Statistic Rate Monotonic Scheduling

In this paper we present Statistical Rate Monotonic Scheduling (SRMS), a generalization of the classical RMS results of Liu and Layland that allows scheduling periodic tasks with highly variable execution times and statistical QoS requirements. Similar to RMS, SRMS has two components: a feasibility test and a scheduling algorithm. The feasibility test for SRMS ensures that using SRMS' scheduling algorithms, it is possible for a given periodic task set to share a given resource (e.g. a processor, communication medium, switching device, etc.) in such a way that such sharing does not result in the violation of any of the periodic tasks QoS constraints. The SRMS scheduling algorithm incorporates a number of unique features. First, it allows for fixed priority scheduling that keeps the tasks' value (or importance) independent of their periods. Second, it allows for job admission control, which allows the rejection of jobs that are not guaranteed to finish by their deadlines as soon as they are released, thus enabling the system to take necessary compensating actions. Also, admission control allows the preservation of resources since no time is spent on jobs that will miss their deadlines anyway. Third, SRMS integrates reservation-based and best-effort resource scheduling seamlessly. Reservation-based scheduling ensures the delivery of the minimal requested QoS; best-effort scheduling ensures that unused, reserved bandwidth is not wasted, but rather used to improve QoS further. Fourth, SRMS allows a system to deal gracefully with overload conditions by ensuring a fair deterioration in QoS across all tasks---as opposed to penalizing tasks with longer periods, for example. Finally, SRMS has the added advantage that its schedulability test is simple and its scheduling algorithm has a constant overhead in the sense that the complexity of the scheduler is not dependent on the number of the tasks in the system. We have evaluated SRMS against a number of alternative scheduling algorithms suggested in the literature (e.g. RMS and slack stealing), as well as refinements thereof, which we describe in this paper. Consistently throughout our experiments, SRMS provided the best performance. In addition, to evaluate the optimality of SRMS, we have compared it to an inefficient, yet optimal scheduler for task sets with harmonic periods.National Science Foundation (CCR-970668

Optimal Time Utility Based Scheduling Policy Design for Cyber-Physical Systems

Classical scheduling abstractions such as deadlines and priorities do not readily capture the complex timing semantics found in many real-time cyber-physical systems. Time utility functions provide a necessarily richer description of timing semantics, but designing utility-aware scheduling policies using them is an open research problem. In particular, optimal utility accrual scheduling design is needed for real-time cyber-physical domains. In this paper we design optimal utility accrual scheduling policies for cyber-physical systems with periodic, non-preemptable tasks that run with stochastic duration. These policies are derived by solving a Markov Decision Process formulation of the scheduling problem. We use this formulation to demonstrate that our technique improves on existing heuristic utility accrual scheduling policies

Extending the Path Analysis Technique to Obtain a Soft WCET

This paper discusses an efficient approach to statically compute a WCET that is "soft" rather than "hard". The goal of most timing analysis is to determine a guaranteed WCET; however this execution time may be far above the actual distribution of observed execution times. A WCET estimate that bounds the execution time 99% of the time may be more useful for a designer in a soft real-time environment. This paper discusses an approach to measure the execution time distribution by a hardware simulator, and a path-based timing analysis approach to derive a static estimation of this same distribution. The technique can find a soft WCET for loops having any number of paths

An Analytical Solution for Probabilistic Guarantees of Reservation Based Soft Real-Time Systems

We show a methodology for the computation of the probability of deadline miss for a periodic real-time task scheduled by a resource reservation algorithm. We propose a modelling technique for the system that reduces the computation of such a probability to that of the steady state probability of an infinite state Discrete Time Markov Chain with a periodic structure. This structure is exploited to develop an efficient numeric solution where different accuracy/computation time trade-offs can be obtained by operating on the granularity of the model. More importantly we offer a closed form conservative bound for the probability of a deadline miss. Our experiments reveal that the bound remains reasonably close to the experimental probability in one real-time application of practical interest. When this bound is used for the optimisation of the overall Quality of Service for a set of tasks sharing the CPU, it produces a good sub-optimal solution in a small amount of time.Comment: IEEE Transactions on Parallel and Distributed Systems, Volume:27, Issue: 3, March 201

Meta-QoS performance of earliest-deadline-first and rate-monotonic scheduling of smoothed video data in a client-server environment

In this paper we present an extensive performance study of two modified EDF and RM scheduling algorithms which are enhanced to provide quality of service (QoS) guarantees for smoothed video data. With a probabilistic definition of QoS, we incorporate admission control conditions into the two algorithms. Furthermore, we also include a counter-based scheduling module as the core scheduling mechanism which adaptively adjusts the actual QoS levels assigned to requests. Our theoretical analysis of the two enhanced algorithms, called QEDF and QRM, shows that the QRM algorithm is more robust than the QEDF algorithm for different workload and utilization conditions. We also propose to use a new metric called meta-QoS to quantify the overall performance of a packet scheduler given a set of simultaneous requests. In our experiments, we find that the QRM algorithm can sustain a rather stable level of meta-QoS even when the workload and utilization levels are increased. On the other hand, the QEDF algorithm is found to be less desirable for a high level of utilization and a large number of requests.published_or_final_versio

The SRMS Workbench

The SRMS Workbench is a software system developed to demonstrate the notion of Statistical QoS employed in SRMS [AtlasBestavros:1998]. The SRMS Workbench includes: (1) the SRMS schedulability analyzer (QoS negotiator), and (2) a SRMS simulator (Basic SRMS + all extensions). These two components are packaged into a Java Applet that can be executed remotely on any Java-capable Internet browser. For comparison, other scheduling algorithms, including RMS [LiuLayland:1973] and SSJAC [AtlasBestavros:1998] are included. Through a simple GUI, the SRMS Workbench allows users to specify a set of periodic tasks, each with (a) its own period, (b) the distributional characteristics of its periodic resource requirements (e.g. Poisson, Pareto, Normal, Exponential, Gamma, etc.), (c) its desired QoS as a lower bound on the percentage of deadlines to be met, and (d) a criticality/importance index indicating the value of the task (relative to other tasks in the task set). Once the task set is specified, the SRMS Workbench allows the user to check for schedulability under SRMS. If the task set is schedulable, the SRMS Workbench generates the appropriate allowance for each task and allows the user to create an animated simulation of the task system, which can be executed and profiled. If the task set is not schedulable, the SRMS Workbench informs the user of that fact and suggests (as part of the QoS negotiation) an alternative set of feasible QoS requirements that reflects the specified criticality/importance index of the tasks in the task set. The SRMS Workbench is available on the Web at http://www.cs.bu.edu/groups/realtime/SRMSworkbenc

Soft real-time communications over Bluetooth under interferences from ISM devices

Bluetooth is a suitable technology to support soft real-time applications like multimedia streams at the personal area network level. In this paper, we analytically evaluate the worst-case deadline failure probability of Bluetooth packets under co-channel interference as a way to provide statistical guarantees when transmitting soft real-time traffic using ACL links. We consider the interference from independent Bluetooth devices, as well as from other devices operating in the ISM band like 802.11b/g and Zigbee. Finally, we show as an example how to use our model to obtain some results for the transmission of a voice stream.Ministerio de Ciencia y Tecnología TIC2001-1868-C03-0

An exact stochastic analysis of priority-driven periodic real-time systems and its approximations.

Abstract This paper describes a stochastic analysis framework which computes the response time distribution and the deadline miss probability of individual tasks, even for systems with a maximum utilization greater than one. The framework is uniformly applied to fixed-priority and dynamic-priority systems and can handle tasks with arbitrary relative deadlines and execution time distributions