The growing size of neural language models has led to increased attention in model compression.
The two predominant approaches arepruning, which gradually removes weights from a pre-trained model, anddistillation, which trains a smaller compact model to match a larger one.
Pruning methods can significantly reduce the model size but hardly achieve large speedups as distillation. However, distillation methods require large amounts of unlabeled data and are expensive to train.
In this work, we propose a task-specific structured pruning method CoFi111CoFi is pronounced as.(Coarse- andFine-grained Pruning) , which delivers highly parallelizable subnetworks and matches the distillation methods in both accuracy and latency, without resorting to any unlabeled data. Our key insight is to jointly prune coarse-grained (e.g., layers)  and fine-grained (e.g., heads and hidden units)  modules, which controls the pruning decision of each parameter with masks of different granularity. We also devise a layerwise distillation strategy to transfer knowledge from unpruned to pruned models during optimization. Our experiments on GLUE and SQuAD datasets show that CoFi yields models with over 10speedups with a small accuracy drop, showing its effectiveness and efficiency compared to previous pruning and distillation approaches.222Our code and models are publicly available athttps://github.com/princeton-nlp/CoFiPruning.