GRaSP: Latency-Aware Optimization for Geo-Distributed Retrieval-Augmented Generation

Overview & Synopsis

Retrieval-Augmented Generation (RAG) is increasingly served across geo-distributed cloud and edge infrastructures, where wide-area latency, cross-region egress cost, and data-residency rules compete with retrieval quality. Conventional pipelines rank by semantic similarity alone and ignore where content lives, inflating tail latency and cost. We formalize the Geo-RAG Selection Problem (GRaSP), a drop-in re-scoring layer that decays relevance by latency and adds an optional cost penalty under a cardinality budget and per-region caps, with no retraining. We first settle its complexity: the base formulation is a linear objective over a partition matroid, solved optimally by an O(N log N) greedy algorithm, rather than NP-hard as a knapsack framing suggests, with hardness entering only under an additive egress budget. On a five-node CloudLab testbed emulating a four-region WAN over the Natural Questions corpus (2.68M passages), GRaSP's benefit is placement-dependent: under uniform placement it cuts fetch latency at negligible quality cost; under topical (residency-like) placement it traces a recall-latency Pareto frontier that dominates relevance-only, nearest-k, and a latency-budgeted baseline. At a moderate latency setting, GRaSP lowers P90 fetch latency by 41% while retaining 78% of relevance-only's recall, with sub-millisecond selection overhead, stable to 200 queries/s. Findings generalize across four public BEIR benchmarks. In end-to-end question answering, GRaSP reduces context-fetch latency by 43% while remaining within 0.062 absolute EM of relevance-only retrieval.