Streamlining Large Language Models by Pruning Unimportant Layers

Core Concepts

Certain consecutive layers in large language models have minimal impact on hidden states, allowing for effective layer pruning without significant performance degradation.

Abstract

The content discusses a method for compressing large language models (LLMs) by pruning unimportant layers. The key observations are:

Different layers in LLMs have varying degrees of impact on the hidden states, with some consecutive layers exhibiting minimal perturbation.
The authors propose using the cosine similarity between layer input and output hidden states as a metric to identify less important layers.
The proposed LLM-Streamline framework consists of two steps:

Layer pruning: Removing a set of consecutive layers with the lowest importance based on the target sparsity.
Layer replacement: Training a lightweight model, such as a multi-layer perceptron (MLP), to substitute the pruned layers and mitigate performance degradation.

Experiments on various LLM architectures (OPT, Llama2) show that LLM-Streamline can maintain 92% of the original model's performance on classification tasks and 68% on generation tasks, while reducing the model size by 25%.
The authors also explore the impact of different lightweight models, the number of lightweight models, and the amount of training data on the performance of the pruned models.

Stats

The content does not provide specific numerical data or metrics, but rather discusses the general approach and findings of the proposed LLM-Streamline method.

Quotes

None.

Key Insights Distilled From

Compressing Large Language Models by Streamlining the Unimportant Layer

by Xiaodong Che... at arxiv.org 03-29-2024

https://arxiv.org/pdf/2403.19135.pdf

Deeper Inquiries

How does the performance of LLM-Streamline scale with larger model sizes (e.g., models with over 10 billion parameters)

The performance of LLM-Streamline is expected to scale well with larger model sizes, such as models with over 10 billion parameters. The key insight of identifying less important layers based on the cosine similarity between input and output hidden states remains valid regardless of the model size. In larger models, there may be more redundancy and layers with minimal impact on the hidden states, making the layer pruning approach even more effective. Additionally, the use of lightweight models, such as MLPs, to replace pruned layers can help maintain performance while reducing the overall parameter count. Therefore, LLM-Streamline is likely to continue outperforming existing pruning methods for larger language models.

What are the potential drawbacks or limitations of the layer pruning approach, and how could they be addressed in future work

While LLM-Streamline offers significant advantages in compressing large language models, there are potential drawbacks and limitations to consider. One limitation is the trade-off between model size reduction and performance retention. Pruning too many layers or using overly simplistic lightweight models may lead to a significant drop in performance. To address this, future work could focus on optimizing the selection of layers for pruning and the design of lightweight models to better capture the information from pruned layers. Additionally, the method's reliance on cosine similarity as a metric for layer importance may not capture all aspects of a layer's contribution to the model. Exploring alternative metrics or combining multiple criteria could enhance the accuracy of layer pruning decisions.

Could the insights from this work be applied to other types of large neural networks beyond language models, such as vision transformers or multimodal models

The insights from this work on layer pruning in large language models can be applied to other types of large neural networks beyond language models. For instance, in vision transformers, similar layer redundancy may exist, allowing for the identification and removal of less important layers to compress the model. The concept of using lightweight models to replace pruned layers can also be extended to vision transformers to maintain performance while reducing parameters. Furthermore, multimodal models that combine text and image inputs could benefit from a similar layer pruning approach to streamline the model architecture and improve efficiency without sacrificing performance. By adapting the principles of LLM-Streamline to these domains, researchers can explore more efficient and compact architectures for various types of large neural networks.