Rethinking Self-Distillation for Multi-Teacher Capability Merging

LLM SFT Other LLM
将多个从同一基础检查点(base checkpoint)出发训练得到的专家模型能力进行融合,已成为当前前沿语言模型后训练中日益普遍的做法。近期研究趋势表明,多教师在线策略蒸馏(MOPD)在性能上优于传统的离线策略方法。然而,尽管MOPD带来了更高的推理开销与环境交互成本,我们发现其报告的精度提升在很大程度上源于特定的训练设计选择和超参数优化,而非该算法本身固有的优势。我们在两种多教师设定、四种模型及十一个基准任务上开展了一项受控的自蒸馏研究,系统比较了离线策略方法(即监督微调SFT与软标签蒸馏)与混合式教师前缀蒸馏(hybrid teacher-prefix distillation)及MOPD的性能表现。结果表明,这四种方法所达到的精度几乎完全相同。但MOPD所需的训练GPU小时数却是SFT的14.8至23.1倍。此外,我们重新审视了近期发表的四组关于在线策略与离线策略蒸馏的对比实验,发现:当SFT基线模型采用经拒绝采样(rejection sampling)生成的教师轨迹进行训练,并使用独立调优的超参数时,原先报告的在线策略方法优势显著缩小。作为无需额外训练的替代方案,我们还发现,简单的权重合并(weight-merging)方法仅需数分钟CPU时间即可完成合并,便能有效复现各专家模型的能力;不过,其精度会随模型规模减小及任务间干扰加剧而有所下降。总体而言,我们的结果对近期归因于MOPD所取得的性能提升提出了质疑,并建议:通过审慎调优更高效的离线策略基线方法,同样可成为一种切实可行的替代路径。
Combining capabilities of multiple expert models trained starting from the same base checkpoint has become increasingly common in frontier language-model post-training. Recent trends suggest that multi-teacher on-policy distillation (MOPD) outperforms conventional off-policy methods. However, despite the higher inference and environment interaction costs incurred by MOPD, we find that much of its reported accuracy gain is due to certain training design choices and hyperparameter optimization, rather than the algorithm itself. We conduct a controlled self-distillation study across two multi-teacher settings, four models, and eleven benchmarks, comparing off-policy methods, namely supervised fine-tuning (SFT) and soft-label distillation, to hybrid teacher-prefix distillation and MOPD. We found that all four methods achieve \textit{nearly identical} accuracy. However, MOPD uses $14.8$--$23.1\times$ SFT's training GPU-hours. We also revisit four recently published comparisons between on-policy and off-policy distillation and find that the reported on-policy gains shrink substantially when SFT baselines are trained on rejection-sampled teacher trajectories and use independently tuned hyperparameters. As a training-free alternative, we also find that simple weight-merging methods can recover expert capabilities with minutes of CPU merging time, although their accuracy degrades as model size decreases and task interference increases. Overall, our results question recent gains reported due to MOPD and suggest careful tuning of more efficient off-policy baselines as a viable alternative.
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