On Randomized Online Span Minimization
We study the online Busy Time scheduling model on a single machine of unbounded capacity, with non-preemptive jobs. In our setting, flexible jobs arrive online with a processing time and deadline, both of which become known to the algorithm at the job's arrival time. The goal is to schedule jobs on the machine to finish all jobs by their deadlines, so that the total time when the machine is turned on (busy time, also called span in this setting) is minimized. We present a randomized online algorithm with an exact competitive ratio of exactly (1 + e) < 3.72, where e is Euler's number, against an oblivious adversary, and show that no randomized algorithm can have a competitive ratio better than $e$ against an oblivious adversary. This lower bound holds even for algorithms that are allowed to restart jobs and are given lookahead. Previous work either dealt with special cases, or with deterministic algorithms, for which the known upper bound on the competitive ratio is 5 and the known lower bound is 4. Our findings offer fresh insights into randomization in online energy-aware scheduling. In the setting where jobs have uniform processing times, and a job that is started by the algorithm must be finished, we show that no deterministic algorithm can do better than 2, even when the jobs are agreeable. This deterministic lower bound also holds for uniform processing times and restarts in the scenario where a job's deadline is only revealed at its starting deadline. For Capacitated Busy Time with p_max-lookahead, we obtain a deterministic online algorithm with competitive ratio at most 8 and a randomized online algorithm with competitive ratio at most (4+ e) < 6.72, against an oblivious adversary.