Paper:
We present BlockBERT, a lightweight and efficient BERT model that is designed to better modeling long-distance dependencies. Our model extends BERT by introducing sparse block structures into the attention matrix to reduce both memory consumption and training time, which also enables attention heads to capture either short-or long-range contextual information. We conduct experiments on several benchmark question answering datasets with various paragraph lengths. Results show that BlockBERT uses 18.7-36.1% less memory and reduces the training time by 12.0-25.1%, while having comparable and sometimes better prediction accuracy, compared to an advanced BERT-based model, RoBERTa.

Review:
The authors propose BlockBERT, a model that makes the attention matrix of Transformer models sparse by introducing block structure. This has the dual benefit of reducing memory and reducing training time. The authors show on various question answering tasks that their model is competitive with RoBERTa. 
1. Can the authors add additional details about the training of their model in Section 4.1? It is not clear for example the vocabulary size - is that the RoBERTa vocabulary size of around 50k or the BERT vocabulary size that is smaller? I believe this will affect the memory and training speed. 
2. For the datasets such as TriviaQA and SearchQA, how is RoBERTa finetuned on these tasks? By doing the window approach? 
3. The authors compare to RoBERTa and Sparse BERT as baselines for the section on performance. However, can the authors also include metrics on training time and memory in Table 2 for Sparse BERT as well as other sparse attention transformer architectures proposed (for example the Correia paper or the Sukhbaatar paper)? It is not clear the savings from this architecture compared to sparse Transformers in general. 
4. The analysis in Section 4.3 is quite unclear due to how compressed the description is and how tiny the graphs are. 
5. The authors mention that attention heads can be sparsified due to the memory usage and quadratic time complexity. Other work has also shown that the attention heads are quite redundant and can be pruned away, so attention head dropout is effective. For example https://arxiv.org/abs/1905.09418