Paper:
Adversarial training, a method for learning robust deep networks, is typically assumed to be more expensive than traditional training due to the necessity of constructing adversarial examples via a first-order method like projected gradient decent (PGD). In this paper, we make the surprising discovery that it is possible to train empirically robust models using a much weaker and cheaper adversary, an approach that was previously believed to be ineffective, rendering the method no more costly than standard training in practice. Specifically, we show that adversarial training with the fast gradient sign method (FGSM), when combined with random initialization, is as effective as PGD-based training but has significantly lower cost. Furthermore we show that FGSM adversarial training can be further accelerated by using standard techniques for efficient training of deep networks, allowing us to learn a robust CIFAR10 classifier with 45% robust accuracy at = 8/255 in 6 minutes, and a robust ImageNet classifier with 43% robust accuracy at = 2/255 in 12 hours, in comparison to past work based on "free" adversarial training which took 10 and 50 hours to reach the same respective thresholds.

Review:
This paper revisits Random+FGSM method to train robust models against strong PGD evasion attacks. Coupled together with tricks for accelerating natural training, such as cyclic learning rate, mixed precision, the robust models can be trained faster than previous methods. 
+The experimental results are impressive. The trained model is robust (at Madry’s PGD level), and the total training procedure is fast (6 min for CIFAR-10 and 12 hr for ImageNet).
+ The method is simple, and I guess reproducible. 
+The paper shows surprising facts of a well-known method.
+The paper is generally well-written and easy to follow.
I do have some concerns of the work
- The paper is empirical and the techniques are combinations of previous methods. Even for the surprising fact that Random+FGSM, it has been discussed in several previous papers, for example,  ICLR 2019 Defensive Quantization: When Efficiency Meets Robustness  https://openreview.net/forum?id=ryetZ20ctX. So the main contribution of the paper is limited to show RFGSM works well when combined with optimization tricks like cyclic learning rate. 
-In previous methods claiming random+FGSM can train robust model, their method seems to be slightly different from Alg 3 in page 4 of this paper. The alg in this paper seems to be identical to Madry’s implementation of R-FGSM, which is shown not robust to PGD attacks. See discussions in https://openreview.net/forum?id=rJzIBfZAb and https://openreview.net/forum?id=ryetZ20ctX. I would like the authors to clarify their method to resolve such conflicts and make it clear how R-FGSM can be as robust as PGD as in table 1. 
-The first two paragraphs of section 4.1 seem to be inaccurate. One important trick in the “adversarial training for free” paper is to replay each minibatch m times. It is hard to say how much nonzero initialization helps. According to “universal adversarial training” (https://arxiv.org/pdf/1811.11304.pdf). It may help, but cannot compete with Madry’s PGD training when defending against PGD attacks. 
-I am not sure if using a larger norm 1.25 * \epsilon is a fair comparison. A baseline of PGD training bounded by 1.25 * \epsilon would help. 
-Could the authors combine table 4 and 5 for easy comparison of robust accuracy and training time? Did the authors try the optimization tricks on ImageNet for the baseline free adversarial training method?
================== after rebuttal =================
I change my rating to weak accept. I tend to accept for the following reason
(1) there seems to be no obvious flaw in the authors implementation. I quickly skimmed their code, and looks like a few researchers have tried their code and responded in public discussion. The surprising robustness of RFGSM, though the originality is questionable and the technical difference comparing to previous methods are subtle, seems to hold true. 
(2) The authors work hard to address the comments. 
I still have some concerns, mainly regarding the fairness of experimental comparison. 
(1) As pointed out in public discussion, the success of the proposed RFGSM relies on early stopping. I am wondering if early stopping also helps other methods since it turns out to be some sort of selection procedure.
(2) The authors did not update time in table 1 for CIFAR-10 results, which I consider almost no extra efforts. I am wondering how much more time each method needs from 45% in figure 2 to higher robust accuracy in table 1.
(3) I cannot understand why the proposed method is a particular good fit with cyclic LR and low precision tricks comparing to other methods.