The Curse of Depth in Large Language Models
Authors: name: Wenfang Sun, name: Xinyuan Song, name: Pengxiang Li, name: Lu Yin, name: Yefeng Zheng, name: Shiwei Liu

In this paper, we introduce the Curse of Depth, a concept that highlights,
explains, and addresses the recent observation in modern Large Language
Models(LLMs) where nearly half of the layers are less effective than expected.
We first confirm the wide existence of this phenomenon across the most popular
families of LLMs such as Llama, Mistral, DeepSeek, and Qwen. Our analysis,
theoretically and empirically, identifies that the underlying reason for the
ineffectiveness of deep layers in LLMs is the widespread usage of Pre-Layer
Normalization (Pre-LN). While Pre-LN stabilizes the training of Transformer
LLMs, its output variance exponentially grows with the model depth, which
undesirably causes the derivative of the deep Transformer blocks to be an
identity matrix, and therefore barely contributes to the training. To resolve
this training pitfall, we propose LayerNorm Scaling, which scales the variance
of output of the layer normalization inversely by the square root of its depth.
This simple modification mitigates the output variance explosion of deeper
Transformer layers, improving their contribution. Our experimental results,
spanning model sizes from 130M to 1B, demonstrate that LayerNorm Scaling
significantly enhances LLM pre-training performance compared to Pre-LN.
Moreover, this improvement seamlessly carries over to supervised fine-tuning.
All these gains can be attributed to the fact that LayerNorm Scaling enables
deeper layers to contribute more effectively during training.