Expand and Prune: Maximizing Trajectory Diversity for Effective GRPO in Generative Models

• URL: http://arxiv.org/abs/2512.15347v1
• Date: Wed, 17 Dec 2025 11:44:34 GMT
• Title: Expand and Prune: Maximizing Trajectory Diversity for Effective GRPO in Generative Models
• Authors: Shiran Ge, Chenyi Huang, Yuang Ai, Qihang Fan, Huaibo Huang, Ran He,
• Abstract summary: Group Relative Policy Optimization is a powerful technique for aligning generative models.<n>But its effectiveness is bottlenecked by the conflict between large group sizes and prohibitive computational costs.<n>We propose Pro-GRPO, a novel dynamic framework that integrates latent feature-based trajectory pruning into the sampling process.
• Score: 37.48289959306949
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Group Relative Policy Optimization (GRPO) is a powerful technique for aligning generative models, but its effectiveness is bottlenecked by the conflict between large group sizes and prohibitive computational costs. In this work, we investigate the trade-off through empirical studies, yielding two key observations. First, we discover the reward clustering phenomenon in which many trajectories collapse toward the group-mean reward, offering limited optimization value. Second, we design a heuristic strategy named Optimal Variance Filtering (OVF), and verify that a high-variance subset of trajectories, selected by OVF can outperform the larger, unfiltered group. However, this static, post-sampling OVF approach still necessitates critical computational overhead, as it performs unnecessary sampling for trajectories that are ultimately discarded. To resolve this, we propose Pro-GRPO (Proactive GRPO), a novel dynamic framework that integrates latent feature-based trajectory pruning into the sampling process. Through the early termination of reward-clustered trajectories, Pro-GRPO reduces computational overhead. Leveraging its efficiency, Pro-GRPO employs an "Expand-and-Prune" strategy. This strategy first expands the size of initial sampling group to maximize trajectory diversity, then it applies multi-step OVF to the latents, avoiding prohibitive computational costs. Extensive experiments on both diffusion-based and flow-based models demonstrate the generality and effectiveness of our Pro-GRPO framework.

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