Nearest Neighbor Machine Translation is Meta-Optimizer on Output Projection Layer

Ruize Gao, Zhirui Zhang, Yichao Du, Lemao Liu, Rui Wang


Abstract
Nearest Neighbor Machine Translation (kNN-MT) has achieved great success in domain adaptation tasks by integrating pre-trained Neural Machine Translation (NMT) models with domain-specific token-level retrieval. However, the reasons underlying its success have not been thoroughly investigated. In this paper, we comprehensively analyze kNN-MT through theoretical and empirical studies. Initially, we provide new insights into the working mechanism of kNN-MT as an efficient technique to implicitly execute gradient descent on the output projection layer of NMT, indicating that it is a specific case of model fine-tuning. Subsequently, we conduct multi-domain experiments and word-level analysis to examine the differences in performance between kNN-MT and entire-model fine-tuning. Our findings suggest that: (i) Incorporating kNN-MT with adapters yields comparable translation performance to fine-tuning on in-domain test sets, while achieving better performance on out-of-domain test sets; (ii) Fine-tuning significantly outperforms kNN-MT on the recall of in-domain low-frequency words, but this gap could be bridged by optimizing the context representations with additional adapter layers.
Anthology ID:
2023.emnlp-main.964
Volume:
Proceedings of the 2023 Conference on Empirical Methods in Natural Language Processing
Month:
December
Year:
2023
Address:
Singapore
Editors:
Houda Bouamor, Juan Pino, Kalika Bali
Venue:
EMNLP
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
15592–15608
Language:
URL:
https://aclanthology.org/2023.emnlp-main.964
DOI:
10.18653/v1/2023.emnlp-main.964
Bibkey:
Cite (ACL):
Ruize Gao, Zhirui Zhang, Yichao Du, Lemao Liu, and Rui Wang. 2023. Nearest Neighbor Machine Translation is Meta-Optimizer on Output Projection Layer. In Proceedings of the 2023 Conference on Empirical Methods in Natural Language Processing, pages 15592–15608, Singapore. Association for Computational Linguistics.
Cite (Informal):
Nearest Neighbor Machine Translation is Meta-Optimizer on Output Projection Layer (Gao et al., EMNLP 2023)
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PDF:
https://aclanthology.org/2023.emnlp-main.964.pdf
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