 Paper Abstract:  Pretraining has been shown to scale well with compute, data size and data diversity. Multitask learning trains on a  mixture of supervised datasets and produces improved performance compared to self-supervised pretraining. Until now,  massively multitask learning required simultaneous access to all datasets in the mixture and heavy compute resources  that are only available to well-resourced teams.  In this paper, we propose ColD Fusion, a method that provides the benefits of multitask learning but leverages distributed  computation and requires limited communication and no sharing of data. Consequentially, ColD Fusion can create a synergistic  loop, where finetuned models can be recycled to continually improve the pretrained model they are based on. We show that  ColD Fusion yields comparable benefits to multitask pretraining by producing a model that (a) attains strong performance on  all of the datasets it was multitask trained on and (b) is a better starting point for finetuning on unseen datasets. We find  ColD Fusion outperforms RoBERTa and even previous multitask models. Specifically, when training and testing on 35 diverse datasets,  ColD Fusion-based model outperforms RoBERTa by 2.45 points in average without any changes to the architecture.   