Paper:
Searching for Activation Functions
The choice of activation functions in deep networks has a significant effect on the training dynamics and task performance. Currently, the most successful and widely-used activation function is the Rectified Linear Unit (ReLU). Although various hand-designed alternatives to ReLU have been proposed, none have managed to replace it due to inconsistent gains. In this work, we propose to leverage automatic search techniques to discover new activation functions. Using a combination of exhaustive and reinforcement learning-based search, we discover multiple novel activation functions. We verify the effectiveness of the searches by conducting an empirical evaluation with the best discovered activation function. Our experiments show that the best discovered activation function, f (x) = x · sigmoid(βx), which we name Swish, tends to work better than ReLU on deeper models across a number of challenging datasets. For example, simply replacing ReLUs with Swish units improves top-1 classification accuracy on ImageNet by 0.9% for Mobile NASNet-A and 0.6% for Inception-ResNet-v2. The simplicity of Swish and its similarity to ReLU make it easy for practitioners to replace ReLUs with Swish units in any neural network.

Review:
Authors propose a reinforcement learning based approach for finding a non-linearity by searching through combinations from a set of unary and binary operators. The best one found is termed Swish unit; x * sigmoid(b*x). 
The properties of Swish like allowing information flow on the negative side and linear nature on the positive have been proven to be important for better optimization in the past by other functions like LReLU, PLReLU etc. As pointed out by the authors themselves for b=1 Swish is equivalent to SiL proposed in Elfwing et. al. (2017).
In terms of experimental validation, in most cases the increase is performance when using Swish as compared to other models are very small fractions. Again, the authors do state that "our results may not be directly comparable to the results in the corresponding works due to differences in our training steps."   
Based on the Figure 6 authors claim that the non-monotonic bump of Swish on the negative side is very important aspect. More explanation is required on why is it important and how does it help optimization. Distribution of learned b in Swish for different layers of a network can interesting to observe.