Fault-tolerant Real-time Systems Research Articles

Enhancing fault tolerance: dual Q-learning with dynamic scheduling

<span>Cloud computing has revolutionized IT delivery by offering scalable on-demand internet services encompassing software, platforms, and infrastructure. However, cloud services face significant performance challenges due to their susceptibility to failures given their vast operational scale. Implementing fault tolerance in dynamic cloud services is a key challenge, with complex configurations and dependencies complicating deployment. This paper introduces an innovative approach that combines double deep Q-learning (DDQL) with a dynamic fault-tolerant real-time scheduling algorithm (DFTRTSA) to enhance fault tolerance in real-time systems. DDQL, an extension of deep Q-learning, optimizes the fault-tolerance decision-making process. The algorithm adjusts scheduling strategies dynamically based on system conditions and errors. The fusion of DDQL and DFTRTSA aims to create a resilient and adaptive fault-tolerant mechanism, ensuring uninterrupted operation while meeting real-time requirements. This adaptive approach efficiently manages resources, meets deadlines, and gracefully handles errors, as demonstrated through experiments. Our DDQL-DFTRTSA method outperforms conventional fault-tolerant mechanisms in defect tolerance, energy efficiency, downtime reduction, and system dependability. It proves to be an ideal solution for real-time systems in dynamic and unpredictable environments.</span>

Energy management of fault-tolerant real-time embedded systems through switching-activity-based techniques

Energy management and fault tolerance are essential aspects in designing most real-time embedded systems, and dynamic voltage and frequency scaling (DVFS) is a fundamental approach in energy management. Using DVFS can lead to a considerable decrease in the system's reliability as the rate of transient faults increases due to low voltage and frequency. This study focuses on finding and implementing methods that address the following three issues in hard real-time embedded systems: energy consumption, fault tolerance, and time limitations. Switching activity depends on the nature of tasks. Since the switching activity is task-specific, the energy consumption of a task can be significantly greater than that of other tasks. As far as we know, previous studies on the power management of fault-tolerant real-time systems have not considered switching activity, so the impact of this factor is considered in this study. Multiple heuristic methods are proposed based on the switching activity factor to minimize energy consumption while sustaining the system's original reliability. In the proposed methods, the order of task execution is determined based on task switching activities. The methods are developed for both periodic and frame-based task models. The results show that the proposed methods can decrease the energy consumption by 12.7 %∼35.5 % for the periodic task model and 17.4 %∼45.2 % for the frame-based task model while preserving the system's reliability.

Implementation of processor cores of a fault-tolerant control system on FPGA under external control

The paper describes the choice of a technical solution for the implementation of a fault-tolerant real-time control system under external control. The main idea of the method is to introduce an external control block of the main control system that includes a watchdog and a backup decision-making algorithm. This block was supposed to be executed on an external FPGA-based block. The main system was supposed to be executed on the CPU. The paper proposes a variant of creating the entire system on a single FPGA. It implements both the processor cores for the operation of the main control system and control block. Various variants of cores for implementation on FPGA are considered. The paper studies suitable projects of processor cores for implementation on FPGA. It is concluded that the LEON series cores are suitable for the problem of building a fault-tolerant control system, since they already have technical solutions to improve fault tolerance. It is concluded that the proposed approach to building a fault-tolerant control system with external control can be fully implemented on a single FPGA. It has some advantages over implementation on separate blocks.

Implications of Non-Uniform Deadline Scaling to Quality of Service Under Single Errors

Fault-tolerant real-time systems for emerging critical applications like wearable electronic healthcare monitors, consumer-grade unmanned aerial vehicles, or environmental monitoring have to tolerate errors during operation. If they fail, the consequences are dire. But often their budgets for error mitigation capabilities are low, which requires to limit mitigation capabilities to the most critical functions of a system. Still, less critical functions of a system should operate as long as possible, but current state-of-the-art scheduling approaches either ignore them or suffer from low acceptance rates. Our fully static and verification friendly mixed-criticality approach guarantees mentioned systems that the most critical system functions are always available, and maximizes the time where less critical system functions are operational: We prove that our approach is feasible, and extends the system operation time while providing full service by a factor of 1.93 with a probability of 0.92. With 1.56 higher acceptance rates compared to similar state-of-the-art approaches, the integration of functionalities of different criticality in one fault-tolerant system succeeds more and more often, which especially benefits emerging critical applications with limited budgets for error mitigation.

MODIFIED, COMPLEMENTED TAXONOMY OF FAULTS IN FAULT-TOLERANT REAL-TIME SYSTEMS

This paper presents the main definitions relating to dependability. Basic definitions including reliability, security, maintainability, etc. are described first. They are then supplemented by additional definitions, which address to the threats of dependability (faults, errors, failures). Overlapping dependability standards, renumbering and integration can cause uncertainty when using of a certain definition. For this purpose, authors present complemented fault taxonomy for fault-tolerant real-time systems to eliminate inconsistencies and to unify existing fault taxonomies.

Comparison of Scheduling Algorithms in The Design of Fault Tolerant Real Time Systems

Most of the real time systems have the timing constraints. The main important timing constraints of any real time systems are to meet the deadlines of its application tasks. Not only satisfying the timing constraints of any real-time system, but also the functional correctness of application needs to be guaranteed. Meeting the dead line of application is no use if it deviates from its precise output. Timing constraints of the system can be satisfied by choosing proper task scheduling algorithms and the reliability of the system can be reached by providing fault-tolerance. In this paper, various fault scheduling algorithms like fixed priority, EDF(Earliest dead line first), LLF (Least Laxity First),Rate monotonic etc have been studied and compare the parameters like Worst-case execution times, response time, task missed deadlines, Number of preemption, number of context switches, Deadlock and processor utilization factor.

Schedulability of Fault Tolerant Real Time System Based on Local Optimum Checkpoint Under Priority Mixed Strategy

Against the defect of the real time system fault tolerant model that can only tolerate one fault, the Local optimum checkpoint (LOC) algorithm was proposed. Then according to the schedulability analysis, the worst case response time formula under the fault tolerant priority mixed strategy based on the local optimum checkpoint was deduced. Finally the Fault tolerant priority configuration search algorithm under Mixed strategy based on LOC (FTPCS-MS-LOC) was proposed. The FTPCS-MS-LOC can toleratemultiple transient faults. The algorithm can effectively reduce the search space compared to the enumeration method. The simulation shows that the FTPCS-MSLOC can significantly improve the system fault resilience than the fault tolerant priority inheritance, promotion and demotion strategy, and also the mixed strategy based on the checkpoint interval in the condition of single fault occurred.

Designing fault-tolerant real-time computer systems with diversified bus architecture for nuclear power plants

Fault-tolerant real-time computer (FT-RTC) systems are widely used to perform safe operation of nuclear power plants (NPP) and safe shutdown in the event of any untoward situation. Design requirements for such systems need high reliability, availability, computational ability for measurement via sensors, control action via actuators, data communication and human interface via keyboard or display. All these attributes of FT-RTC systems are required to be implemented using best known methods such as redundant system design using diversified bus architecture to avoid common cause failure, fail-safe design to avoid unsafe failure and diagnostic features to validate system operation. In this context, the system designer must select efficient as well as highly reliable diversified bus architecture in order to realize fault-tolerant system design. This paper presents a comparative study between CompactPCI bus and Versa Module Eurocard (VME) bus architecture for designing FT-RTC systems with switch over logic system (SOLS) for NPP.

Dynamic voltage and frequency scaling algorithm for fault-tolerant real-time systems

Many modern real-time systems (RTSs) are required to provide both fault tolerance and energy-efficiency in addition to their main objective to compute and deliver correct results within a specified period of time. Dynamic voltage and frequency scaling (DVFS) technique is known as one of the most effective low-energy technique for RTSs. However, most existing DVFS techniques only focus on minimizing energy consumption without taking the fault-tolerant capability of RTS into account. To solve this problem, in this paper we developed a new heuristic-based fault-tolerant dynamic voltage and frequency scaling (FT-DVFS) algorithm. The goal of the proposed algorithm is to find frequencies at which each task should be executed such that the energy consumed by the set of task is minimized. Beside energy minimization FT-DVFS algorithm has to meet all real-time requirements of individual tasks and to keep the system’s ability to tolerate transient faults via task re-execution. The simulation results show that the proposed approach could save a significant amount of energy while preserving the required level of system’s fault-tolerance capability when compared with the solutions obtained without energy-minimization.

Discrete feedback-based dynamic voltage scaling for safety critical real-time systems

Recently, the tradeoff between low energy consumption and high fault-tolerance has attracted a lot of attention as a key issue in the design of real-time systems. Dynamic Voltage Scaling (DVS) is commonly employed as one of the most effective low energy techniques for real-time systems. It has been observed that the use of feedback-based methods can improve the effectiveness of DVS-enabled systems. In this paper, we have investigated reducing the energy consumption of fault-tolerant hard real-time systems using the feedback control theory. Our proposed method makes the system capable of selecting the proper frequency and voltage settings in order to reduce the energy consumption, while guaranteeing hard real-time requirements in the presence of unpredictable workload fluctuations and faults. In the proposed method, the available slack-time is exploited by a feedback-based DVS at runtime to reduce the energy consumption. Furthermore, some slack-time is reserved for re-execution in case of faults. The simulation results show that compared with the traditional DVS method, our proposed method not only provides up to 59% energy saving, but also satisfies hard real-time constraints. Our proposed method is also effective in harnessing the static energy. The transition overheads are also taken into account in our simulation experiments.

XFTRTS - A XtratuM Based Fault-Tolerant Real-Time Control System

Nowadays there are a few works which are concerned with the virtualization technology and the fault-tolerance technology, because virtualization system can provide an environment allowing multiple operating systems running in concurrent way. In this paper, we based on a real-time hypervisor-XtratuM, propose the architecture of a fault-tolerant real-time control system (XFTRTS), which provide local backup execution and to support different level diversity including N-version programming on a single host. Furthermore, we implement a prototype of XFTRTS and test its important performance metric–latency, which is within two microseconds.

Optimal Checkpoint Placement on Real-Time Tasks with Harmonic Periods

This paper presents an optimal checkpoint strategy for fault-tolerance in real-time systems where transient faults occur in Poisson distribution. In our environment, multiple real-time tasks with different deadlines and harmonic periods are scheduled in the system by rate-monotonic algorithm, and checkpoints are inserted at a constant interval in each task. When a fault is detected, the system carries out rollback to the latest checkpoint and re-executes tasks. The maximum number of re-executable checkpoints and an equation to check schedulability are derived, and the optimal number of checkpoints is selected to maximize the probability of completing all the tasks within their deadlines.

임의 주기를 가지는 실시간 멀티 태스크를 위한 체크포인트 구간 최적화

This paper presents an optimal checkpoint strategy for fault-tolerance in real-time systems. In our environment, multiple real-time tasks with arbitrary periods are scheduled in the system by Rate Monotonic (RM) algorithm, and checkpoints are inserted at a constant interval in each task while the width of interval is different with respect to the task. We propose a method to determine the optimal checkpoint interval for each task so that the probability of completing all the tasks is maximized. Whenever a fault occurs to a checkpoint interval of a task, the execution time of the task would be prolonged by rollback and re-execution of checkpoints. Our scheme includes the schedulability test to examine whether a task can be completed with an extended execution time. A numerical experiment is conducted to demonstrate the applicability of the proposed scheme.

Feedback fault tolerance of real-time embedded systems

Fault tolerance is an important aspect in real-time computing.In real-time systems, tasks could be faulty due to various causes. Faulty tasks may compromise the safety and performance of the whole system and even cause disastrous consequences. In this paper, we study the possibilities of applying feedback control of software execution to real-time systems for fault tolerance purposes. A new fault tolerance architecture called ORTGA (On-demand Real-Time GuArd) is proposed. We argue the advantages and benefits of using ORTGA for fault tolerance in real-time systems. We also list research problems faced by ORTGA and point out directions for possible solutions. Throughout the paper, we use an example of real-time inverted pendulum control to illustrate ideas, problems and and possible solutions.

Combined time and information redundancy for SEU-tolerance in energy-efficient real-time systems

Recently, the tradeoff between energy consumption and fault-tolerance in real-time systems has been highlighted. These works have focused on dynamic voltage scaling (DVS) to reduce dynamic energy dissipation and on-time redundancy to achieve transient-fault tolerance. While the time redundancy technique exploits the available slack-time to increase the fault-tolerance by performing recovery executions, DVS exploits slack-time to save energy. Therefore, we believe there is a resource conflict between the time-redundancy technique and DVS. The first aim of this paper is to propose the use of information redundancy to solve this problem. We demonstrate through analytical and experimental studies that it is possible to achieve both higher transient fault-tolerance [tolerance to single event upsets (SEUs)] and less energy using a combination of information and time redundancy when compared with using time redundancy alone. The second aim of this paper is to analyze the interplay of transient-fault tolerance (SEU-tolerance) and adaptive body biasing (ABB) used to reduce static leakage energy, which has not been addressed in previous studies. We show that the same technique (i.e., the combination of time and information redundancy) is applicable to ABB-enabled systems and provides more advantages than time redundancy alone.

A Feasible Schedulability Analysis for Fault-Tolerant Real-Time Systems

Based on the worst-case response time schedulability analysis for fault-tolerant real-time systems, a new fault-tolerant priority assignment algorithm is proposed. This algorithm can be used, together with the schedulability analysis, to effectively improve system fault resilience when the two traditional fault-tolerant priority assignment policies can't improve system fault resilience. A fault-tolerant priority configuration search algorithm is also presented for the proposed analysis. The effectiveness of the proposed approach is evaluated by simulation.

Recent advances in modeling response-time distributions in real-time systems

Real-time systems are an important class of process control systems that need to respond to events under time constraints, or deadlines. Such systems may also be required to deliver service in spite of hardware or software faults in their components. This fault-tolerant characteristic is especially critical in systems whose failure can cause economic disaster and/or loss of lives. This paper reports recent research in the area of analytical modeling of the three major characteristics of real-time systems: timeliness, dependability, and external environmental dependencies. The paper starts with a brief introduction to analytical modeling frameworks such as Markov models and stochastic petri nets. This is followed by an examination of advances in modeling response-time distributions, reliability, distributed messaging services, and software fault-tolerance in real-time systems.

Design and Verification of Distributed Recovery Blocks with CSP

A case study on the application of Communicating Sequential Processes (CSP) to the design and verification of fault-tolerant real-time systems is presented. The distributed recovery block (DRB) scheme is a design technique for the uniform treatment of hardware and software faults in real-time systems. Through a simple fault-tolerant real-time system design using the DRB scheme, the case study illustrates a paradigm for specifying fault-tolerant software and demonstrates how the different behavioural aspects of a fault-tolerant real-time system design can be separately and systematically specified, formulated, and verified using an integrated set of formal techniques based on CSP.

VLSI communication interface for time-triggered real-time systems

This paper describes the design of a VLSI communication controller for the time-triggered protocol (TTP/C) as the result of a microelectronics development project done at Carinthia Tech Institute in cooperation with industrial partners. TTP/C is an emerging communication protocol for fault-tolerant real-time systems. Typical applications are safety-critical digital control systems such as drive-by-wire and fly-by-wire. We applied a VHDL-based design flow to implement digital standard cell logic, RAMs, flash memory and analog cells into a 27 mm2 chip using 0.35 μ CMOS technology. First fully-tested production samples are available and proved the design to be first time right. At this year’s SAE (Society of Automotive Engineers) World Congress in Detroit, from more than 5000 innovative products presented, the TTP controller IC was nominated as ‘Top Product of the Year’.

Modelling and simulation for mechatronic design in automotive systems

This paper gives an overview of current industry based projects in the field of vehicle modelling and simulation for the mechatronic design of automotive systems. It shows the wide range of applications for analysis and synthesis during the development process, including vehicle systems, vehicle dynamics, occupant safety, adaptive cruise control, hardware-in-the-loop and fault tolerant real-time systems.