Sandwich: Joint Configuration Search and Hot-Switching for Efficient CPU LLM Serving

Abstract

CPUs are critical for LLM serving due to their availability, cost efficiency, and edge applicability. However, efficient CPU serving is hindered by conflicting prefill/decode resource demands under non-disaggregated deployment constraints--existing solutions fail to avoid cross-phase interference, ignore sub-NUMA hardware structures, and deliver suboptimal dynamic-shape kernel performance. We propose Sandwich, a full-stack CPU LLM serving system with three core innovations addressing these challenges: (1) seamless phase-wise plan switching to eliminate cross-phase interference; (2) TopoTree, a tree-based hardware abstraction for automated substructure-aware (e.g., LLC slices) partial core allocation; (3) fast-start-then-finetune dynamic-shape tensor program generation. Across five x86/ARM CPU platforms, Sandwich achieves an average 2.01x end-to-end speedup and up to 3.40x latency reduction over state-of-the-art systems. Its kernels match static compiler performance with three orders of magnitude lower tuning cost.