Guiding Frozen Language Models with Learned Soft Prompts

Large pre-trained language models, which are continuing to grow in size, achieve state-of-art results on many natural language processing (NLP) benchmarks. Since the development of GPT and BERT, standard practice has been to fine-tune models on downstream tasks, which involves adjusting every weight in the network (i.e., model tuning). However, as models become larger, storing and serving a tuned copy of the model for each downstream task becomes impractical.

An appealing alternative is to share across all downstream tasks a single frozen pre-trained language model, in which all weights are fixed. In an exciting development, GPT-3 showed convincingly that a frozen model can be conditioned to perform different tasks through “in-context” learning. With this approach, a user primes the model for a given task through prompt design, i.e., hand-crafting a text prompt with a description or examples of the task at hand. For instance, to condition a model for sentiment analysis, one could attach the prompt, “Is the following movie review positive or negative?” before the input sequence, “This movie was amazing!

Sharing the same frozen model across tasks greatly simplifies serving and allows for efficient mixed-task inference, but unfortunately, this is at the expense of task performance. Text prompts require manual effort to design, and even well-designed prompts still far underperform compared to model tuning. For instance, the performance of a frozen GPT-3 175B parameter model on the SuperGLUE benchmark is 5 points below a fine-tuned T5 model that uses 800 times fewer parameters.

In “The Power of Scale for Parameter-Efficient Prompt Tuning”, presented at EMNLP 2021, we explore prompt tuning, a more efficient and effective method for conditioning frozen models using tunable soft prompts. Just like engineered text prompts, soft prompts are concatenated to the input text. But rather than selecting from existing vocabulary items, the “tokens” of the soft prompt are learnable vectors. This means a soft prompt can be optimized end-to-end over a training dataset. In addition to removing the need for manual design, this allows the prompt to condense information from datasets containing thousands or millions of examples. By comparison, discrete text prompts are typically limited to under 50 examples due to constraints on model input length. We are also excited to release the code and checkpoints to fully reproduce our experiments.

Prompt tuning retains the strong task performance of model tuning, while keeping the pre-trained model frozen, enabling efficient multitask serving.

Prompt Tuning
To create a soft prompt for a given task, we first initialize the prompt as a fixed-length sequence of vectors (e.g., 20 tokens long). We attach these vectors to the beginning of each embedded input and feed the combined sequence into the model. The model’s prediction is compared to the target to calculate a loss, and the error is back-propagated to calculate gradients, however we only apply these gradient updates to our new learnable vectors — keeping the core model frozen. While soft prompts learned in this way are not immediately interpretable, at an intuitive level, the soft prompt is extracting evidence about how to perform a task from the labeled dataset, performing the same role as a manually written text prompt, but without the need to be constrained to discrete language.

Our codebase, implemented in the new JAX-based T5X framework, makes it easy for anyone to replicate this procedure, and provides practical hyperparameter settings, including a large learning rate (0.3), which we found was important for achieving good results.

Since soft prompts have a small parameter footprint (we train prompts with as few as 512 parameters), one can easily pass the model a different prompt along with each input example. This enables mixed-task inference batches, which can streamline serving by sharing one core model across many tasks.

Left: With model tuning, incoming data are routed to task-specific models. Right: With prompt tuning, examples and prompts from different tasks can flow through a single frozen model in large batches, better utilizing serving resources.

Improvement with Scale
When evaluated on SuperGLUE and using a frozen T5 model, prompt tuning significantly outperforms prompt design using either GPT-3 or T5. Furthermore, as model size increases, prompt tuning catches up to the performance level of model tuning. Intuitively, the larger the pre-trained model, the less of a “push” it needs to perform a specific task, and the more capable it is of being adapted in a parameter-efficient way.

As scale increases, prompt tuning matches model tuning, despite tuning 25,000 times fewer parameters.

The effectiveness of prompt tuning at large model scales is especially important, since serving separate copies of a large model can incur significant computational overhead. In our paper, we demonstrate that larger models can be conditioned successfully even with soft prompts as short as 5 tokens. For T5 XXL, this means tuning just 20 thousand parameters to guide the behavior of an 11 billion parameter model.

Resilience to Domain Shift
Another advantage of prompt tuning is its resilience to domain shift. Since model tuning touches every weight in the network, it has the capacity to easily overfit on the provided fine-tuning data and may not generalize well to variations in the task at inference time. By comparison, our learned soft prompts have a small number of parameters, so the solutions they represent may be more generalizable.

To test generalizability, we train prompt tuning and model tuning solutions on one task, and evaluate zero-shot on a closely related task. For example, when we train on the Quora Question Pairs task (i.e., detecting if two questions are duplicates) and evaluate on MRPC (i.e., detecting if two sentences from news articles are paraphrases), prompt tuning achieves +3.2 points higher accuracy than model tuning.

Train    Eval    Tuning    Accuracy    F1
QQP    MRPC    Model    73.1 ±0.9    81.2 ±2.1
Prompt    76.3 ±0.1    84.3 ±0.3
MRPC    QQP    Model    74.9 ±1.3    70.9 ±1.2
Prompt    75.4 ±0.8    69.7 ±0.3   
On zero-shot domain transfer between two paraphrase detection tasks, prompt tuning matches or outperforms model tuning, depending on the direction of transfer.

Looking Forward
Prompt-based learning is an exciting new area that is quickly evolving. While several similar methods have been proposed — such as Prefix Tuning, WARP, and P-Tuningwe discuss their pros and cons and demonstrate that prompt tuning is the simplest and the most parameter efficient method.

In addition to the Prompt Tuning codebase, we’ve also released our LM-adapted T5 checkpoints, which we found to be better-suited for prompt tuning compared to the original T5. This codebase was used for the prompt tuning experiments in FLAN, and the checkpoints were used as a starting point for training the BigScience T0 model. We hope that the research community continues to leverage and extend prompt tuning in future research.

This project was a collaboration between Brian Lester, Rami Al-Rfou and Noah Constant. We are grateful to the following people for feedback, discussion and assistance: Waleed Ammar, Lucas Dixon, Slav Petrov, Colin Raffel, Adam Roberts, Sebastian Ruder, Noam Shazeer, Tu Vu and Linting Xue.

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