Optimizing SLO-oriented LLM Serving with PD-Multiplexing

Abstract: Modern LLM services demand high throughput and stringent SLO guarantees across two distinct inference phases-prefill and decode-and complex multi-turn workflows. However, current systems face a fundamental tradeoff: out-of-place compute partition enables per-phase SLO attainment, while in-place memory sharing maximizes throughput via KV cache reuse. Moreover, existing in-place compute partition also encounters low utilization and high overhead due to phase-coupling design. We present Drift, a new LLM serving framework that resolves this tension via PD multiplexing, enabling in-place and phase-decoupled compute partition. Drift leverages low-level GPU partitioning techniques to multiplex prefill and decode phases spatially and adaptively on shared GPUs, while preserving in-place memory sharing. To fully leverage the multiplexing capability, Drift introduces an adaptive gang scheduling mechanism, a contention-free modeling method, and a SLO-aware dispatching policy. Evaluation shows that Drift achieves an average $5.1\times$ throughput improvement (up to $17.5\times$) over state-of-the-art baselines, while consistently meeting SLO targets under complex LLM workloads.