Diverse Schedules for Makespan Minimization on Unrelated Machines

Abstract

We study makespan minimization on unrelated machines with the additional goal of constructing diverse schedules: Instead of outputting a single schedule, we seek a collection of schedules with provable makespan guarantees and substantially different job assignments. Committing to a single schedule in advance may be undesirable. Processing times may be uncertain, and constraints or domain knowledge may emerge later, so $k$ schedules of similar quality offer meaningful alternatives. We measure pairwise diversity by the weighted Hamming distance between job assignments and study the two standard objectives in the diverse-solutions literature: max-sum diversity, which maximizes the total pairwise distance between schedules, and max-min diversity, which maximizes the smallest pairwise distance. For the max-sum objective, we show that optimal diversity can be achieved in polynomial time without sacrificing the classic makespan approximation factor. For every feasible makespan target $T$, we can compute $k$ schedules of makespan at most $2T$ whose max-sum diversity is optimal among all $k$ schedules of makespan at most $T$. Therefore, we match the classic factor-$2$ makespan guarantee of Lenstra, Shmoys, and Tardos (Math. Prog., 1990) and preserve optimal diversity. For the max-min objective, we show that the diversity problem is inherently harder. Namely, we show that, even without makespan constraints, deciding whether three schedules of given pairwise diversity exist is NP-hard. We complete this with two complementary bicriteria approximation algorithms for every fixed $k$ and $\varepsilon \in (0,1)$. The first preserves almost all of the optimal max-min diversity with makespan at most $(3\lceil12/\varepsilon\rceil+1)T$. The second keeps the makespan arbitrarily close to the classic $2$ guarantee while achieving a $\frac{1}{2}(1-\varepsilon)$ fraction of the optimal max-min diversity.