Closing the Gap for Makespan Scheduling via Sparsification Techniques

TL;DR

This paper presents a near-optimal algorithm for makespan scheduling on identical machines, achieving a running time close to the theoretical lower bound, by discovering a new structural property of optimal solutions that enables efficient enumeration.

Contribution

The paper introduces a novel structural result on configuration-IP solutions, leading to an improved algorithm with tight running time bounds under ETH for makespan scheduling.

Findings

Achieves a running time of $2^{ ilde{O}(1/ au)} + O(n ext{log} n)$, tight under ETH.

Develops a structural theorem showing the existence of sparse, symmetric optimal solutions.

Extends the structural approach to related machines and broader objective functions, yielding efficient PTAS.

Abstract

Makespan scheduling on identical machines is one of the most basic and fundamental packing problems studied in the discrete optimization literature. It asks for an assignment of $n$ jobs to a set of $m$ identical machines that minimizes the makespan. The problem is strongly NP-hard, and thus we do not expect a $(1+\epsilon)$-approximation algorithm with a running time that depends polynomially on $1/\epsilon$. Furthermore, Chen et al. [3] recently showed that a running time of $2^{(1/\epsilon)^{1-\delta}}+\text{poly}(n)$ for any $\delta>0$ would imply that the Exponential Time Hypothesis (ETH) fails. A long sequence of algorithms have been developed that try to obtain low dependencies on $1/\epsilon$, the better of which achieves a running time of $2^{\tilde{O}(1/\epsilon^2)}+O(n\log n)$ [11]. In this paper we obtain an algorithm with a running time of $2^{\tilde{O}(1/\epsilon)}+O(n\log n)$, which is tight under ETH up to logarithmic factors on the exponent. Our main technical contribution is a new structural result on the configuration-IP. More precisely, we show the existence of a highly symmetric and sparse optimal solution, in which all but a constant number of machines are assigned a configuration with small support. This structure can then be exploited by integer programming techniques and enumeration. We believe that our structural result is of independent interest and should find applications to other settings. In particular, we show how the structure can be applied to the minimum makespan problem on related machines and to a larger class of objective functions on parallel machines. For all these cases we obtain an efficient PTAS with running time $2^{\tilde{O}(1/\epsilon)} + \text{poly}(n)$.