Adversarial Training Versus Weight Decay
Adversarial Training Versus Weight Decay
Angus Galloway and Thomas Tanay and Graham W. Taylor
2018

Paper summary
davidstutz
Galloway et al. provide a theoretical and experimental discussion of adversarial training and weight decay with respect to robustness as well as generalization. In the following I want to try and highlight the most important findings based on their discussion of linear logistic regression. Considering the softplus loss $\mathcal{L}(z) = \log(1 + e^{-z})$, the learning problem takes the form:
$\min_w \mathbb{E}_{x,y \sim p_{data}} [\mathcal{L}(y(w^Tx + b)]$
where $y \in \{-1,1\}$. This optimization problem is also illustrated in Figure 1 (top). Now considering $L_2$ weight decay can also be seen to be equivalent to scaling the softplus loss. In particular, Galloway et al. Argue that $w^Tx + b = \|w\|_2 d(x)$ where $d(x)$ is the (signed) Euclidean distance to the decision boundary. (This follows directly from the fact that $d(x) = \frac{w^Tx +b}{\|w\|w_2}$.) Then, the problem can be rewritten as
$\min_w \mathbb{E}_{x,y \sim p_{data}} [\mathcal{L}(yd(x) \|w\|_2)]$
This can be understood as a scaled version of the softplus loss; adding a $L_2$ weight decay term basically controls the level of scaling. This is illustrated in Figure 1 (middle) for different levels of scaling. Finally, adversarial training means training on the worst-case example for a given $\epsilon$. In practice, for the linear logistic regression model, this results in training on $x - \epsilon y \frac{w}{\|w\|_2}$ - which can easily be understood when considering that the attacker can cause the most disturbance when changing the samples in the direction of $-w$ for label $1$. Then,
$y (w^T(x - \epsilon y \frac{w}{\|w\|_2}) + b) = y(w^Tx + b) - \epsilon \|w\|_2 = \|w\|_2 (yd(x) - \epsilon)$,
which results in a shift of the data by $\epsilon$ - as illustrated in Figure 1 (bottom). Overall, show that weight decay acts as scaling the objective and adversarial training acts as shifting the data (or equivalently the objective).
In the non-linear case, decaying weights is argued to be equivalent to decaying the logits. Effectively, this results in a temperature parameter for the softmax function resulting in smoother probability distributions. Similarly, adversarial training (in a first-order approximation) can be understood as effectively reducing the probability attributed to the correct class. Here, again, weight decay results in a scaling effect and adversarial training in a shifting effect. In conclusion, adversarial training is argued to be only effective with small perturbation sizes (i.e., if the shift is not too large), weil weight decay is also beneficial for generalization. However, from reading the paper, it is unclear what the actual recommendation on both methods is.
In the experimental section, the authors focus on two models, a wide residual network and a very constrained 4-layer convolutional neural network. Here, their discussion shifts slightly to the complexity of the employed model. While not stated very explicitly, one of the take-aways is that the simpler model might be more robust, especially for fooling images.
https://i.imgur.com/FKT3a2O.png
https://i.imgur.com/wWwFKqn.png
https://i.imgur.com/oaTfqHJ.png
Figure 1: Illustration of the linear logistic regression argument. Top: illustration of linear logistic regression where $\xi$ is the loss $\mathcal{L}$, middle: illustration of the impact of weight decay/scaling, bottom: illustration of the impact of shift for adversarial training.
Also find this summary at [davidstutz.de](https://davidstutz.de/category/reading/).
Adversarial Training Versus Weight Decay

Angus Galloway and Thomas Tanay and Graham W. Taylor

arXiv e-Print archive - 2018 via Local arXiv

Keywords: cs.LG, stat.ML

**First published:** 2018/04/10 (1 year ago)

**Abstract:** Performance-critical machine learning models should be robust to input
perturbations not seen during training. Adversarial training is a method for
improving a model's robustness to some perturbations by including them in the
training process, but this tends to exacerbate other vulnerabilities of the
model. The adversarial training framework has the effect of translating the
data with respect to the cost function, while weight decay has a scaling
effect. Although weight decay could be considered a crude regularization
technique, it appears superior to adversarial training as it remains stable
over a broader range of regimes and reduces all generalization errors. Equipped
with these abstractions, we provide key baseline results and methodology for
characterizing robustness. The two approaches can be combined to yield one
small model that demonstrates good robustness to several white-box attacks
associated with different metrics.
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Angus Galloway and Thomas Tanay and Graham W. Taylor

arXiv e-Print archive - 2018 via Local arXiv

Keywords: cs.LG, stat.ML

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