Additive Approximation Schemes for Load Balancing Problems

TL;DR

This paper introduces additive approximation schemes for load balancing problems, providing new methods that yield solutions with controlled absolute error, especially useful when traditional multiplicative approximations are infeasible.

Contribution

It presents a novel relaxation called slot-MILP and a local-search algorithm, enabling additive approximations for complex load balancing problems.

Findings

Additive schemes can outperform multiplicative ones when OPT is large.

The slot-MILP relaxation efficiently approximates solutions under certain load bounds.

The algorithms achieve an additive error proportional to the maximum job processing time.

Abstract

In this paper we introduce the concept of additive approximation schemes and apply it to load balancing problems. Additive approximation schemes aim to find a solution with an absolute error in the objective of at most $\epsilon h$ for some suitable parameter $h$. In the case that the parameter $h$ provides a lower bound an additive approximation scheme implies a standard multiplicative approximation scheme and can be much stronger when $h \ll$ OPT. On the other hand, when no PTAS exists (or is unlikely to exist), additive approximation schemes can provide a different notion for approximation. We consider the problem of assigning jobs to identical machines with lower and upper bounds for the loads of the machines. This setting generalizes problems like makespan minimization, the Santa Claus problem (on identical machines), and the envy-minimizing Santa Claus problem. For the last problem, in which the objective is to minimize the difference between the maximum and minimum load, the optimal objective value may be zero and hence it is NP-hard to obtain any multiplicative approximation guarantee. For this class of problems we present additive approximation schemes for $h = p_{\max}$, the maximum processing time of the jobs. Our technical contribution is two-fold. First, we introduce a new relaxation based on integrally assigning slots to machines and fractionally assigning jobs to the slots (the slot-MILP). We identify structural properties of (near-)optimal solutions of the slot-MILP, which allow us to solve it efficiently, assuming that there are $O(1)$ different lower and upper bounds on the machine loads (which is the relevant setting for the three problems mentioned above). The second technical contribution is a local-search based algorithm which rounds a solution to the slot-MILP introducing an additive error on the target load intervals of at most $\epsilon\cdot p_{\max}$.