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How to Fine-Tune LLMs with Prefix Tuning
Prefix tuning is a parameter-efficient method for adapting large language models (LLMs) to specific tasks without modifying their pre-trained weights. Instead of updating the entire model during fine-tuning, prefix tuning introduces learnable prefix parameters —continuous vectors that act as task-specific prompts. These prefixes are prepended to the input sequence and passed through all layers of the model, guiding the LLM’s behavior during inference. This approach keeps the original model parameters frozen, reducing computational costs while enabling task adaptation. The core idea stems from optimizing these prefixes to encode task-relevant information, such as instructions or contextual cues. For example, in natural language generation tasks, the prefixes might encode signals like “summarize” or “translate to French,” allowing the model to generate outputs aligned with the desired objective. Unlike traditional fine-tuning, which updates all model weights, prefix tuning isolates changes to these small, task-specific parameters, making it computationally efficient and scalable for large models. As mentioned in the section, this method falls under broader categories like prompt-based tuning, which focuses on soft instruction signals. Prefix tuning offers several advantages over conventional fine-tuning methods. First, it significantly reduces the number of parameters that need training. Studies show that prefix parameters typically account for less than 0.1% of an LLM’s total parameters, drastically cutting memory and computational requirements. This efficiency is critical for deploying large models on resource-constrained systems or when training data is limited.