Non-Preemptive Earliest-Deadline-First Scheduling Policy: A Performance Study

M. Kargahi, A. Movaghar
13th IEEE International Symposium on Modeling, Analysis, and Simulation of Computer and Telecommunication Systems  
This paper introduces an analytical method for approximating the performability of a firm realtime system modeled by a multi-server queue. The service discipline in the queue is earliestdeadline-first (EDF), which is an optimal scheduling algorithm. Real-time jobs with exponentially distributed relative deadlines arrive according to a Poisson process. All jobs have deadlines until the end of service and are served non-preemptively. An important performance measure to calculate is the loss
more » ... e is the loss probability. The performance of the system is approximated by a Markovian model in the long run. A key parameter, namely, the loss rate when there are n jobs in the system is used in the model, which is estimated by partitioning the system into two subsystems. The resulting model can then be solved analytically using standard Markovian solution techniques. The number of servers in the system may change due to failure or repair. The performability of the system is evaluated in the presence of such structural changes. The latter measure is approximated by a Markov reward model, considering the loss probability as the reward rate. Comparing numerical and simulation results, we find that the existing errors are relatively small. Kargahi and Movaghar 38 systems wherein jobs missing their deadlines can continue their execution with a degraded value are categorized as SRT, while systems wherein such jobs are of no value and are usually thrown away are called FRT. In general, scheduling policies can be classified into two broad categories: preemptive and nonpreemptive. In a preemptive scheduling policy, processing of the currently running job can be interrupted by a higher priority job, whereas in a non-preemptive scheduling policy, an arriving higher priority job is scheduled only after the completion of the current job execution. Though preemptive scheduling policies can guarantee higher system utilization, there are scenarios where the properties of some hardware or software devices make preemption either impossible or prohibitively expensive. For example, in high speed packet switching networks, preemption requires the retransmission of the preempted packet. Scheduling over a shared media such as LAN, WLAN and field buses described in EN 50170 (1996) such as CAN bus which is discussed in CAN-CIA (1992) and Livani and Kaiser (1998) is inherently non-preemptive, because each node in the network has to ensure that the shared channel is free before it can begin transmission. Besides its extensive use in communication systems, non-preemptive processor scheduling is also used in light weight multi-tasking kernels and is beneficial in multimedia applications as depicted in Dolev and Keizelman (1999) . Non-preemptive scheduling policies for real-time embedded systems have also the benefits of more accurate response time analysis, ease of implementation, reduced run-time overhead, and guaranteeing exclusive access to shared resources and data which eliminate both the need for synchronization and its associated overheads.
doi:10.1109/mascots.2005.44 dblp:conf/mascots/KargahiM05 fatcat:ccffphdlejgzhm4mu6cgebbkkm