A case for conservative workload modeling: Parallel job scheduling with daily cycles of activity

Computer workloads have many attributes. When modeling these workloads it is often difficult to decide which attributes are important, and which can be abstracted away. In many cases, the modeler only includes attributes that are believed to be important, and ignores the rest. We argue, however, that this can lead to impaired workloads and unreliable system evaluations. Using parallel job scheduling as a case study, and daily cycles of activity as the attribute in dispute, we present two schedulers whose simulated performance seems identical without cycles, but then becomes significantly different when daily cycles are included in the workload. We trace this to the ability of one scheduler to prioritize interactive jobs, which leads to implicitly delaying less critical work to nighttime, when it can utilize resources that otherwise would have been left idle. Notably, this was not a design feature of this scheduler, but rather an emergent property that was not anticipated in advance.