block to the decode node—costs that differ by an order of magnitude and depend on prefix length, model width, and fabric bandwidth. Meanwhile, admitting a block to the global KV pool requires an additional transfer at compute time, so a poorly chosen keep decision wastes both memory and bandwidth even before reuse occurs. We present KVLearn, a learning-based retention framework that makes keep/evict decisions as first-class cost-optimization choices in disaggregated LLM serving. KVLearn consists of three components: (i) a lightweight Prefix Reuse Predictor (PRP) that estimates reuse probability from structural and temporal prefix features without touching model weights; (ii) a Cost-Aware Retention Score (CARS) that translates reuse probability into a keep/admit signal by accounting for per-block recompute, transfer, and storage costs; and (iii) an Adaptive Threshold Controller (ATC) that adjusts the admission threshold online using closed-loop feedback from observed hit rates and memory pressure. We integrate KVLearn into a globally disaggregated serving topology and evaluate it on both text and multimodal workloads, where image/video-derived tokens create large, expensive-to-recompute KV blocks under heterogeneous reuse distributions. KVLearn reduces end-to-end time-to-first-token (TTFT) by up to 56% vs. No-Cache (recompute-only), up to 38% vs. LRU-Pool, and up to 33% vs. Mooncake-style disaggregated baselines. Inter-node KV transfer volume is cut by up to 53% vs. LRU-Pool. On MM-Session, throughput stays within ~5% of oracle. Our code implementation of KVLearn is available at https://github.com/FastLM/KVLearn.