When Perplexity Lies: Generation-Focused Distillation of Hybrid Sequence Models

TL;DR

This paper introduces a new hybrid model architecture and distillation pipeline that emphasizes generation-based evaluation, revealing that traditional log-likelihood metrics can mislead model quality assessments.

Contribution

The authors propose Hybrid-KDA architecture and GenDistill pipeline, demonstrating the importance of generation-based evaluation for effective distillation of sequence models.

Findings

Log-likelihood evaluation underestimates the teacher-student gap.

Generation-based evaluation can reverse conclusions from perplexity-only metrics.

Dataset choice and training strategies significantly impact generation quality.

Abstract

Converting a pretrained Transformer into a more efficient hybrid model through distillation offers a promising approach to reducing inference costs. However, achieving high-quality generation in distilled models requires careful joint design of both the student architecture and the distillation process. Many prior distillation works evaluate downstream multiple-choice benchmarks by ranking candidate answers with log-likelihood rather than requiring autoregressive generation, which can obscure important differences in model quality. For example, we show that a 7B parameter distilled model that nearly matches its teacher to within 0.2\,pp under log-likelihood scoring actually falls behind by 20.8\,pp when the model must generate answers autoregressively. We propose a Hybrid Kimi Delta Attention (Hybrid-KDA) architecture paired with GenDistill, a multi-stage distillation pipeline, and use generation-based evaluation throughout to guide design decisions. Applying this approach to Qwen3-0.6B, we systematically ablate six design axes: training objective, loss masking, training duration, dataset selection, parameter freezing, and architecture choice. We find that log-likelihood-based evaluation consistently underestimates the gap between teacher and student, and can in some cases reverse the ranking of design choices, meaning that conclusions drawn from perplexity-only evaluation may be misleading. Among the factors we study, dataset selection, completion-only masking, and freezing attention layers during post-training have the largest impact on generation quality. Our best Hybrid-KDA model retains 86--90\% of teacher accuracy on knowledge benchmarks while reducing KV cache memory by up to 75\% and improving time-to-first-token by 2--4$\times$ at 128K-token contexts.