Simulate Before You Scale: High-Fidelity LLM Inference Planning Without GPUs
Deploying large language model (LLM) inference at scale requires jointly selecting hardware layout, parallelism strategy, scheduling policy, admission control, and request routing to meet latency and throughput targets. These choices interact: a routing decision that improves cache locality can concentrate load, and an admission threshold that triggers only at saturation allows queues to grow deep enough to degrade critical-tier traffic. Evaluating candidates on GPU clusters is too expensive for iterative exploration, yet existing predictive tools either ignore batching and queueing dynamics or model only fragments of the pipeline. In this paper, we present BLIS, a deterministic, CPU-only discrete-event simulator that models the full inference serving pipeline (admission, routing, scheduling, prefill/decode execution, KV-cache management, and preemption). BLIS pairs analytical basis functions with learned scaling coefficients and overhead terms, fit once globally, to predict step latency without per-model or per-GPU profiling. Across 36 experiments spanning six models and three GPU types, BLIS achieves 6.7% median end-to-end error while running around 200 times faster than real execution. In a configuration search over 420 serving configurations for Llama-2-70B, BLIS identifies a 4-GPU deployment that meets a 300ms mean TTFT service-level objective (SLO), whereas competing tools either require double the GPUs or recommend configurations that fail under load. Two policies evolved entirely in simulation were validated on real hardware and merged into llm-d, a widely adopted open-source platform for distributed LLM serving: an admission policy that reduces critical-tier TTFT P90 by up to 82%, and a flow-control policy that reduces E2E P50 latency by 32-48% on overloaded workloads.