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Title: Boosting Ticket: Towards Practical Pruning for Adversarial Training with Lottery Ticket Hypothesis

Paper content:
Recent research has proposed the lottery ticket hypothesis, suggesting that for a deep neural network, there exist trainable sub-networks performing equally or better than the original model with commensurate training steps.
While this discovery is insightful, finding proper sub-networks requires iterative training and pruning.
The high cost incurred limits the applications of the lottery ticket hypothesis.
We show there exists a subset of the aforementioned sub-networks that converge significantly faster during the training process and thus can mitigate the cost issue.
We conduct extensive experiments to show such sub-networks consistently exist across various model structures for a restrictive setting of hyperparameters (e.g., carefully selected learning rate, pruning ratio, and model capacity).
As a practical application of our findings, we demonstrate that such sub-networks can help in cutting down the total time of adversarial training, a standard approach to improve robustness, by up to 49% on CIFAR-10 to achieve the state-of-the-art robustness.
Pruning has served as an important technique for removing redundant structure in neural networks (Han et al., 2015b; a; Li et al., 2016; He et al., 2017) .
Properly pruning can reduce cost in computation and storage without harming performance.
However, pruning was until recently only used as a post-processing procedure, while pruning at initialization was believed ineffective (Han et al., 2015a; Li et al., 2016) .
Recently, proposed the lottery ticket hypothesis, showing that for a deep neural network there exist sub-networks, when trained from certain initialization obtained by pruning, performing equally or better than the original model with commensurate convergence rates.
Such pairs of sub-networks and initialization are called winning tickets.
This phenomenon indicates it is possible to perform pruning at initialization.
However, finding winning tickets still requires iterative pruning and excessive training.
Its high cost limits the application of winning tickets.
Although shows that winning tickets converge faster than the corresponding full models, it is only observed on small networks, such as a convolutional neural network (CNN) with only a few convolution layers.
In this paper, we show that for a variety of model architectures, there consistently exist such sub-networks that converge significantly faster when trained from certain initialization after pruning.
We call these boosting tickets.
We observe the standard technique introduced in for identifying winning tickets does not always find boosting tickets.
In fact, the requirements are more restrictive.
We extensively investigate underlining factors that affect such boosting effect, considering three stateof-the-art large model architectures: VGG-16 (Simonyan & Zisserman, 2014) , ResNet-18 (He et al., 2016) , and WideResNet (Zagoruyko & Komodakis, 2016) .
We conclude that the boosting effect depends principally on three factors:
(i) learning rate,
(ii) pruning ratio, and
(iii) network capacity; we also demonstrate how these factors affect the boosting effect.
By controlling these factors, after only one training epoch on CIFAR-10, we are able to obtain 90.88%/90.28% validation/test accuracy (regularly requires >30 training epochs) on WideResNet-34-10 when 80% parameters are pruned.
We further show that the boosting tickets have a practical application in accelerating adversarial training, an effective but expensive defensive training method for obtaining robust models against adversarial examples.
Adversarial examples are carefully perturbed inputs that are indistinguishable from natural inputs but can easily fool a classifier (Szegedy et al., 2013; Goodfellow et al., 2015) .
We first show our observations on winning and boosting tickets extend to the adversarial training scheme.
Furthermore, we observe that the boosting tickets pruned from a weakly robust model can be used to accelerate the adversarial training process for obtaining a strongly robust model.
On CIFAR-10 trained with WideResNet-34-10, we manage to save up to 49% of the total training time (including both pruning and training) compared to the regular adversarial training process.
Our contributions are summarized as follows:
1. We demonstrate that there exists boosting tickets, a special type of winning tickets that significantly accelerate the training process while still maintaining high accuracy.
2. We conduct extensive experiments to investigate the major factors affecting the performance of boosting tickets.
3. We demonstrate that winning tickets and boosting tickets exist for adversarial training scheme as well.
4. We show that pruning a non-robust model allows us to find winning/boosting tickets for a strongly robust model, which enables accelerated adversarial training process.
2 BACKGROUND AND RELATED WORK
Not knowledge distillation.
It may seem that winning tickets and boosting tickets behave like knowledge distillation (Ba & Caruana, 2014; Hinton et al., 2015) where the learned knowledge from a large model is transferred to a small model.
This conjecture may explain the boosting effects as the pruned model quickly recover the knowledge from the full model.
However, the lottery ticket framework seems to be distinctive to knowledge distillation.
If boosting tickets simply transfer knowledge from the full model to the pruned model, then an FGSM-based adversarially trained model should not find tickets that improves the robustness of the sub-model against PGD attacks, as the full model itself is vulnerable to PGD attacks.
Yet in Section 4.1 we observe an FGSM-based adversarially trained model still leads to boosting tickets that accelerates PGD-based adversarial training.
We believe the cause of boosting tickets requires further investigation in the future.
Accelerate adversarial training.
Recently, Shafahi et al. (2019) propose to reduce the training time for PGD-based adversarial training by recycling the gradients computed for parameter updates and constructing adversarial examples.
While their approach focuses on reducing the computational time for each epoch, our method focuses more on the convergence rate (i.e., reduce the number of epochs required for convergence).
Therefore, our approach is compatible with theirs, making it a promising future direction to combine both to further reduce the training time.
In this paper, we investigate boosting tickets, sub-networks coupled with certain initialization that can be trained with significantly faster convergence rate.
As a practical application, in the adversarial training scheme, we show pruning a weakly robust model allows to find boosting tickets that can save up to 49% of the total training time to obtain a strongly robust model that matches the state-ofthe-art robustness.
Finally, it is an interesting direction to investigate whether there is a way to find boosting tickets without training the full model beforehand, as it is technically not necessary.

assistant: text: We show the possibility of pruning to find a small sub-network with significantly higher convergence rate than the full model.