Abstract
We present a Flexible Alternating Direction Method of Multipliers (F-ADMM) algorithm for solving optimization problems involving a strongly convex objective function that is separable into \(n \ge 2\) blocks, subject to (non-separable) linear equality constraints. The F-ADMM algorithm uses a Gauss–Seidel scheme to update blocks of variables, and a regularization term is added to each of the subproblems. We prove, under common assumptions, that F-ADMM is globally convergent and that the iterates converge linearly. We also present a special case of F-ADMM that is partially parallelizable, which makes it attractive in a big data setting. In particular, we partition the data into groups, so that each group consists of multiple blocks of variables. By applying F-ADMM to this partitioning of the data, and using a specific regularization matrix, we obtain a hybrid ADMM (H-ADMM) algorithm: the grouped data is updated in a Gauss–Seidel fashion, and the blocks within each group are updated in a Jacobi (parallel) manner. Convergence of H-ADMM follows directly from the convergence properties of F-ADMM. Also, we describe a special case of H-ADMM that may be applied to functions that are convex, rather than strongly convex. Numerical experiments demonstrate the practical advantages of these new algorithms.
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Notes
The Gaussian back substitution step involves a vector of size \(N+m - N_1\) and a square upper triangular matrix of the same size. Recall that \(x_i \in {{\mathbf {R}}}^{N_i}\), so \(N_1\) is the dimension of \(x_1\).
The precise constant depends on the ‘reduceall’ implementation of MPI.
Indeed, the theory in [8] holds under weaker assumptions, but the condition \(P_1,P_2 \succ 0\) is sufficient for the discussion here.
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Acknowledgments
The authors would like to thank Professor Wotao Yin and Mr Damek Davis for their helpful discussion and feedback on an earlier version of this work. We would also like to thank the anonymous reviewers, whose comments greatly improved the presentation of this paper.
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Daniel P. Robinson has received support from the U.S. National Science Foundation under Grant No. DMS–1217153.
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Robinson, D.P., Tappenden, R. A Flexible ADMM Algorithm for Big Data Applications. J Sci Comput 71, 435–467 (2017). https://doi.org/10.1007/s10915-016-0306-6
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DOI: https://doi.org/10.1007/s10915-016-0306-6