AdapterFusion: Non-Destructive Task Composition for Transfer Learning
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/2005.00247
Summary
Two standard ways of transferring knowledge from several NLU tasks into one pretrained transformer both have drawbacks. Sequential fine-tuning tends to erase what was learned earlier (catastrophic forgetting). Multi-task learning needs every dataset available at once, requires full retraining whenever a task is added, and makes it hard to balance large and small tasks. The authors propose AdapterFusion, which splits transfer into two phases. In the knowledge-extraction phase, a small adapter module is trained for each task while the pretrained weights stay frozen. Adapters can be trained independently (ST-A) or jointly (MT-A). In the knowledge-composition phase, a new Fusion layer inside every transformer layer learns how to weight and mix the outputs of all stored adapters for a chosen target task, without modifying them. On 16 varied NLU tasks, including sentiment, commonsense reasoning, paraphrase detection and entailment, this setup outperforms full fine-tuning and both single-task and multi-task adapter baselines. The method matters because cheap, separately trained adapters can be reused without destructive interference. Code and adapters are released via AdapterHub.ml.
Key points
- The method has two stages: adapters are trained first, then a Fusion layer learns to mix them. It works with both ST-A and MT-A. Composing ST-A is the more efficient option, since those adapters can be trained in parallel and reused.
- On the 16-task benchmark, AdapterFusion outperforms full fine-tuning, ST-A, and the multi-task adapter approach of Stickland and Murray (2019).
- An analysis of the Fusion weights shows two patterns:
- Low-resource tasks such as CB, RTE and MRPC lean heavily on adapters trained on large datasets like MNLI and QQP.
- Tasks that gain nothing (e.g., HellaSwag, SciTail, QQP) mostly attend to their own adapter, which acts as an implicit no-op option.
- Fusion weights in layer 12 are spread across many tasks more than in earlier layers. The likely reason is that the last adapters behave more like an extension of the prediction head.
- Adapters are lightweight: in the Houlsby et al. design they add about 3.6% of the pretrained model's parameters. Follow-up work (Rücklé et al., 2020a) reports two further findings:
- Fusion cost grows linearly with the number of adapters.
- Pruning rarely used adapters barely hurts accuracy while speeding up inference.
Cited by
- Bias and Fairness in Large Language Models: A Survey
- Parameter-Efficient Transfer Learning with Diff Pruning
- Parameter-Efficient Fine-Tuning for Large Models: A Comprehensive Survey
- DADA: Dialect Adaptation via Dynamic Aggregation of Linguistic Rules
- MAD-X: An Adapter-Based Framework for Multi-Task Cross-Lingual Transfer
- Lifting the Curse of Multilinguality by Pre-training Modular Transformers
- AdapterDrop: On the Efficiency of Adapters in Transformers
- How Good is Your Tokenizer? On the Monolingual Performance of Multilingual Language Models
- AuthorMix: Modular Authorship Style Transfer via Layer-wise Adapter Mixing
- AdaLoRA: Adaptive Budget Allocation for Parameter-Efficient Fine-Tuning
Cite
@misc{pfeiffer2020adapterfusion,
title = {{AdapterFusion: Non-Destructive Task Composition for Transfer Learning}},
author = {Pfeiffer, Jonas and Kamath, Aishwarya and Rücklé, Andreas and Cho, Kyunghyun and Gurevych, Iryna},
year = {2020},
doi = {10.48550/arxiv.2005.00247},
eprint = {2005.00247},
archivePrefix = {arXiv},
url = {https://doi.org/10.48550/arxiv.2005.00247}
}References
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