Stochastic scheduling with Bernoulli-type jobs through policy stratification

TL;DR

This paper develops a Polynomial-Time Approximation Scheme (PTAS) for stochastic scheduling with Bernoulli-type jobs on identical machines, showing near-optimal solutions are feasible when job-size diversity is limited.

Contribution

It introduces a stratified policy approach that approximates optimal scheduling decisions with negligible cost increase, extending to a quasi-polynomial approximation for general cases.

Findings

PTAS for Bernoulli-type job scheduling with bounded job-size parameters

Dynamic programming computes optimal stratified policies

Quasi-polynomial O(log N) approximation for arbitrary job sizes

Abstract

This paper addresses the problem of computing a scheduling policy that minimizes the total expected completion time of a set of $N$ jobs with stochastic processing times on $m$ parallel identical machines. When all processing times follow Bernoulli-type distributions, Gupta et al. (SODA '23) exhibited approximation algorithms with an approximation guarantee $\tilde{\text{O}}(\sqrt{m})$, where $m$ is the number of machines and $\tilde{\text{O}}(\cdot)$ suppresses polylogarithmic factors in $N$, improving upon an earlier ${\text{O}}(m)$ approximation by Eberle et al. (OR Letters '19) for a special case. The present paper shows that, quite unexpectedly, the problem with Bernoulli-type jobs admits a PTAS whenever the number of different job-size parameters is bounded by a constant. The result is based on a series of transformations of an optimal scheduling policy to a "stratified" policy that makes scheduling decisions at specific points in time only, while losing only a negligible factor in expected cost. An optimal stratified policy is computed using dynamic programming. Two technical issues are solved, namely (i) to ensure that, with at most a slight delay, the stratified policy has an information advantage over the optimal policy, allowing it to simulate its decisions, and (ii) to ensure that the delays do not accumulate, thus solving the trade-off between the complexity of the scheduling policy and its expected cost. Our results also imply a quasi-polynomial $\text{O}(\log N)$-approximation for the case with an arbitrary number of job sizes.