GACL: Exemplar-Free Generalized Analytic Continual Learning

Class incremental learning (CIL) trains a network on sequential tasks with separated categories in each task but suffers from catastrophic forgetting, where models quickly lose previously learned knowledge when acquiring new tasks. The generalized CIL (GCIL) aims to address the CIL problem in a more real-world scenario, where incoming data have mixed data categories and unknown sample size distribution. Existing attempts for the GCIL either have poor performance or invade data privacy by saving exemplars. In this paper, we propose a new exemplar-free GCIL technique named generalized analytic continual learning (GACL). The GACL adopts analytic learning (a gradient-free training technique) and delivers an analytical (i.e., closed-form) solution to the GCIL scenario. This solution is derived via decomposing the incoming data into exposed and unexposed classes, thereby attaining a weight-invariant property, a rare yet valuable property supporting an equivalence between incremental learning and its joint training. Such an equivalence is crucial in GCIL settings as data distributions among different tasks no longer pose challenges to adopting our GACL. Theoretically, this equivalence property is validated through matrix analysis tools. Empirically, we conduct extensive experiments where, compared with existing GCIL methods, our GACL exhibits a consistently leading performance across various datasets and GCIL settings. Source code is available at https://github.com/CHEN-YIZHU/GACL.

Paper

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Peer review

Reviewer tsV28/10 · confidence 5/52024-07-03

Summary

This paper proposes a new exemplar-free GCIL technique named generalized analytic continual learning (GACL). It adopts analytic learning (a gradient-free training technique), and delivers an analytical (i.e., closed-form) solution to the GCIL scenario, which is derived via decomposing the incoming data into exposed and unexposed classes. The GACL attains a weight-invariant property, supporting an equivalence between the incremental learning and its joint training. This method is both therectically and empircally validated.

Strengths

1. This is very interesting technique that obtains an equivalence between the GCIL and the joint training. Although a pre-trained network is needed, the weight-invariant property is very valuable in this area. 2. Another key contribution of GACL is that it is both accurate and exemplar-free, and this is powerful and significant. 3. The approach is also well-motivated and clearly explained. 4. The paper provides a comprehensive literature review, effectively contextualizing its contribution and facilitating the reader's understanding. 5. It is very clear theoretically regarding how the algorithm is parted into W_unexposed and W_ECLG.

Weaknesses

1. Could you explain why the avg accuracy on CIFAR-100 is much lower than the last accuracy? 2. You need to explain more regarding "weight-invariance" in Theorem 3.1. It is not very easy to follow for those who are not in this area of research. 3. In this paper, the authors assume that the pre-trained backbone is generalizable so that the model can be frozen throughout the learning process. However, there are also some cases where the pre-trained model cannot generalize to downstream tasks. It would also be interesting to see the scenario where the pre-trained backbone yields a large domain gap.

Questions

1. Continual learning has many variations (e.g., domain/task/class -incremental learning). Since the experiments only focus on generalized class-incremental learning, it would be better to adjust the title accordingly. 2. Since generalized CIL is also a specific case of class-incremental learning, it would be better to discuss why the proposed method is specially designed for GCIL. For example, when the data stream only contains new classes, will GCIL still work? 3. As the authors discussed in the main paper, the proposed method is a super case of ACIL and other analytical methods. As many readers would like to know, the authors are suggested to compare this method to other analytical methods to show how this method stands against other analytical works.

Rating

8

Confidence

5

Soundness

4

Presentation

3

Contribution

4

Limitations

Yes

Reviewer tsV22024-08-11

I thank the authors for providing the rebuttal. After reading the rebuttal and other reviewers' comments, all my previous concerns have been adequately addressed. I will keep my positive rating.

Authorsrebuttal2024-08-12

Thank you for the response

Thank you for taking the time to read our response! We are glad that our response addressed your concerns!

Reviewer qU327/10 · confidence 4/52024-07-09

Summary

Class Incremental Learning (CIL) faces the problem of catastrophic forgetting when training a network, i.e., the model loses previous knowledge when learning a new task. Generalized CIL (GCIL) aims to address more realistic scenarios, but existing methods are either ineffective or violate data privacy. This paper propose a new exemplar-free GCIL technique called Generalized Analytic Continual Learning (GACL), which provides a closed-form solution through analytic learning. GACL achieves equivalence between incremental learning and co-training by disaggregating the input data to keep the weights constant. This approach is theoretically validated and performs well across multiple datasets and settings.

Strengths

* The paper is well-written and easy to follow. * The proposed method outperforms other methods on several GCIL benchmarks. * This paper provides a detailed proof of the theorem.

Weaknesses

* The experimental setup is not clear. Given that the most recent method MVP [1] compared in this paper is the GCIL method for online CIL, were the experiments in this paper also conducted in an online scenario? * The technical improvements on the previous work of ACL are somewhat incremental. [1] Jun-Yeong Moon, Keon-Hee Park, Jung Uk Kim, and Gyeong-Moon Park. Online class incremental learning on stochastic blurry task boundary via mask and visual prompt tuning. ICCV 2023.

Questions

1. What was the motivation for introducing ACL into GCIL? The claims in this paper seem to be an extended experiment of ACL in the GCIL scenario. It is difficult to generate further inspiration to the reader for solving the GCIL problem. 2. The proposed method utilize the DeiT-S/16 as backbone but previous works typically use ViT-B/16. Regarding the comparison methodology, what backbone network was used to obtain the reported results? 3. What are the technical improvements and contributions of this paper over previous ACL approaches such as ACIL [2] and DS-AL [3]? [2] Huiping Zhuang, Zhenyu Weng, Hongxin Wei, RENCHUNZI XIE, Kar-Ann Toh, and Zhiping Lin. ACIL: Analytic class-incremental learning with absolute memorization and privacy protection. NeurIPS 2022 [3] Huiping Zhuang, Run He, Kai Tong, Ziqian Zeng, Cen Chen, and Zhiping Lin. DS-AL: A dual-stream analytic learning for exemplar-free class-incremental learning. AAAI 2024.

Rating

7

Confidence

4

Soundness

3

Presentation

3

Contribution

2

Limitations

It is hard to foresee any potential negative societal impact of this theoretical work.

Reviewer qU322024-08-09

Official Comment by Reviewer

Thanks to the authors for the response. This rebuttal addresses my concerns well. It fully explains the motivation for introducing ACL to address GCIL and the improvements this paper makes to existing ACL methods in response to the GCIL problem. Taking into account the comments of other reviewers and the authors' rebuttal, I decide to increase my rating.

Authorsrebuttal2024-08-09

Thank you for the response

Thank you for taking the time to read our response and increasing your score! We are glad to hear that the response addressed your concern.

Reviewer Xb3R6/10 · confidence 4/52024-07-11

Summary

This paper proposes a new exempler-free generalized continual learning (GCIL), named generalized analytic continual learning (GACL) technique. It does not depend on gradient-based tranining, which avoids the task-recency bias leading to the forgetting issue. It also delivers an closed-form solution to the GCIL scenario that provides identical solutions to its joint training. Entensive experiments demonstrate that the proposed GACL achieves consistently leading performance.

Strengths

- The paper proposes very effective method that completely avoids forgetting in the gradient-free training technique. - Extensive experiments are included explaining that GACL works good overall in the three datasets compared with many studies. - Ablation studies are coducted that clarifies the differences among datasets and the reason why GACL is slightly worse than other baselines at early tasks. They also analyzed the contributions of ECLG module and its robustness. - The paper is well written and easy to read.

Weaknesses

- Missing baselines. As described in the Section 2, many ACL methods have been proposed. Comparing GACL with those methods (especially RanPAC) derives more solid results. - Comparison to the joint-traning is not included. Although it is argued that GACL can bridge the gap between continual learning and joint-training, such result is not provided in the experiments.

Questions

- I believe that the proposed gradient-free training scheme is beneficial also in the perspective of training speed. Does GACL run more fast than other (ACL) methods?

Rating

6

Confidence

4

Soundness

3

Presentation

4

Contribution

2

Limitations

- As described, GACL cannot update backbone weights, which can be a problem. However, as new backbones emerge frequently, showing that GACL can work well not only with the used DeiT-S/16 but also others makes the arguments more convincing, instead.

Reviewer vKCe5/10 · confidence 4/52024-07-12

Summary

This paper deals with the generalized CIL (GCIL) problem where incoming data have mixed data categories and unknown sample size distribution. The author proposes generalized analytic continual learning (GACL) which adopts a pre-trained and fixed backbone and uses least squares to get a closed-form solution. Experiments verified that the GACL achieves better performance compared to other baselines.

Strengths

* The generalized class incremental learning is an important and valuable problem. * This paper is generally clear and easy to follow. * Experiments show that GACL is effective in addressing forgetting in GCIL.

Weaknesses

* Main concern. Most of the content on page 4 and page 5 (e.g., Theorem 3.1) in Section 3 is overly similar to existing ACL works [7, 8]. The main difference claimed by the authors, i.e., the ECLG module that corresponds to the exposed-class gain, is a trivial extension of the original ACL [7]. * Experiments. This paper adopts the pre-trained backbone developed in [40, 41]. However, many baselines (Table 2 in [40]) are missing in experiments. The GACL method is similar to RanPAC [25]. Experiments are needed to compare the performance of GACL with RanPAC. * Comparison of the memory cost of weights in the buffer layer and other replay-based method.

Questions

* How and why the dimensionality of the random projection affects the performance.

Rating

5

Confidence

4

Soundness

3

Presentation

3

Contribution

2

Limitations

Yes.

Authorsrebuttal2024-08-12

More updates

Dear Reviewer, We would like to provde our updates during the discussion period, in which we have still been working on the response. 1) We have managed to produce the results in comparison with RanPAC as follows. Overall, the RanPAC is a very strong-performing counterpart with average and final results comparable to ours. However, it is **NOT designed for GCIL/online tasks**, and hence **gives limited performance in ACC_AUC** (area under the curve of accuracy), which measures the online performance. For instance, on CIFAR-100, the GACL obtains 66.79% while RanPAC only has 51.62%. | | Buffer | CIFAR100 | | | ImageNet-R | | | Tiny-ImageNet | | | |:----------------------:|:------:|:--------:|:-------:|:-------:|:-------------:|:-------:|:-------:|:-------------:|:--------:|:-------:| | | | ACC_AUC | ACC_AVG | ACC_LAST |ACC_AUC | ACC_AVG | ACC_LAST |ACC_AUC |ACC_AVG | ACC_LAST | | RanPAC (NeurIPS,2023) | 0 | 51.62 | 63.45 | 77.83 | 42.39 | 61.68 | 57.77 | 62.80 | 82.80 | 78.54 | | GACL | 0 | 66.79 | 63.94 | 77.34 |57.02 |62.26 |57.68 |77.65 |82.95 |77.80 | 2) Regrarding your concern of novelty we have clarified this in "**Response to W_1**" in our rebuttal. Reviewer qU32 also pointed out this issue, and **has aknowledged our clarification**. Could you have a look and let us know if more is required to clarify this?

Reviewer vKCe2024-08-13

I appreciated the author's responses and have raised my score to 5.

Authorsrebuttal2024-08-14

Thank you for the response

Thank you for taking the time to read our response! We are glad that our response addressed your concerns.

Authorsrebuttal2024-08-12

Comparison update

Dear Reviewer, We would like to provde our updates during the discussion period, in which we have still been working on the response regarding the comparion with the RanPAC. We have managed to produce the results in comparison with RanPAC as follows. Overall, the RanPAC is a very strong-performing counterpart with average and final results comparable to ours. However, it is **NOT designed for GCIL/online tasks**, and hence **gives limited performance in ACC_AUC** (area under the curve of accuracy), which measures the online performance. For instance, on CIFAR-100, the GACL obtains 66.79% while RanPAC only has 51.62%. | | Buffer | CIFAR100 | | | ImageNet-R | | | Tiny-ImageNet | | | |:----------------------:|:------:|:--------:|:-------:|:-------:|:-------------:|:-------:|:-------:|:-------------:|:--------:|:-------:| | | | ACC_AUC | ACC_AVG | ACC_LAST |ACC_AUC | ACC_AVG | ACC_LAST |ACC_AUC |ACC_AVG | ACC_LAST | | RanPAC (NeurIPS,2023) | 0 | 51.62 | 63.45 | 77.83 | 42.39 | 61.68 | 57.77 | 62.80 | 82.80 | 78.54 | | GACL | 0 | 66.79 | 63.94 | 77.34 |57.02 |62.26 |57.68 |77.65 |82.95 |77.80 | Let us know if more clarification is required!

Program Chairsdecision2024-09-25

Decision

Accept (poster)

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