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Efficient Autoregressive Decoding with Adaptive Feed Forward Skipping in Large Language Models

Core Concepts
FFN-SkipLLM is a novel fine-grained skip strategy that can skip ~25-30% of feed-forward network (FFN) blocks in autoregressive large language models (LLMs) with marginal change in performance on knowledge-intensive tasks.
The content discusses the challenges faced by autoregressive large language models (LLMs) in terms of their massive scale and high computational requirements for token-by-token generation. To address this, the authors propose FFN-SkipLLM, a novel fine-grained skip strategy that focuses on skipping the computationally expensive feed-forward network (FFN) blocks in LLMs. The key observations that motivate FFN-SkipLLM are: Monotonically increasing cosine similarity between tensors before and after the FFN blocks across layers, indicating redundant computation. Existence of "cold regions" (initial and final layers) where FFN blocks should not be skipped to maintain performance. Leveraging the "attention sink" phenomenon to stabilize the initial token generation before applying FFN skipping. FFN-SkipLLM uses an input-adaptive approach to identify the non-cold regions where FFN blocks can be skipped based on the cosine similarity threshold. It can skip ~25-30% of FFN blocks with marginal change in performance on knowledge-intensive tasks like Factoid-QA, multi-turn conversations, and variable-length text summarization, while avoiding the KV cache issues associated with layer-skipping methods. The authors extensively evaluate FFN-SkipLLM against baselines and show its effectiveness in retaining the model's ability to access factual knowledge, maintain coherence, consistency, fluency, and relevance in the generated outputs.
The GPT-175B model requires 325 GB of GPU memory and a minimum of five A100 (80GB) GPUs. FFN blocks hold approximately two-thirds of the parameter budget of a LLaMa layer.
"Unlike prior layer-skipping methods, we focus on only skipping computationally expensive FFN blocks based on our observation of their monotonically increasing saturation within the middle layers of LLMs." "Our proposed FFN-SkipLLM uses a simple cosine similarity metric across tensors to capture the trend of FFN saturation and decide an input-adaptive skipping of FFN blocks." "Our extensive knowledge-intensive experiments such as Factoid-QA, Multi-turn conversations and Variable-length in-context text summarization, reveal that FFN-SkipLLM can skip ~25-30% of FFN blocks of LLMs with a marginal change in performance and reduce hallucination and token collapse."

Key Insights Distilled From

by Ajay Jaiswal... at 04-08-2024

Deeper Inquiries

How can the FFN-SkipLLM approach be extended to handle even higher skip ratios (e.g., >35%) without significant performance degradation?

To extend the FFN-SkipLLM approach to handle higher skip ratios without significant performance degradation, several strategies can be considered: Fine-tuning Techniques: Implement parameter-efficient fine-tuning techniques to optimize the model for higher skip ratios. This involves adjusting the training process to adapt the model to effectively skip a larger percentage of FFN blocks while maintaining performance. Dynamic Skipping Thresholds: Implement dynamic skipping thresholds based on token characteristics or context. By dynamically adjusting the skipping criteria based on the input, the model can intelligently decide when to skip FFN blocks, potentially allowing for higher skip ratios without sacrificing performance. Adaptive Warm-Up: Enhance the warm-up mechanism in FFN-SkipLLM to better prepare the model for higher skip ratios. By improving the initial token processing and KV cache stabilization, the model can handle larger skip ratios more effectively. Ensemble Methods: Explore ensemble methods where multiple models with different skip ratios are combined to leverage the strengths of each model. This can help mitigate the performance degradation associated with higher skip ratios by combining the outputs of multiple models. By incorporating these strategies and potentially exploring new techniques tailored to handling higher skip ratios, the FFN-SkipLLM approach can be extended to achieve efficient skipping while maintaining performance integrity.

What are the potential trade-offs between the computational savings achieved through FFN skipping and the potential loss of model expressivity?

The trade-offs between computational savings through FFN skipping and potential loss of model expressivity include: Performance vs. Efficiency: The primary trade-off is between model performance and computational efficiency. While FFN skipping can significantly reduce computation and speed up inference, there is a risk of sacrificing model performance, especially in tasks requiring complex reasoning or nuanced understanding. Information Loss: Skipping FFN blocks may lead to information loss or incomplete processing of tokens, impacting the model's ability to generate accurate and contextually relevant responses. This trade-off can affect the overall quality of the model's outputs. Generalization: By skipping FFN blocks, the model may lose the ability to generalize effectively across different tasks or datasets. This trade-off can limit the model's adaptability and robustness in diverse scenarios. Complexity Reduction: While FFN skipping simplifies the computational complexity of the model, it may also reduce the model's capacity to handle intricate patterns or relationships in the data. This trade-off can affect the model's ability to capture nuanced features. Balancing these trade-offs is crucial in optimizing the FFN skipping approach to strike a harmonious balance between computational efficiency and model expressivity.

How can the insights from FFN-SkipLLM be applied to improve the efficiency of other types of large neural networks beyond autoregressive language models?

The insights from FFN-SkipLLM can be applied to enhance the efficiency of other types of large neural networks by: Layer Skipping Strategies: Implementing similar fine-grained skipping strategies for computationally expensive components in other neural network architectures. By identifying redundant computations and selectively skipping certain components, the efficiency of the network can be improved. Dynamic Skipping Mechanisms: Introducing dynamic skipping mechanisms based on input characteristics or task requirements in different neural network models. This adaptive approach can optimize the computational resources used during inference. KV Cache Optimization: Developing strategies to handle KV caching issues in other neural network architectures by exploring alternative methods to stabilize the cache or mitigate the impact of missing information due to early exits. Ensemble Techniques: Leveraging ensemble techniques to combine models with different skipping strategies or efficiencies. This approach can enhance overall performance and efficiency by capitalizing on the strengths of each model. By applying the principles and methodologies of FFN-SkipLLM to other large neural networks, researchers and practitioners can enhance the efficiency and effectiveness of various models across different domains and tasks.