Large language model (LLM) personalization typically relies on modeling each user in isolation, conditioning on their historical interactions to adapt model behavior. However, this user-centric formulation overlooks the collective knowledge shared across users, limiting generalization for users with sparse histories and amplifying overfitting for those with highly skewed behaviors. We argue that effective personalization requires leveraging both individual preferences and population-level patterns. To this end, we propose LoGo, a Local–Global knowledge framework that augments user-specific signals with a global knowledge encoding collective behavioral trends. LoGo models global knowledge through a temporally evolving process that captures how population-wide preferences change over time, and a community-aware structure that organizes users into coherent groups with shared interests. To balance potentially conflicting local and global signals, LoGo employs a mediator module that adaptively fuses the two knowledge sources. Experiments on five personalization benchmarks show that LoGo consistently enhances personalization quality, outperforming existing methods by improving generalization in users with limited histories and mitigating bias in users with abundant histories. These results demonstrate the central role of collective knowledge in advancing LLM personalization. Our code is publicly available at https://github.com/Zehong-Wang/LoGo.

Aligning large language models (LLMs) with human preferences has become a critical step in their development. Recent research has increasingly focused on test-time alignment, where additional compute is allocated during inference to enhance LLM safety and reasoning capabilities. However, these test-time alignment techniques often incur substantial inference costs, limiting their practical application. We are inspired by the speculative sampling acceleration, which leverages a small draft model to efficiently predict future tokens, to address the efficiency bottleneck of test-time alignment. We introduce the reward-shifted speculative sampling (SSS) algorithm, in which the draft model is aligned with human preferences, while the target model remains unchanged. We theoretically demonstrate that the distributional shift between the aligned draft model and the unaligned target model can be exploited to recover the RLHF optimal solution without actually obtaining it, by modifying the acceptance criterion and bonus token distribution. Our algorithm achieves superior gold reward scores at a significantly reduced inference cost in test-time weak-to-strong alignment experiments, thereby validating both its effectiveness and efficiency.