What Deep CNNs Benefit from Global Covariance Pooling: An Optimization Perspective

Recent works have demonstrated that global covariance pooling (GCP) has the ability to improve performance of deep convolutional neural networks (CNNs) on visual classification task. Despite considerable advance, the reasons on effectiveness of GCP on deep CNNs have not been well studied. In this paper, we make an attempt to understand what deep CNNs benefit from GCP in a viewpoint of optimization. Specifically, we explore the effect of GCP on deep CNNs in terms of the Lipschitzness of optimization loss and the predictiveness of gradients, and show that GCP can make the optimization landscape more smooth and the gradients more predictive. Furthermore, we discuss the connection between GCP and second-order optimization for deep CNNs. More importantly, above findings can account for several merits of covariance pooling for training deep CNNs that have not been recognized previously or fully explored, including significant acceleration of network convergence (i.e., the networks trained with GCP can support rapid decay of learning rates, achieving favorable performance while significantly reducing number of training epochs), stronger robustness to distorted examples generated by image corruptions and perturbations, and good generalization ability to different vision tasks, e.g., object detection and instance segmentation. We conduct extensive experiments using various deep CNN models on diversified tasks, and the results provide strong support to our findings.

ECA-Net: Efficient Channel Attention for Deep Convolutional Neural Networks

Channel attention has recently demonstrated to offer great potential in improving the performance of deep convolutional neural networks (CNNs). However, most existing methods dedicate to developing more sophisticated attention modules to achieve better performance, inevitably increasing the computational burden. To overcome the paradox of performance and complexity trade-off, this paper makes an attempt to investigate an extremely lightweight attention module for boosting the performance of deep CNNs. In particular, we propose an Efficient Channel Attention (ECA) module, which only involves $k (k < 9)$ parameters but brings clear performance gain. By revisiting the channel attention module in SENet, we empirically show avoiding dimensionality reduction and appropriate cross-channel interaction are important to learn effective channel attention. Therefore, we propose a local cross-channel interaction strategy without dimension reduction, which can be efficiently implemented by a fast 1D convolution. Furthermore, we develop a function of channel dimension to adaptively determine kernel size of 1D convolution, which stands for coverage of local cross-channel interaction. Our ECA module can be flexibly incorporated into existing CNN architectures, and the resulting CNNs are named by ECA-Net. We extensively evaluate the proposed ECA-Net on image classification, object detection and instance segmentation with backbones of ResNets and MobileNetV2. The experimental results show our ECA-Net is more efficient while performing favorably against its counterparts. The source code and models can be available at https://github.com/BangguWu/ECANet.