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Random block coordinate descent methods for linearly constrained optimization over networks

In this paper we develop random block coordinate gradient descent methods for minimizing large scale linearly constrained separable convex problems over networks. Since we have coupled constraints in the problem, we devise an algorithm that updates in parallel $τ\geq 2$ (block) components per iteration. Moreover, for this method the computations can be performed in a distributed fashion according to the structure of the network. However, its complexity per iteration is usually cheaper than of the full gradient method when the number of nodes $N$ in the network is large. We prove that for this method we obtain in expectation an $ε$-accurate solution in at most $\mathcal{O}(\frac{N}{τε})$ iterations and thus the convergence rate depends linearly on the number of (block) components $τ$ to be updated. For strongly convex functions the new method converges linearly. We also focus on how to choose the probabilities to make the randomized algorithm to converge as fast as possible and we arrive at solving a sparse SDP. Finally, we describe several applications that fit in our framework, in particular the convex feasibility problem. Numerically, we show that the parallel coordinate descent method with $τ>2$ accelerates on its basic counterpart corresponding to $τ=2$.