Protected hard real-time

Real-time systems are those systems in which the correctness of the system depends not only on the logical results of computation but also on the time at which the results are produced [Stankovic 1988]. They span a broad spectrum of complexity from very simple microcontrollers in embedded systems (a microprocessor controlling an automobile engine) to highly sophisticated, complex, and distributed systems (air traffic control for the continental United States). Other real-time systems include command and control systems, process control systems, and multimedia and high-speed communication systems. Some real-time systems are beginning to add expert systems and other AI technology, creating additional requirements and complexities. At least three major trends in the real-time and embedded systems field have had a major impact on its technology: the increased growth and sophistication of embedded systems, the development of more scientific and technological results for hard real-time systems, and the advent of distributed multimedia. Most embedded systems consist of a small microcontroller and limited software situated within some product such as a microwave oven or automobile. However, with the increasing sophistication of such systems, powerful microcontrollers and digital signal processing (DSP) chips are commonly used, as are off-the-shelf real-time operating systems and design and debugging tools. Many people working with embedded systems deal on a daily basis with sensors and data acquisition technology and systems; others construct architectures based on single-board computers (many are still 68000-based, but RISC processors are being used more and more) and buses such as the VME bus. Many people are involved with the programming and debugging of embedded systems, largely using the C programming language and cross development and debugging platforms. Embedded systems may or may not have real-time constraints. Hard real-time systems are those in which missing an important deadline can cause severe consequences, even death. In this area, many fundamental results have been developed. For example, in real-time scheduling, rate monotonic analysis has enabled careful evaluation of many practical systems; the concept and analysis of competitive algorithms has provided important scheduling bounds and limits; and on-line planning has added flexible and dynamic capabilities to real-time systems. Operating systems research has produced predictable primitives, time-constrained synchronization techniques, and reservation and admission-control paradigms. Many other results exist in real-time architecture, fault tolerance, communication protocols, specification and design tools, formal verification, databases and object-oriented systems. Emphasis on all these areas is expected to increase in the foreseeable future. Many hard real-time systems are embedded systems. Distributed multimedia have produced a new set of soft real-time requirements. Real-time principles lie at the heart of distributed multimedia, but without the concomitant high reliability

Operating systems offering virtual memory and protected address spaces have been an elusive target of static worst-case execution time (WCET) analysis. This is due to a combination of size, unstructured code and tight coupling with hardware. As a result, hard real-time systems are usually developed without memory protection, perhaps utilizing a lightweight real-time executive to provide OS abstractions. This paper presents a WCET analysis of seL4, a third-generation micro kernel. seL4 is the world's first formally-verified operating-system kernel, featuring machine-checked correctness proofs of its complete functionality. This makes seL4 an ideal platform for security-critical systems. Adding temporal guarantees makes seL4 also a compelling platform for safety- and timing-critical systems. It creates a foundation for integrating hard real-time systems with less critical time-sharing components on the same processor, supporting enhanced functionality while keeping hardware and development costs low. We believe this is one of the largest code bases on which a fully context-aware WCET analysis has been performed. This analysis is made possible due to the minimalistic nature of modern micro kernels, and properties of seL4's source code arising from the requirements of formal verification.

9 - Software Structure of Controller

With the wide-spread using of LinuxOS, the Real-time requirement in certain applications is heighly demanded. So the Xenomai comes up, it has a higher priority than Linux kernel, so it can deal with the tasks which need real-time requirement.In order to set up the real-time Linux based on the ARM platform, the mini2440 development board contained s3c2440 as its MCU is used to set up the real-time operation system. Design and test the performs of the system. The results show that the embeded real-time Linux system based on the Xenomai has the good quality of real-time and can be used in the soft real-time system or the hard real-time system.

This paper examines a class of real-time control systems in which each control task triggers its next release based on the value of the last sampled state. Prior work used simulations to demonstrate that self-triggered control systems can be remarkably robust to task delay. This paper derives bounds on a task's sampling period and deadline to quantify how robust the control system's performance will be to variations in these parameters. In particular we establish inequality constraints on a control task's period and deadline whose satisfaction ensures that the closed-loop system's induced L 2 gain lies below a specified performance threshold. The results apply to linear time-invariant systems driven by external disturbances whose magnitude is bounded by a linear function of the system state's norm. The plant is regulated by a full-information H infin controller. These results can serve as the basis for the design of soft real-time systems that guarantee closed-loop control system performance at levels traditionally seen in hard real-time systems.

In hard real-time systems, the importance of meeting a task execution deadline is a central issue for correctness and reliability of such systems. The focus of this research is to design and implement a real-time system which utilizes a set of mobile robots that are performing individual and common critical tasks. Specifically, a scheduling scheme consists of local and global schedulers is developed to schedule critical tasks using a transputer-based machine. To guarantee execution of critical tasks within the requested time frame, each task is provided with a Safety Time Factor (STF) based on its worst-case computation time. This STF can be added to the execution time of a newly arrived task if its deadline and the status of scheduled tasks allow. The local scheduler takes the advantages of accumulated safety times and provides alternatives for scheduling sporadic critical tasks. The performance of the model is evaluated in terms of the number of critical tasks that are guaranteed and executed by local processors. >

This paper introduces a novel lightweight clock synchronization protocol (LCSP) and BUB clock synchronization algorithm based on real-time micro-kernel. Unlike other clock synchronization approaches which rely on redundant exchanged messages, this approach takes advantages of the traffic smoothing technique at each end node to suppress or block other traffics during the clock synchronization duration. The BUB algorithm efficiently decreases the number of synchronization messages exchanged during each synchronization round and can easily extend to large distributed real-time systems. Performance evaluation results prove that the LCSP protocol and the BUB algorithm can provide perfect synchronization precision and are appropriate for most distributed hard real-time and embedded control systems, when integrated with the traffic smoothing technique.

Real-time computer systems are characterized by the existence of timing constraints as well as logical correctness requirements on computations they carry out. The timing constraints are statically determined prior to system operation time from the characteristics of physical systems they interact with. In hard real-time systems, a timing failure is considered catastrophic and a guarantee should be given prior to runtime that every timing constraint will be satisfied throughout the system operation time. Examples are found in application domains such as avionics, process control, automated manufacturing, robotics, etc. In this paper, the issues and approaches on real-time system design will be addressed in uni-processor environment.

Evaluation of Level of Confidence and Optimization of Roll-back Recovery with Checkpointing for Real-Time Systems

Failure is a systemic error that affects overall system performance and may eventually crash across the entire configuration. In Real-Time Systems (RTS), deadline is the key to successful completion of the program. If tasks effectively meet the deadline, it means the system is working in pristine order. However, missing the deadline means a systemic fault due to which the system can crash (hard RTS) or degrade inclusive performance (soft RTS). To fine-tune the RTS, tolerance is the critical issue and must be handled with extreme care. This article explains the context of fault tolerance with improvised Joint EDF-RMS algorithm in RTS. The backup method has been derived to prevent the system from being recursively migrating the same task. If any task migrates three times, this migrated task will get shifted to the backup queue. This backup queue assigns the task to a backup processor and is destined for final execution. For performance evaluation purposes, a relative graph between fault and failure rates, failure and total processor utilization along with other averages have been evaluated. Furthermore, these archived results are compared with fault-tolerant Earliest Deadline First (EDF) and Rate Monotonic Scheduling (RMS) algorithms independently in relatively similar conditions. These comparisons show better performance against overloading conditions.

Task scheduling in real-time systems is a concept by which we can schedule the tasks according to their priorities. In hard real-time systems, the task scheduling is more important due to the chance of system failure. We discuss EDF task scheduling which is an optimal technique for the uniprocessor systems. The existing result of the QPA algorithm was applied on the uniprocessor system to check the schedulability of the tasks in uniprocessor system. We have extended the QPA algorithm to check the schedulability of the task sets in multiprocessor systems. Our algorithm is based on QPA algorithm and the utilization balanced algorithm. The proposed algorithm checks the absolute deadlines much faster than the existing QPA algorithm for uniprocessor real-time systems.

EPICS is used worldwide mostly for controlling accelerators and large experimental physics facilities. Although EPICS is well fit for the design and development of automation systems, which are typically VME or PLC-based systems, and for soft real-time systems, it may present several drawbacks when used to develop hard real-time systems/applications especially when general purpose operating systems as plain Linux are chosen. This is in particular true in fusion research devices typically employing several hard real-time systems, such as the magnetic control systems, that may require strict determinism, and high performance in terms of jitter and latency. Serious deterioration of important plasma parameters may happen otherwise, possibly leading to an abrupt termination of the plasma discharge. The MARTe framework has been recently developed to fulfill the demanding requirements for such real-time systems that are alike to run on general purpose operating systems, possibly integrated with the low-latency real-time preemption patches. MARTe has been adopted to develop a number of real-time systems in different Tokamaks. In this paper, we first summarize differences and similarities between EPICS IOC and MARTe. Then we report on a set of performance measurements executed on an x86 64 bit multicore machine running Linux with an IO control algorithm implemented in an EPICS IOC and in MARTe.

Recent trends towards increasing complexity in distributed embedded real-time systems pose challenges in designing and implementing a reliable system such as a self-driving car. The conventional way of improving reliability is to use redundant hardware to replicate the whole (sub)system. Although hardware replication has been widely deployed in hard real-time systems such as avionics, space shuttles and nuclear power plants, it is significantly less attractive to many applications because the amount of necessary hardware multiplies as the size of the system increases. The growing needs of flexible system design are also not consistent with hardware replication techniques. To address the needs of dependability through redundancy operating in real-time, we propose a layer called SAFER(System-level Architecture for Failure Evasion in Real-time applications) to incorporate configurable task-level fault-tolerance features to tolerate fail-stop processor and task failures for distributed embedded real-time systems. To detect such failures, SAFER monitors the health status and state information of each task and broadcasts the information. When a failure is detected using either time-based failure detection or event-based failure detection, SAFER reconfigures the system to retain the functionality of the whole system. We provide a formal analysis of the worst-case timing behaviors of SAFER features. We also describe the modeling of a system equipped with SAFER to analyze timing characteristics through a model-based design tool called SysWeaver. SAFER has been implemented on Ubuntu 10.04 LTS and deployed on Boss, an award-winning autonomous vehicle developed at Carnegie Mellon University. We show various measurements using simulation scenarios used during the 2007 DARPA Urban Challenge. Finally, we present a case study of failure recovery by SAFER when node failures are injected.

Accelerating the time to market for new embedded real-time systems is an imperative strategy nowadays. However, releasing poor-quality designs to the market can be dangerous, since a missed deadline in hard real-time systems can be catastrophic. Thus, early detection of potential problems in these systems is mandatory, since it may reduce the risks of fault propagations from early specification to the final code. This paper presents Calau, an environment for modeling and analyzing embedded real-time systems. Calau supports the mapping process of SysML State Machine diagram into a Time Petri Net with Energy consumption in order to analyze timing and energy consumption requirements in the early phases of embedded real-time systems development life cycle. It allows designers who do not have expertise in formal model, to design and analyze embedded system specifications quantitatively and qualitatively. We show the applicability of our tool by detailing an example.

Task migration is a potent instrument to exploit multi-core processors. Like full preemption, full migration is particularly advantageous as it allows the scheduler to relocate tasks at arbitrary times between cores. However, in hard real-time systems, migration is accompanied by a tremendous drawback: poor predictability and thus inevitable overapproximations in the worst-case execution-time analysis. This is due to the non-constant size of the tasks' resident set and the costs associated with its transfer between cores. As a result, migration is banned in many real-time systems, regressing the developer to a static allocation of tasks to cores with disadvantageous effects on the overall utilization and schedulability. In previous work, we successfully alleviated the shortcomings of full migration in real-time systems by reducing the associated costs and increasing its predictability. By employing static analysis, we were able to identify beneficial migration points and thus generate static schedules migrating tasks at these identified points. In ongoing work, we extend this approach to dynamic scheduling by providing information about advantageous migration points to an operating system which then makes migration decisions at runtime.