A Case Against Periodic Jukebox Scheduling

摘要

This paper presents the jukebox early quantum scheduler (JEQS). JEQS is a periodic jukebox scheduler for a Video-on-Demand system. JEQS uses the jukebox robots in a cyclic way and the time is divided in constant units called quanta. A quantum is the maximum time needed to unload and load all the drives. An RSM is loaded in a drive for a fixed period of time, corresponding to the time needed to switch the media on the other drives. During this time the drive can read data from it. JEQS is based on the scheduling theory on early quantum tasks (EQT). An early quantum task executes its first instance in the next quantum after its arrival and the rest of the instances are scheduled in a normal periodic way with the release time immediately after the first execution. Although JEQS is an efficient periodic scheduler, that can guarantee the execution of most tasks in the next cycle after the requests arrive, we show that using JEQS results in much longer response times than using a-periodic schedulers. Furthermore, we show that the bad performance of JEQS is intrinsic to any periodic jukebox scheduler. The only advantage of using a periodic scheduler is that the scheduling algorithms are less complex. However, the simplicity of the algorithms clearly does not outweigh the unacceptably long response times.