Combining ADMM and the Augmented Lagrangian Method
for Efficiently Handling Many Constraints
Abstract
Many machine learning methods entail minimizing a loss-function that is the sum of the losses
for each data point. The form of the loss function is exploited algorithmically, for instance in
stochastic gradient descent (SGD) and in the alternating direction method of multipliers (ADMM).
However, there are also machine learning methods where the entailed optimization problem features the data points not in the objective function
but in the form of constraints, typically one constraint per data point. Here, we address the problem of solving convex optimization problems with
many convex constraints. Our approach is an extension of ADMM. The straightforward implementation of ADMM for solving constrained optimization problems in a distributed fashion solves constrained subproblems on different compute nodes
that are aggregated until a consensus solution is
reached. Hence, the straightforward approach has
three nested loops: one for reaching consensus,
one for the constraints, and one for the unconstrained problems. Here, we show that solving the
costly constrained subproblems can be avoided. In
our approach, we combine the ability of ADMM
to solve convex optimization problems in a distributed setting with the ability of the augmented
Lagrangian method to solve constrained optimization problems. Consequently, our algorithm only
needs two nested loops. We prove that it inherits the convergence guarantees of both ADMM and
the augmented Lagrangian method. Experimental
results corroborate our theoretical findings