%0 Conference Proceedings %T On Attention Redundancy: A Comprehensive Study %A Bian, Yuchen %A Huang, Jiaji %A Cai, Xingyu %A Yuan, Jiahong %A Church, Kenneth %Y Toutanova, Kristina %Y Rumshisky, Anna %Y Zettlemoyer, Luke %Y Hakkani-Tur, Dilek %Y Beltagy, Iz %Y Bethard, Steven %Y Cotterell, Ryan %Y Chakraborty, Tanmoy %Y Zhou, Yichao %S Proceedings of the 2021 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies %D 2021 %8 June %I Association for Computational Linguistics %C Online %F bian-etal-2021-attention %X Multi-layer multi-head self-attention mechanism is widely applied in modern neural language models. Attention redundancy has been observed among attention heads but has not been deeply studied in the literature. Using BERT-base model as an example, this paper provides a comprehensive study on attention redundancy which is helpful for model interpretation and model compression. We analyze the attention redundancy with Five-Ws and How. (What) We define and focus the study on redundancy matrices generated from pre-trained and fine-tuned BERT-base model for GLUE datasets. (How) We use both token-based and sentence-based distance functions to measure the redundancy. (Where) Clear and similar redundancy patterns (cluster structure) are observed among attention heads. (When) Redundancy patterns are similar in both pre-training and fine-tuning phases. (Who) We discover that redundancy patterns are task-agnostic. Similar redundancy patterns even exist for randomly generated token sequences. (“Why”) We also evaluate influences of the pre-training dropout ratios on attention redundancy. Based on the phase-independent and task-agnostic attention redundancy patterns, we propose a simple zero-shot pruning method as a case study. Experiments on fine-tuning GLUE tasks verify its effectiveness. The comprehensive analyses on attention redundancy make model understanding and zero-shot model pruning promising. %R 10.18653/v1/2021.naacl-main.72 %U https://aclanthology.org/2021.naacl-main.72 %U https://doi.org/10.18653/v1/2021.naacl-main.72 %P 930-945