Optimal Learning Rates for Localized SVMs
This work provides a theoretical foundation for localized SVMs, addressing scalability issues for practitioners in machine learning, though it is incremental as it builds on existing chunk-based methods.
The paper tackles the computational inefficiency of support vector machines (SVMs) in large-scale applications by proposing a localized SVM approach based on input space partitioning, deriving minimax optimal learning rates for least squares regression with Gaussian kernels and showing comparable test performance to global SVMs with significantly reduced computational requirements.
One of the limiting factors of using support vector machines (SVMs) in large scale applications are their super-linear computational requirements in terms of the number of training samples. To address this issue, several approaches that train SVMs on many small chunks of large data sets separately have been proposed in the literature. So far, however, almost all these approaches have only been empirically investigated. In addition, their motivation was always based on computational requirements. In this work, we consider a localized SVM approach based upon a partition of the input space. For this local SVM, we derive a general oracle inequality. Then we apply this oracle inequality to least squares regression using Gaussian kernels and deduce local learning rates that are essentially minimax optimal under some standard smoothness assumptions on the regression function. This gives the first motivation for using local SVMs that is not based on computational requirements but on theoretical predictions on the generalization performance. We further introduce a data-dependent parameter selection method for our local SVM approach and show that this method achieves the same learning rates as before. Finally, we present some larger scale experiments for our localized SVM showing that it achieves essentially the same test performance as a global SVM for a fraction of the computational requirements. In addition, it turns out that the computational requirements for the local SVMs are similar to those of a vanilla random chunk approach, while the achieved test errors are significantly better.