On Teacher Hacking in Language Model Distillation
Authors: name: Daniil Tiapkin, name: Daniele Calandriello, name: Johan Ferret, name: Sarah Perrin, name: Nino Vieillard, name: Alexandre Ramé, name: Mathieu Blondel

Post-training of language models (LMs) increasingly relies on the following
two stages: (i) knowledge distillation, where the LM is trained to imitate a
larger teacher LM, and (ii) reinforcement learning from human feedback (RLHF),
where the LM is aligned by optimizing a reward model. In the second RLHF stage,
a well-known challenge is reward hacking, where the LM over-optimizes the
reward model. Such phenomenon is in line with Goodhart's law and can lead to
degraded performance on the true objective. In this paper, we investigate
whether a similar phenomenon, that we call teacher hacking, can occur during
knowledge distillation. This could arise because the teacher LM is itself an
imperfect approximation of the true distribution. To study this, we propose a
controlled experimental setup involving: (i) an oracle LM representing the
ground-truth distribution, (ii) a teacher LM distilled from the oracle, and
(iii) a student LM distilled from the teacher. Our experiments reveal the
following insights. When using a fixed offline dataset for distillation,
teacher hacking occurs; moreover, we can detect it by observing when the
optimization process deviates from polynomial convergence laws. In contrast,
employing online data generation techniques effectively mitigates teacher
hacking. More precisely, we identify data diversity as the key factor in
preventing hacking. Overall, our findings provide a deeper understanding of the
benefits and limitations of distillation for building robust and efficient LMs.