More or Less: When and How to Build Convolutional Neural Network Ensembles

Convolutional neural network models are applied to many tasks with rising complexity and data sizes. Researchers and practitioners seek to scale the representational power of these models by adding more parameters. Increasing parameters, though, requires additional critical resources such as memory and compute time leading to increased training and inference cost. Thus a consistent challenge is to obtain as high as possible accuracy within a parameter budget. As neural network designers navigate this complex landscape, they are guided by conventional wisdom. This comes from empirical studies of the design space. We identify a critical part of this design space that is not well-understood: That is how to decide between the alternatives of expanding a single convolutional network model or increasing the number of networks and using them together in an ensemble. We study this question in detail through extensive experimentation across various network architectures and data sets. Crucially, we provide a robust assessment, missing in previous studies, by fixing the number of parameters. We also consider a holistic set of metrics such as training time, inference time, and memory usage. Contrary to conventional wisdom, we show that when we perform a holistic and robust assessment, we uncover a wide design space, where ensembles not only provide better accuracy but also train faster, and deploy at a speed comparable to single convolutional networks with the same total number of parameters.