Quasi-static scheduling for real-time systems with hard and soft tasks

Abstract

This report addresses the problem of scheduling for real-time systems that include both hard and soft tasks. In order to capture the relative importance of soft tasks and how the quality of results is affected when missing a soft deadline, we use utility functions associated to soft tasks. Thus the aim is to find the execution order of tasks that makes the total utility maximum and guarantees hard deadlines. We consider intervals rather than fixed execution times for tasks. Since a purely off-line solution is too pessimistic and a purely on-line approach incurs an unacceptable overhead due to the high complexity of the problem, we propose a quasi-static approach where a number of schedules are prepared at design-time and the decision of which of them to follow is taken at run-time based on the actual execution times. We propose an exact algorithm as well as different heuristics for the problem addressed in this report.

Synthesis of Flexible Fault-Tolerant Schedules with Preemption for Mixed Soft and Hard Real-Time Systems

“…We have proposed a novel quasi-static scheduling strategy, where a set of fault-tolerant schedules is synthesized off-line and, at run time, the scheduler will select the right schedule based on the occurrence of faults and the actual execution times of processes, such that hard deadlines are guaranteed and the overall system utility is maximized. The online overhead of quasistatic scheduling is very low, compared to traditional online scheduling approaches [4]. The proposed scheduling strategy can also handle overload situations with dropping of soft processes.…”

“…After the list L of schedulable processes is created, the next step is to find which process out of the schedulable processes is the best to schedule first. We calculate priorities for all unscheduled soft processes using the MU priority function presented in [4] (line 16). The MU priority function computes for each soft process P i the value, which constitutes of the utility produced by P i , scheduled as early as possible, and the sum of utility contributions of the other soft processes delayed because of P i .…”

“…Traditionally, hard and soft real-time systems have been scheduled using very different techniques [12]. However, many applications have both hard and soft timing constraints [3], and therefore researchers have proposed techniques for addressing mixed hard/soft real-time systems [3,5,4].…”

Quasi-Static Scheduling for Multiprocessor Real-Time Systems with Hard and Soft Tasks

“…Another quasi-static solution, similar to the one discussed above, is {Ω, Ω } but with Ω t 1 ; [2,7] −−−→ Ω which actually gives better results (it outperforms the static schedule Ω in 56 % of the cases and yields an average total utility 1.1, yet guaranteeing no hard deadline miss). Thus the most important question in the quasi-static approach discussed in this report is how to compute at design-time the set of schedules and switching points such that they deliver the highest quality (utility).…”

“…The on-line overhead, though, can be significant when the system workload is high. In a previous work we have discussed quasi-static scheduling for hard/soft systems in the particular case of monoprocessor systems [7], a problem whose analysis complexity is significantly lower than when considering multiple processors.…”