Firm Real-time Systems Research Articles

Scheduling the tasks of multiple AGVs in a fault-tolerant control way

The paper concerns a development of an approach for implementing fault-tolerant control (FTC) of automated guided vehicles (AGVs). The main challenge considered in the article is the way of taking into account in the scheduling algorithm the delays occurring in the system resulting from the use of transport means. Mostly, multiple AGVs are designed and implemented as a firm real-time system in which delays are treated as faults. The proposed framework allows to compensate the faults in the real time. In the paper a multiple AGVs that transport item from an output of a production system to a high bay warehouse is presented. The article presents the non-deterministic nature of a multi-AGV system and includes a mathematical description of the system using (max, +) algebra. Having a analytical description of AGVs system, a new FTC strategy is proposed. Moreover, in the paper an illustrative case study showing the performance of the developed approach is presented.

An Energy-Efficient Periodic Resource Model for Bounded Delay-Tolerant Real-Time Systems

In the past few years, the energy consumption of real-time systems has become an essential topic of research, especially with the rise of the internet of things. In hard real-time systems, sensitivity to timing is the focal feature of system behaviors. Real-time systems must consider time constraints (mainly deadlines) to deliver the proper results. Therefore, typical real-time systems are inefficient in resource reservation, where resource supply is always higher than the workload demand and thus consumes more power than required. In firm and soft real-time systems, some deadline violations can be tolerated in a specific time interval. The worst-case execution time can be delayed to abound range. Therefore, instead of over-provisioning resources when considering the demand cannot be higher than supply, we propose a new algorithm that can efficiently estimate the resource reservation and reduce power consumption. The proposed algorithm is theoretically studied, and results show that while still satisfying the delay requirements of control systems, significant energy savings (up to 78.5%) could be achieved.

Performance maximization of energy-variable self-powered (m, k)-firm real-time systems

Many of today’s embedded devices, e.g. pacemakers or nodes in a monitoring sensor network, are expected to work energy perpetual, i.e. to be self-powered by energy harvesting from renewable sources. The major issue of such systems is the uncertainty of the available energy, influencing the application performance predictability. In many such applications, different performance levels are defined according to the patterns of job skipping. This paper proposes a performance maximization method for self-powered energy-intermittent (m, k)-firm systems via appropriate switching between the performance levels. To formally examine the impact of performance switch time on the performance-related criteria, we introduce the energy supply and energy demand functions. A sufficient schedulability test for one hyperperiod is also proposed for the preemptive fixed-priority as-soon-as-possible ($$PFP_{ASAP}$$) scheduling algorithm under variable-rate energy harvesting. We then propose a performance maximization heuristic, and compare its effectiveness to the optimal performance. The extensive simulations show that our proposed method is fast, whereas it effectively approximates the optimal solution.

Analytical Leakage-Aware Thermal Modeling of a Real-Time System

We consider a firm real-time system with a single processor working in two power modes depending on whether it is idle or executing a job. The system is equipped with dynamic thermal management through a cooling subsystem which can switch between two cooling modes. Real-time jobs which arrive to the system have stochastic properties and are prone to soft errors. A successful job is one that enters the system and completes its execution with no timing or soft error. Appropriateness of the system is evaluated based on its performance, temperature behavior, reliability, and energy consumption. It is noteworthy that these criteria have mutual interactions to each other: the stochastic nature of the system affects the success ratio of jobs beside the system dynamic power, the leakage as well as dynamic power impacts the processor temperature, this temperature affects the leakage power, the cooling subsystem power, and the soft error rate, which the latter in turn impacts the system reliability and the success ratio of jobs. This paper proposes an analytical evaluation method with a Markovian view to the system which considers these reciprocal effects. A number of simulation experiments are carried out to validate the accuracy of the proposed method.

Timed-automata based schedulability analysis for distributed firm real-time systems: a case study

The growing level of complexity of modern embedded systems, which are increasingly distributed and heterogeneous, calls for new design approaches able to reconcile mathematical rigour with flexibility, user-friendliness and scalability. In the last few years, Timed Automata (TA) have emerged as a very promising formalism for the availability of very effective verification tools. However, their adoption in the industrial community is still slow. The reluctance of industrial practitioners is partly motivated by persistent concerns on the ease of use of this formalism, on the scalability of the verification process and on the quality of the feedback that the designer receives. In this paper, we discuss these issues by showing the application of the TA formalism on a case study of industrial complexity. We expose the generality of the approach, the efficiency of state of the art tools, but also the limitations in the semantics and in dealing with free design parameters.

Performance Optimization Based on Analytical Modeling in a Real-Time System with Constrained Time/Utility Functions

We consider a single-processor firm real-time (FRT) system with exponential interarrival and execution times for jobs with relative deadlines following a general distribution. The scheduling policy of the system is first-come first-served (FCFS) and the capacity of the system is arbitrary. This system is subject to an arbitrary-shaped time/utility function (TUF), which determines the accrued utility of each job according to its completion time. It is considered that the system power consumption at different working states is predetermined for each processor speed. We have proposed an exact analytical method for the calculation of specific performance and power-related measures of the system. The resulting analytical formulations for the performance measures are functions of the processor speed and system capacity. These measures are optimized through appropriate selections of the speed using derivatives and the capacity employing numerical search methods. Some experimental results are presented for different unimodal TUFs in systems with deterministic and exponential relative deadlines. For the latter distribution, the results are compared against similar results obtained through simulation for the nonpreemptive earliest-deadline-first (NP-EDF) scheduling policy. The comparisons show that FCFS is superior to NP-EDF for some measures and TUFs.

Dynamic routing of real-time jobs among parallel EDF queues: A performance study

This paper introduces an analytical method for approximating the performance of a firm real-time system consisting of a number of parallel infinite-capacity single-server queues. The service discipline for the individual queues is earliest-deadline-first (EDF). Real-time jobs with exponentially distributed relative deadlines arrive according to a Poisson process. Jobs either all have deadlines until the beginning of service or deadlines until the end of service. Upon arrival, a job joins a queue according to a state-dependent stationary policy, where the state of the system is the number of jobs in each queue. Migration among the queues is not allowed. An important performance measure to consider is the overall loss probability of the system. The system is approximated by a Markovian model in the long run. The resulting model can then be solved analytically using standard Markovian solution techniques. Comparing numerical and simulation results for at least three different stationary policies, we find that the existing errors are relatively small.

Firm real-time system scheduling based on a novel QoS constraint

Many real-time systems have firm real-time requirements which allow occasional deadline violations but discard any jobs that are not finished by their deadlines. To measure the performance of such a system, a quality of service (QoS) metric is needed. Examples of often used QoS metrics for firm real-time systems are average deadline miss rates and (m, k)-firm constraints. However, for certain applications, these metrics may not be adequate measures of system performance. This paper introduces a novel QoS constraint for firm real-time systems. The new QoS constraint generalizes existing firm real-time constraints. Furthermore, using networked control system as an example, we show that this constraint can be directly related to the control system's performance. We then present three different scheduling approaches with respect to this QoS constraint. Experimental results are provided to show the effectiveness of these approaches.

On Optimistic Concurrency Control for Real-Time Database Systems

The performance of database transaction processing system can be profoundly affected by the concurrency control method employed since it is necessary to preserve database integrity in a multi-user environment. In addition to satisfying the consistency requirement as in traditional database system, real-time database systems must also satisfy timing constraints. In this study we present a virtual run policy for the restarted first run optimistic transactions and compare its performance with optimistic concurrency control in firm real-time database system.

Weakly hard real-time systems

In a hard real-time system, it is assumed that no deadline is missed, whereas, in a soft or firm real-time system, deadlines can be missed, although this usually happens in a nonpredictable way. However, most hard real-time systems could miss some deadlines provided that it happens in a known and predictable way. Also, adding predictability on the pattern of missed deadlines for soft and firm real-time systems is desirable, for instance, to guarantee levels of quality of service. We introduce the concept of weakly hard real-time systems to model real-time systems that can tolerate a clearly specified degree of missed deadlines. For this purpose, we define four temporal constraints based on determining a maximum number of deadlines that can be missed during a window of time (a given number of invocations). This paper provides the theoretical analysis of the properties and relationships of these constraints. It also shows the exact conditions under which a constraint is harder to satisfy than another constraint. Finally, results on fixed priority scheduling and response-time schedulability tests for a wide range of process models are presented.

Firm real-time systems

Firm real-time systems

Software architecture for a firm real-time database system

Abstract An actual development of a real-time database management system (RT-DBMS) must confront many new and practical problems which have been often ignored in previous work based on simulation. In addition to transaction scheduling, an RT-DBMS must deal with resource and data contention and enforce timing constraints. This paper discusses how current real-time technology has been applied to architect an RT-DBMS on a real-time microkernel operating system and how the problems have been addressed. We present the current software architecture of our StarBase RT-DBMS and address how it can be extended to support both guaranteed and non-guaranteed transaction processing.