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Register a new userQPPAL: A two-phase proximal augmented Lagrangian method for high dimensional convex quadratic programming problems
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In this paper, we aim to solve high dimensional convex quadratic programming (QP) problems with a large number of quadratic terms, linear equality and inequality constraints. In order to solve the targeted problems to a desired accuracy efficiently, we develop a two-phase proximal augmented Lagrangian method, with Phase I to generate a reasonably good initial point to warm start Phase II to obtain an accurate solution efficiently. More specifically, in Phase I, based on the recently developed symmetric Gauss-Seidel (sGS) decomposition technique, we design a novel sGS based semi-proximal augmented Lagrangian method for the purpose of finding a solution of low to medium accuracy. Then, in Phase II, a proximal augmented Lagrangian algorithm is proposed to obtain a more accurate solution efficiently. Extensive numerical results evaluating the performance of our proposed algorithm against the highly optimized commercial solver Gurobi and the open source solver OSQP are presented to demonstrate the high efficiency and robustness of our proposed algorithm for solving various classes of large-scale convex QP problems.
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We propose a semi-proximal augmented Lagrangian based decomposition method for convex composite quadratic conic programming problems with primal block angular structures. Using our algorithmic framework, we are able to naturally derive several well known augmented Lagrangian based decomposition methods for stochastic programming such as the diagonal quadratic approximation method of Mulvey and Ruszczy{n}ski. Moreover, we are able to derive novel enhancements and generalizations of these well known methods. We also propose a semi-proximal symmetric Gauss-Seidel based alternating direction method of multipliers for solving the corresponding dual problem. Numerical results show that our algorithms can perform well even for very large instances of primal block angular convex QP problems. For example, one instance with more than $300,000$ linear constraints and $12,500,000$ nonnegative variables is solved in less than a minute whereas Gurobi took more than 3 hours, and another instance {tt qp-gridgen1} with more than $331,000$ linear constraints and $986,000$ nonnegative variables is solved in about 5 minutes whereas Gurobi took more than 35 minutes.
The augmented Lagrangian method (ALM) is a fundamental tool for solving the canonical convex minimization problem with linear constraints, and efficiently and easily how to implement the original ALM is affirmatively significant. Recently, He and Yuan have proposed a balanced version of ALM [B.S. He and X.M. Yuan, arXiv:2108.08554, 2021], which reshapes the original ALM by balancing its subproblems and makes the benchmark ALM easier to implement without any additional condition. In practice, the balanced ALM updates the new iterate by a primal-dual order. In this note, exploiting the variational inequality structure of the most recent balanced ALM, we propose a dual-primal version of the balanced ALM for linearly constrained convex minimization problems. The novel proposed method generates the new iterate by a dual-primal order and enjoys the same computational difficulty with the original primal-dual balanced ALM. Furthermore, under the lens of the proximal point algorithm, we conduct the convergence analysis of the novel introduced method in the context of variational inequalities. Numerical tests on the basic pursuit problem demonstrate that the introduced method enjoys the same high efficiency with the prototype balanced ALM.
Shape-constrained convex regression problem deals with fitting a convex function to the observed data, where additional constraints are imposed, such as component-wise monotonicity and uniform Lipschitz continuity. This paper provides a unified framework for computing the least squares estimator of a multivariate shape-constrained convex regression function in $mathbb{R}^d$. We prove that the least squares estimator is computable via solving an essentially constrained convex quadratic programming (QP) problem with $(n+1)d$ variables, $n(n-1)$ linear inequality constraints and $n$ possibly non-polyhedral inequality constraints, where $n$ is the number of data points. To efficiently solve the generally very large-scale convex QP, we design a proximal augmented Lagrangian method (proxALM) whose subproblems are solved by the semismooth Newton method (SSN). To further accelerate the computation when $n$ is huge, we design a practical implementation of the constraint generation method such that each reduced problem is efficiently solved by our proposed proxALM. Comprehensive numerical experiments, including those in the pricing of basket options and estimation of production functions in economics, demonstrate that our proposed proxALM outperforms the state-of-the-art algorithms, and the proposed acceleration technique further shortens the computation time by a large margin.
This paper considers the problem of minimizing a convex expectation function over a closed convex set, coupled with a set of inequality convex expectation constraints. We present a new stochastic approximation type algorithm, namely the stochastic approximation proximal method of multipliers (PMMSopt) to solve this convex stochastic optimization problem. We analyze regrets of a stochastic approximation proximal method of multipliers for solving convex stochastic optimization problems. Under mild conditions, we show that this algorithm exhibits ${rm O}(T^{-1/2})$ rate of convergence, in terms of both optimality gap and constraint violation if parameters in the algorithm are properly chosen, when the objective and constraint functions are generally convex, where $T$ denotes the number of iterations. Moreover, we show that, with at least $1-e^{-T^{1/4}}$ probability, the algorithm has no more than ${rm O}(T^{-1/4})$ objective regret and no more than ${rm O}(T^{-1/8})$ constraint violation regret. To the best of our knowledge, this is the first time that such a proximal method for solving expectation constrained stochastic optimization is presented in the literature.
In this paper we consider a class of convex conic programming. In particular, we propose an inexact augmented Lagrangian (I-AL) method for solving this problem, in which the augmented Lagrangian subproblems are solved approximately by a variant of Nesterovs optimal first-order method. We show that the total number of first-order iterations of the proposed I-AL method for computing an $epsilon$-KKT solution is at most $mathcal{O}(epsilon^{-7/4})$. We also propose a modified I-AL method and show that it has an improved iteration-complexity $mathcal{O}(epsilon^{-1}logepsilon^{-1})$, which is so far the lowest complexity bound among all first-order I-AL type of methods for computing an $epsilon$-KKT solution. Our complexity analysis of the I-AL methods is mainly based on an analysis on inexact proximal point algorithm (PPA) and the link between the I-AL methods and inexact PPA. It is substantially different from the existing complexity analyses of the first-order I-AL methods in the literature, which typically regard the I-AL methods as an inexact dual gradient method. Compared to the mostly related I-AL methods cite{Lan16}, our modified I-AL method is more practically efficient and also applicable to a broader class of problems.
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