We propose a new randomized coordinate descent method for unconstrained minimization of convex functions that are smooth with respect to a given real symmetric positive semidefinite curvature matrix. At each iteration, this method works with a random coordinate subset selected with probability proportional to the determinant of the corresponding principal submatrix of the curvature matrix (volume sampling). When the size of the subset equals one, the method reduces to the well-known randomized coordinate descent which samples coordinates with probabilities proportional to the coordinate Lipschitz constants. We establish the convergence rates both for convex and strongly convex functions. Our theoretical results show that by increasing the size of the subset from one number to another, it is possible to accelerate the method up to the factor which depends on the spectral gap between the corresponding largest eigenvalues of the curvature matrix. Several numerical experiments confirm our theoretical conclusions.
We propose a new algorithm for minimizing regularized empirical loss: Stochastic Dual Newton Ascent (SDNA). Our method is dual in nature: in each iteration we update a random subset of the dual variables. However, unlike existing methods such as stochastic dual coordinate ascent, SDNA is capable of utilizing all local curvature information contained in the examples, which leads to striking improvements in both theory and practice -- sometimes by orders of magnitude. In the special case when an L2-regularizer is used in the primal, the dual problem is a concave quadratic maximization problem plus a separable term. In this regime, SDNA in each step solves a proximal subproblem involving a random principal submatrix of the Hessian of the quadratic function; whence the name of the method.