MPNet: Masked and Permuted Pre-training for Language Understanding
Written for the Research library (claude-opus-5-5) from the paper text, in our own words: the license "arXiv — non-exclusive licence to distribute" does not allow redistributing the text itself. Original — arxiv.org/pdf/2004.09297
Summary
BERT and XLNet each use a pre-training objective with its own weakness. BERT's masked language modeling (MLM) predicts each masked token independently of the others. XLNet's permuted language modeling (PLM) captures dependencies between predicted tokens, but it hides the positions of tokens that come later. As a result, the model sees an incomplete sentence during pre-training, which does not match what it gets at fine-tuning. The authors show that both objectives can be described in one framework: a permuted sequence is split into a part used as context and a part to be predicted. Building on this, MPNet predicts tokens one after another in permuted order, as PLM does. It also adds mask symbols that carry the positions of all predicted tokens (called "position compensation"), so the model always knows the full sentence layout. This is implemented with two-stream self-attention. MPNet was pre-trained on over 160GB of text and fine-tuned on GLUE, SQuAD, RACE and IMDB. With the same base model size, it beats BERT, XLNet and RoBERTa. The work matters because it combines the strengths of both objectives in one model, and the model is publicly released.
Key points
- On GLUE dev sets (BERT-BASE setting), MPNet scores 4.8, 3.4 and 1.5 points higher than BERT, XLNet and RoBERTa respectively.
- On RACE, MPNet pre-trained on only Wikipedia + BooksCorpus (16GB) reaches 70.4 accuracy, compared with 65.0 for BERT and 66.8 for XLNet. With the full 160GB corpus it reaches 76.1. On IMDB, its error rate drops to 4.4 (BERT: 5.4, XLNet: 4.9).
- Ablation (16GB, 1M steps): removing position compensation turns MPNet into PLM and costs 0.6–2.3 points. Reducing it to plain MLM costs 0.5–3.7 points, measured on SQuAD v1.1/v2.0, MNLI and SST-2.
- MPNet also clearly beats BERT, XLNet and RoBERTa on SQuAD v1.1 and v2.0 (single model, no data augmentation).
- Published at NeurIPS 2020. The code and pre-trained models are available at github.com/microsoft/MPNet.
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Cite
@misc{song2020mpnet,
title = {{MPNet: Masked and Permuted Pre-training for Language Understanding}},
author = {Song, Kaitao and Tan, Xu and Qin, Tao and Lu, Jianfeng and Liu, Tie-Yan},
year = {2020},
doi = {10.48550/arxiv.2004.09297},
eprint = {2004.09297},
archivePrefix = {arXiv},
url = {https://doi.org/10.48550/arxiv.2004.09297}
}References
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