Pretrained language models (PTLMs) are typically learned over a large, static corpus and further fine-tuned for various downstream tasks. However, when deployed in the real world, a PTLM-based model must deal with data from a new domain that deviates from what the PTLM was initially trained on, or newly emerged data that contains out-of-distribution information. In this paper, we study a lifelong language model pretraining challenge where a PTLM is continually updated so as to adapt to emerging data. Over a domain-incremental research paper stream and a chronologically ordered tweet stream, we incrementally pretrain a PTLM with different continual learning algorithms, and keep track of the downstream task performance (after fine-tuning) to analyze its ability of acquiring new knowledge and preserving learned knowledge. Our experiments show continual learning algorithms improve knowledge preservation, with logit distillation being the most effective approach. We further show that continual pretraining improves generalization when training and testing data of downstream tasks are drawn from different time steps, but do not improve when they are from the same time steps. We believe our problem formulation, methods, and analysis will inspire future studies towards continual pretraining of language models.

本研究通过使用不同的持续学习算法对预先训练的语言模型进行不断的增量预训练，并通过评估模型对新数据的适应能力以及对早期数据所学知识的保留能力来研究生命周期语言模型预训练挑战，结果表明采用基于蒸馏的方法可以最有效地保留早期领域的下游任务性能。这些算法还可以提高知识转移能力，使模型在最新数据上实现更好的下游性能，并在由于时间而存在训练和评估之间的分布差异时，提高时态的泛化能力。

终身预训练：持续适应新出现的语料库的语言模型