arXiv:2607.18163cs.LGcs.AI2026-07

用平滑的输出重置机制替代PPO的截断目标,提升语言模型微调稳定性

OR Else: A Differentiable Trust Region for Policy Optimization

论文配图:OR Else: A Differentiable Trust Region for Policy Optimization
图 1 · 摘自论文原文
  • 引入输出重置(OR)实现单侧平滑饱和,替代传统截断策略
  • 在GAE设置下,PPO-OR比PPO-clip平均高0.305分,但种子间差异更大
  • GRPO-OR虽未提升均值分数,但优化更稳定、残差趋近零、过冲减少

本文研究在大语言模型微调中,用平滑的输出重置(OR)机制替代PPO和GRPO中的截断代理目标。PPO-OR与GRPO-OR在回溯相对词元对数比率空间中使用平方边距损失,优势符号决定更新方向,跨过有利边界后直接残差为零。在Llama-3.2-1B-Instruct模型上,基于Anthropic hh-rlhf数据集,使用共享奖励模型与三个随机种子进行测试。在广义优势估计(GAE)下,PPO-OR训练时奖励模型得分比PPO-clip平均高0.305,但种子间波动更大;在组相对优势下,GRPO-OR虽均值未提升,但表现出更小的种子间差异、接近零的终端OR残差及下降的过冲比例,而原方法轨迹仍不稳定。两种组相对方法的回溯到当前对数比率位移显著大于GAE方法,且OR并未一致降低该位移。因此,OR改变了优化行为,但收益因方法而异。在G=2时,GRPO-OR的诊断指标未带来分数提升,更大分组是否改变结果尚不确定。所有报告分数均为训练时奖励模型评分,非保留人类偏好表现。

原文摘要 · Abstract (English)

PPO and the GRPO baseline studied here use clipped surrogate objectives whose favorable-direction saturation introduces an abrupt change in the scalar objective's derivative. We ask whether Output Reset (OR), a smooth one-sided saturation rule, offers a useful alternative for large language model post-training. PPO-OR and GRPO-OR replace the clipped policy term with an OR squared-margin loss in rollout-relative token log-ratio space; the advantage sign determines the update direction, and a token contributes zero direct OR residual after crossing the favorable margin. We compare PPO-clip with PPO-OR under generalized advantage estimation (GAE), and GRPO with GRPO-OR under group-relative advantages, using \texttt{Llama-3.2-1B-Instruct} on Anthropic \texttt{hh-rlhf} with one shared reward model and three seeds per method. Under GAE, PPO-OR has a mean final training-time reward-model score $0.305$ higher than PPO-clip, with a larger observed across-seed spread. Under group-relative advantages, GRPO-OR does not have a higher mean score, but shows a smaller observed spread, a near-zero terminal OR residual, and a declining overshoot fraction, while the matched GRPO clipped-objective trace remains variable. Both group-relative methods exhibit substantially larger rollout-to-current log-ratio displacement than the GAE methods, and OR does not consistently reduce it. Thus, OR changes optimization behavior in both matched comparisons, but the observed reward effect differs between them. At $G=2$, the GRPO-OR diagnostics do not translate into a reward-score gain. Whether larger groups change this outcome remains open. The reported scores are training-time reward-model measurements, not held-out human-preference performance.

强化学习策略优化大模型微调稳定训练

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