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Efficient Structural Pruning of Pre-trained Language Models via Multi-Objective Neural Architecture Search


Core Concepts
Neural architecture search can be effectively used to find sub-networks of pre-trained language models that balance model efficiency and generalization performance.
Abstract
The paper explores using neural architecture search (NAS) for structural pruning of pre-trained language models (PLMs) like BERT and RoBERTa. The goal is to find sub-networks of the pre-trained model that optimally trade-off efficiency (e.g., model size or latency) and generalization performance. The key insights are: NAS offers a distinct advantage over other pruning strategies by enabling a multi-objective approach to identify the Pareto optimal set of sub-networks. This allows automating the compression process and selecting the best model that meets the requirements, instead of running the pruning process multiple times to find the right threshold. The authors propose four different search spaces to prune transformer-based architectures, which exhibit varying degrees of pruning complexity. They show that simpler search spaces like SMALL and LAYER can often outperform more expressive but harder to explore spaces like LARGE. The authors evaluate weight-sharing based NAS approaches, which train a single super-network and then search for sub-networks within it. This substantially reduces the computational cost compared to standard NAS where each sub-network is fine-tuned independently. Empirically, the NAS-based pruning methods achieve competitive or better performance compared to other structural pruning approaches like head pruning and layer dropping, especially for larger datasets. Overall, the paper demonstrates the effectiveness of using NAS for structural pruning of pre-trained language models to balance model efficiency and performance.
Stats
The pre-trained BERT-base and RoBERTa-base models consist of 12 layers, 12 heads, and 3072 units. The authors evaluate the pruning methods on 8 text classification tasks, including textual entailment, sentiment analysis, and question answering.
Quotes
"Neural architecture search (NAS) offers a distinct advantage over other pruning strategies by enabling a multi-objective approach to identify the Pareto optimal set of sub-networks, which captures the nonlinear relationship between model size and performance instead of just obtaining a single solution." "Empirically, the NAS-based pruning methods achieve competitive or better performance compared to other structural pruning approaches like head pruning and layer dropping, especially for larger datasets."

Deeper Inquiries

How could the proposed NAS-based pruning approach be extended to handle other types of pre-trained models beyond just transformer-based architectures

The proposed NAS-based pruning approach can be extended to handle other types of pre-trained models beyond just transformer-based architectures by adapting the search space and methodology to suit the specific architecture. For instance, for convolutional neural networks (CNNs) commonly used in computer vision tasks, the search space could be defined to prune convolutional layers, filters, or even specific regions of the input image. The methodology could involve fine-tuning a super-network with shared weights and then searching for sub-networks that balance efficiency and performance using multi-objective optimization. By customizing the search space and training process to the characteristics of different types of pre-trained models, the NAS-based pruning approach can be effectively applied to a variety of architectures.

What are the potential drawbacks or limitations of the multi-objective NAS approach compared to single-objective pruning methods, and how could these be addressed

The multi-objective NAS approach has certain drawbacks and limitations compared to single-objective pruning methods. One limitation is the increased computational complexity and time required to search for the Pareto optimal set of sub-networks, especially when dealing with high-dimensional search spaces. This can make the approach less efficient for large-scale models or datasets. Additionally, the multi-objective approach may result in a larger set of potential solutions, making it challenging to select the most suitable sub-network for a specific deployment scenario. To address these limitations, one approach could be to incorporate domain-specific constraints or preferences into the optimization process to guide the search towards solutions that align with specific requirements. Additionally, leveraging techniques such as transfer learning or meta-learning to initialize the search process based on prior knowledge or experience could help accelerate the optimization process and improve the quality of the solutions obtained. Furthermore, exploring more advanced optimization algorithms or strategies tailored to the characteristics of the pruning problem could enhance the efficiency and effectiveness of the multi-objective NAS approach.

Given the success of the NAS-based pruning on text classification tasks, how might this approach translate to other domains like computer vision or speech recognition where pre-trained models are also widely used

The success of the NAS-based pruning approach on text classification tasks can potentially translate to other domains like computer vision or speech recognition where pre-trained models are widely used. In computer vision, the approach could be applied to prune layers, filters, or even specific regions of the input image in pre-trained CNNs. By fine-tuning a super-network with shared weights and searching for sub-networks that balance efficiency and performance, the NAS-based pruning approach can help optimize the deployment of pre-trained models in computer vision tasks. Similarly, in speech recognition tasks using pre-trained models like recurrent neural networks (RNNs) or transformer-based models, the NAS-based pruning approach can be adapted to identify and prune specific components of the model while maintaining performance. By customizing the search space and methodology to suit the architecture and requirements of speech recognition models, the approach can effectively reduce model size and improve efficiency in speech processing tasks. Overall, the NAS-based pruning approach has the potential to be a versatile and effective technique for optimizing pre-trained models across various domains beyond text classification.
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