Random Block Coordinate Descent Methods for Linearly Constrained Optimization over Networks
- 434 Downloads
In this paper we develop random block coordinate descent methods for minimizing large-scale linearly constrained convex problems over networks. Since coupled constraints appear in the problem, we devise an algorithm that updates in parallel at each iteration at least two random components of the solution, chosen according to a given probability distribution. Those computations can be performed in a distributed fashion according to the structure of the network. Complexity per iteration of the proposed methods is usually cheaper than that of the full gradient method when the number of nodes in the network is much larger than the number of updated components. On smooth convex problems, we prove that these methods exhibit a sublinear worst-case convergence rate in the expected value of the objective function. Moreover, this convergence rate depends linearly on the number of components to be updated. On smooth strongly convex problems we prove that our methods converge linearly. We also focus on how to choose the probabilities to make our randomized algorithms converge as fast as possible, which leads us to solving a sparse semidefinite program. We then describe several applications that fit in our framework, in particular the convex feasibility problem. Finally, numerical experiments illustrate the behaviour of our methods, showing in particular that updating more than two components in parallel accelerates the method.
KeywordsConvex optimization over networks Linear coupled constraints Random coordinate descent Distributed computations Convergence analysis
Mathematics Subject Classification90C06 90C25 90C35
This research received funding from UEFISCDI Romania, PNII-RU-TE, project MoCOBiDS, no. 176/01.10.2015. It presents research results of the Belgian Network DYSCO (Dynamical Systems, Control, and Optimization), funded by the Interuniversity Attraction Poles Programme, initiated by the Belgian State, Science Policy Office, and of the Concerted Research Action (ARC) programme supported by the Federation Wallonia-Brussels (contract ARC 14/19-060). Support from two WBI-Romanian Academy grants is also acknowledged. Scientific responsibility rests with the authors.
- 6.Combettes, P.: The convex feasibility problem in image recovery. In: Hawkes, P. (ed.) Advances in Imaging and Electron Physics, pp. 155–270. Academic Press, Cambridge (1996)Google Scholar
- 14.Patrascu, A., Necoara, I.: Efficient random coordinate descent algorithms for large-scale structured nonconvex optimization. J. Global Optim. 61(1), 19–46 (2015)Google Scholar
- 18.Necoara, I., Patrascu, A.: A random coordinate descent algorithm for optimization problems with composite objective function and linear coupled constraints. Comput. Optim. Appl. 57(2), 307–337 (2014)Google Scholar
- 19.Reddi, S., Hefny, A., Downey, C., Dubey, A., Sra, S.: Large-scale randomized-coordinate descent methods with non-separable linear constraints. Tech. rep. (2014). http://arxiv.org/abs/1409.2617.pdf
- 20.Necoara, I., Nesterov, Y., Glineur, F.: A random coordinate descent method on large optimization problems with linear constraints. Tech. rep. (2011). https://acse.pub.ro/person/ion-necoara