Flexible tardiness bounds for sporadic real-time task systems on multiprocessors

The earliest-deadline-first (EDF) scheduling of a sporadic real-time task system on a multiprocessor may require that the total utilization of the task system, U_sum, not exceed (m + 1)/2 on m processors if every deadline needs to be met. In recent work, we considered the alleviation of this under-utilization for task systems that can tolerate deadline misses by bounded amounts (i.e., bounded tardiness). We showed that if U_sum ≤ m and tasks are not pinned to processors, then the tardiness of each task is bounded under both preemptive and non-preemptive EDF. However, the tardiness bounds derived are applicable to every task in the task system, i.e., any task may incur maximum tardiness. In this paper, we consider supporting tasks whose tolerances to tardiness are less than that known to be possible under EDF. We propose a new scheduling policy, called EDF-hl, which is a variant of EDF, and show that under EDF-hl, any tardiness, including zero tardiness, can be ensured for a limited number of privileged tasks, and that bounded tardiness can be guaranteed to the remaining tasks if their utilizations are restricted. EDF-hl reduces to EDF in the absence of privileged tasks. The tardiness bound that we derive is a function of U_sum, in addition to individual task parameters. Hence, tardiness for all tasks can be lowered by lowering U_sum. A simulation-based evaluation of the tardiness bounds that are possible is provided.