Prompt-Based Length Controlled Generation with Reinforcement Learning

Recently, large language models (LLMs) like ChatGPT and GPT-4 have attracted great attention given their surprising improvement and performance. Length controlled generation of LLMs emerges as an important topic, which also enables users to fully leverage the capability of LLMs in more real-world scenarios like generating a proper answer or essay of a desired length. In addition, the autoregressive generation in LLMs is extremely time-consuming, while the ability of controlling this generated length can arbitrarily reduce the inference cost by limiting the length, and thus satisfy different needs. Therefore, we aim to propose a prompt-based length control method to achieve this length controlled generation, which can also be widely applied in GPT-style LLMs. In particular, we adopt reinforcement learning with the reward signal given by either trainable or rule-based reward model, which further affects the generation of LLMs via rewarding a pre-defined target length. Experiments show that our method significantly improves the accuracy of prompt-based length control for summarization task on popular datasets like CNNDM and NYT. We believe this length-controllable ability can provide more potentials towards the era of LLMs.