C3PO: Critical-Layer, Core-Expert, Collaborative Pathway Optimization
  for Test-Time Expert Re-Mixing
Authors: name: Zhongyang Li; name: Ziyue Li; name: Tianyi Zhou
Published: 2025-04-10 17:59:56

Abstract: Mixture-of-Experts (MoE) Large Language Models (LLMs) suffer from severely
sub-optimal expert pathways-our study reveals that naive expert selection
learned from pretraining leaves a surprising 10-20% accuracy gap for
improvement. Motivated by this observation, we develop a novel class of
test-time optimization methods to re-weight or "re-mixing" the experts in
different layers jointly for each test sample. Since the test sample's ground
truth is unknown, we propose to optimize a surrogate objective defined by the
sample's "successful neighbors" from a reference set of samples. We introduce
three surrogates and algorithms based on mode-finding, kernel regression, and
the average loss of similar reference samples/tasks. To reduce the cost of
optimizing whole pathways, we apply our algorithms merely to the core experts'
mixing weights in critical layers, which enjoy similar performance but save
significant computation. This leads to "Critical-Layer, Core-Expert,
Collaborative Pathway Optimization (C3PO)". We apply C3PO to two recent MoE
LLMs and examine it on six widely-used benchmarks. It consistently improves the
base model by 7-15% in accuracy and outperforms widely used test-time learning
baselines, e.g., in-context learning and prompt/prefix tuning, by a large
margin. Moreover, C3PO enables MoE LLMs with 1-3B active parameters to
outperform LLMs of 7-9B parameters, hence improving MoE's advantages on
efficiency. Our thorough ablation study further sheds novel insights on
achieving test-time improvement on MoE.