Tasks In Real-time Systems Research Articles

Real-time task scheduling for energy-aware embedded systems

We present a new approach for scheduling workloads containing periodic tasks in real-time systems. The proposed approach minimizes the total energy consumed by the task set and guarantees that the deadline for every periodic task is met. Energy is a scarce resource for embedded systems, and energy consumption must be carefully balanced against real-time responsiveness. As embedded software becomes a larger component of system-on-a-chip design, energy management using the operating system becomes increasingly important. We present a mixed-integer linear programming model for the NP-complete scheduling problem and solve it for moderate-sized problem instances using a public-domain solver. For larger task sets, we present a novel low-energy earliest-deadline-first (LEDF) scheduling algorithm and apply it to two real-life task sets. We also present extended-LEDF (E-LEDF), a modified version of LEDF that considers more practical scenarios. Our results show that energy can be conserved in embedded real-time systems using energy-aware task scheduling.

Bounds on tardiness in scheduling of precedence-constrained unit real-time task systems

We consider the problem of scheduling unit task systems with deadlines. Both lower and upper bounds on tardiness of schedules for general work-conserving scheduling algorithms, and a refinement of the upper bounds for earliest-deadline-first scheduling algorithm are presented.

Scheduling Real-Time Systems by Means of Petri Nets

We focus on the off-line scheduling of periodic real-time task systems where the first release date of some tasks can differ from the others. We first determine the length of the sequences to construct off-line, which was uninvestigated in the general case. The proposed method is based on the simulation of a Petri net and allows the extraction of optimal schedules regarding several criteria for a chosen set of tasks (e.g. minimizing response time, maximizing importance, ...)

Developing a Testbed for Distributed Real-Time Applications

Fixed priority scheduling is viewed to be a standard technique for the scheduling of tasks in real-time systems on uni-processors. There are, however, also communication protocols which are based upon fixed priority scheduling. This paper discusses a testbed which has been constructed using embedded controller boards connected via a CANbus. The testbed supports experimentation with distributed real-time systems using both the Ada-95 and C programming languages. Support is provided for a higher level CAN communication protocol by a set of interface routines, and for multi-tasking on a single processor through the provision of a kernel of routines based upon the Pthread standards. Results are provided which suggest that the testbed can allow the deterministic scheduling of both tasks and communications, thus facilitating experimentation with distributed real-time systems.

A Feasibility Decision Algorithm for Rate Monotonic andDeadline Monotonic Scheduling

Rate monotonic and deadline monotonic scheduling are commonly used for periodic real-time task systems. This paper discusses a feasibility decision for a given real-time task system when the system is scheduled by rate monotonic and deadline monotonic scheduling. The time complexity of existing feasibility decision algorithms depends on both the number of tasks and maximum periods or deadlines when the periods and deadlines are integers. This paper presents a new necessary and sufficient condition for a given task system to be feasible and proposes a new feasibility decision algorithm based on that condition. The time complexity of this algorithm depends solely on the number of tasks. This condition can also be applied as a sufficient condition for a task system using priority inheritance protocols to be feasible with rate monotonic and deadline monotonic scheduling.

Random trees in queueing systems with deadlines

We survey our research on scheduling aperiodic tasks in real-time systems in order to illustrate the benefits of modelling queueing systems by means of random trees. Relying on a discrete-time single-server queueing system, we investigated deadline meeting properties of several scheduling algorithms employed for servicing probabilistically arriving tasks, characterized by arbitrary arrival and execution time distributions and a constant service time deadline T. Taking a non-queueing theory approach (i.e., without stable-stable assumptions) we found that the probability distribution of the random time sT where such a system operates without violating any task's deadline is approximately exponential with parameter λT = 1μT, with the expectation E[sT] = μT growing exponentially in T. The value μT depends on the particular scheduling algorithm, and its derivation is based on the combinatorial and asymptotic analysis of certain random trees. This paper demonstrates that random trees provide an efficient common framework to deal with different scheduling disciplines and gives an overview of the various combinatorial and asymptotic methods used in the appropriate analysis.

A reservation-based algorithm for scheduling both periodic and aperiodic real-time tasks

This paper considers the problem of scheduling both periodic and aperiodic tasks in real-time systems. A new algorithm, called reservation-based (RB), is proposed for ordering the execution of real-time tasks. This algorithm can guarantee all periodic-task deadlines while minimizing the probability of missing aperiodic-task deadlines. Periodic tasks are scheduled according to the rate monotonic priority algorithm (RMPA), and aperiodic tasks are scheduled by utilizing the processor time left unused by periodic tasks in each unit cycle. The length, u, of a unit cycle is defined as the greatest common divisor of all task periods, and a task is assumed to be invoked at the beginning of a unit cycle. For a set S of periodic tasks, the RB algorithm reserves a fraction R/sub s/ of processor time in each unit cycle for executing aperiodic tasks without missing any periodic-task deadline. The probability of meeting aperiodic-task deadlines is proved to be a monotonic increasing function of R/sub s/. We derive the value of R/sub s/ that maximizes the processor time reservable for the execution of aperiodic tasks without missing any periodic-task deadline. We also show that if the ratio of the computation time to the deadline of each aperiodic task is bounded by R/sub s/, the RB algorithm can meet the deadlines of all periodic and aperiodic tasks. Our analysis and simulation results show that the RB algorithm outperforms all other scheduling algorithms in meeting aperiodic-task deadlines.

Dynamic priority scheduling of periodic and aperiodic tasks in hard real-time systems

This paper addresses the problem of scheduling aperiodic tasks in real-time systems. The proposed scheme combines the Earliest-Deadline-First algorithm for scheduling periodic tasks with the Deferrable Server approach for servicing aperiodic tasks. Necessary and sufficient conditions are derived for guaranteeing feasibility of a given periodic task set when a deferrable server is present. An analytic model is proposed for selecting the best feasible period and computation time of the server to minimize the mean response time of aperiodic tasks. An evaluation of the proposed model using a simulator indicates that the server parameters selected by the model result in mean response times that are close to the best mean response time determined by the simulator.

Algorithms and complexity concerning the preemptive scheduling of periodic, real-time tasks on one processor

We investigate the preemptive scheduling of periodic, real-time task systems on one processor. First, we show that when all parameters to the system are integers, we may assume without loss of generality that all preemptions occur at integer time values. We then assume, for the remainder of the paper, that all parameters are indeed integers. We then give, as our main lemma, both necessary and sufficient conditions for a task system to be feasible on one processor. Although these conditions cannot, in general, be tested efficiently (unless P=NP), they do allow us to give efficient algorithms for deciding feasibility on one processor for certain types of periodic task systems. For example, we give a pseudo-polynomial-time algorithm for synchronous systems whose densities are bounded by a fixed constant less than 1. This algorithm represents an exponential improvement over the previous best algorithm. We also give a polynomial-time algorithm for systems having a fixed number of distinct types of tasks. Furthermore, we are able to use our main lemma to show that the feasibility problem for task systems on one processor is co-NP-complete in the strong sence. In order to show this last result, we first show the Simultaneous Congruences Problem to be NP-complete in the strong sense. Both of these last two results answer questions that have been open for ten years. We conclude by showing that for incomplete task systems, that is, task systems in which the start times are not specified, the feasibility problem is ∑ 2 p -complete.

Aperiodic task scheduling for Hard-Real-Time systems

This thesis develops the Sporadic Server (SS) algorithm for scheduling aperiodic tasks in real-time systems. The SS algorithm is an extension of the rate monotonic algorithm which was designed to schedule periodic tasks. This thesis demonstrates that the SS algorithm is able to guarantee deadlines for hard-deadline aperiodic tasks and provide good responsiveness for soft-deadline aperiodic tasks while avoiding the schedulability penalty and implementation complexity of previous aperiodic service algorithms. It is also proven that the aperiodic servers created by the SS algorithm can be treated as equivalently-sized periodic tasks when assessing schedulability. This allows all the scheduling theories developed for the rate monotonic algorithm to be used to schedule aperiodic tasks. For scheduling aperiodic and periodic tasks that share data, this thesis defines the interactions and schedulability impact of using the SS algorithm with the priority inheritance protocols. For scheduling hard-deadline tasks with short deadlines, an extension of the rate monotonic algorithm and analysis is developed. To predict performance of the SS algorithm, this thesis develops models and equations that allow the use of standard queueing theory models to predict the average response time of soft-deadline aperiodic tasks serviced with a high-priority sporadic server. Implementation methods are also developed to support the SS algorithm in Ada and on the Futurebus+.