Radu Dobrin

FSF: A Real-Time Scheduling Architecture Framework

Scheduling theory generally assumes that real-time systems are mostly composed of activities with hard real-time requirements. Many systems are built today by composing different applications or components in the same system, leading to a mixture of many different kinds of requirements with small parts of the system having hard real-time requirements and other larger parts with requirements for more flexible scheduling and for quality of service. Hard real-time scheduling techniques are extremely pessimistic for the latter part of the application, and consequently it is necessary to use techniques that let the system resources be fully utilized to achieve the highest possible quality. This paper presents a framework for a scheduling architecture that provides the ability to compose several applications or components into the system, and to flexibly schedule the available resources while guaranteeing hard real-time requirements. The framework (called FSF) is independent of the underlying implementation, and can run on different underlying scheduling strategies. It is based on establishing service contracts that represent the complex and flexible requirements of the applications, and which are managed by the underlying system to provide the required level of service.

Reducing the number of preemptions in fixed priority scheduling

Fixed priority scheduling (FPS) has been widely studied and used in a number of applications, mostly due to its flexibility, simple run-time mechanism and small overhead. However, preemption related overhead in FPS may cause undesired high processor utilization, high energy consumption, or, in some cases, even infeasibility. In this paper, we propose a method to reduce the number of preemptions in legacy FPS systems consisting of tasks with priorities, periods and offsets. Unlike other approaches, our algorithm does not require modification of the basic FPS mechanism. Our method analyzes offline a set of periodic tasks scheduled by FPS, detects the maximum number of preemptions that can occur at run-time, and reassigns task attributes such that the tasks are schedulable by the same scheduling mechanism, while achieving a significantly lower number of preemptions. In some cases, there is a cost to pay for a lower number of preemptions in terms of increased amount of tasks and/or reduced task execution flexibility. Our method provides for the ability to choose a user-defined number of preemptions with respect to the cost to pay.

Translating off-line schedules into task attributes for fixed priority scheduling

Off-line scheduling and fixed priority scheduling (FPS) are often considered as complementing and incompatible paradigms. A number of industrial applications demand temporal properties (predictability, jitter constraints, end-to-end deadlines, etc.) that are typically achieved by using off-line scheduling. The rigid off-line scheduling schemes used, however do not provide for flexibility. FPS has been widely studied and used in a number of applications, mostly due to its simple run-time scheduling, and small overhead. It can provide more flexibility, but is limited with respect to predictability, as actual start and completion times of execution depend on run-time events. In this paper we show how off-line scheduling and FPS run-time scheduling can be combined to get the advantages of both the capability to cope with complex timing constraints and flexibility. The paper assumes that a schedule for a set of tasks with complex constraints has been constructed off-line. It presents a method to analyze the off-line schedule and derive an FPS task set with FPS attributes priority, offset, and period, such that the runtime FPS execution matches the off-line schedule. It does so by analyzing the schedule and setting up inequality relations for the priorities of the tasks under FPS. Integer linear programming (ILP) is then used to find a FPS priority assignment that fulfils the relations. In case the priority relations for the tasks of the off-line schedule are not solvable we split tasks into the number of instances, to obtain a new task set with consistent task attributes. Our schedule translation algorithm keeps the number of newly generated artifact tasks minimal.

'State of the Art' in Using Agile Methods for Embedded Systems Development

Agile methods hold a significant promise to reduce cycle times and provide greater value to all key stakeholders involved in the software ecosystem. While these methods appear to be well suited for embedded systems development, their use has not become a widespread practice. In analyzing the state-of-the-art, as captured in published literature, we found that there are technical issues (requirements management, and testing), as well as organizational issues (process tailoring, knowledge sharing & transfer, culture change, and support infrastructure). In this paper, we build preliminary guidance for firms around these six areas and presented as a framework that will enable understanding the expected adoption trajectory.

Implementing off-line message scheduling on controller area network (CAN)

The controller area network (CAN) is widely used in a number of industrial applications. We present a method that shows how off-line scheduled messages can be scheduled on a CAN. The paper assumes that a schedule, for a set of tasks transmitting messages on a CAN, has been constructed off-line. We present a method that analyzes the off-line schedule and derives a set of periodic messages with fixed priorities, which can be scheduled on a CAN. Based on the information provided by the off-line schedule, the method derives inequality relations between the priorities of the messages under fixed priority scheduling protocols. In case the priority relations of the messages are not solvable, we split some messages into a number of artifacts, to obtain a new set of messages with consistent priorities. We use integer linear programming to minimize the final number messages.

Maximizing the Fault Tolerance Capability of Fixed Priority Schedules

Real-time systems typically have to satisfy complex requirements, mapped to the task attributes, eventually guaranteed by the underlying scheduler. These systems consist of a mix of hard and soft tasks with varying criticality, as well as associated fault tolerance requirements. Additionally, the relative criticality of tasks could undergo changes during the system evolution. Time redundancy techniques are often preferred in embedded applications and, hence, it is extremely important to devise appropriate methodologies for scheduling real-time tasks under failure assumptions.In this paper, we propose a methodology to provide a priori guarantees in fixed priority scheduling (FPS) such that the system will be able to tolerate one error per every critical task instance. We do so by using integer linear programming (ILP) to derive task attributes that guarantee re-execution of every critical task instance before its deadline, while keeping the associated costs minimized. We illustrate the effectiveness of our approach, in comparison with fault tolerant (FT) adaptations of the well-known rate monotonic (RM) scheduling, by simulations.

The Global Limited Preemptive Earliest Deadline First Feasibility of Sporadic Real-Time Tasks

The feasibility of preemptive and non-preemptive scheduling has been well investigated on uniprocessor and multiprocessor platforms under both Fixed Priority Scheduling (FPS) and Earliest Deadline First (EDF) paradigms. While feasibility of limited preemptive scheduling under FPS has been addressed on both uniprocssor and multiprocessor platforms, under EDF it has been investigated only on uniprocessors, and a similar analysis for multiprocessor platforms is still missing. In this paper, we introduce global Limited Preemptive Earliest Deadline First (g-LP-EDF) scheduling, and propose the associated feasibility analysis to complete the above described feasibility analysis spectrum. Specifically, we derive a sufficient condition that guarantees g-LP-EDF feasibility of sporadic real-time tasks which directly provides a global Non-Preemptive Earliest Deadline First (g-NP-EDF) feasibility test. We then study the interplay between g-LP-EDF feasibility and processor speed, in order to quantify the sub-optimality of g-NP-EDF in terms of the minimum speed-up required to guarantee g-NP-EDF feasibility of all feasible task sets. The results presented in this paper complement our previous results on uniprocessors, and provide a unified result on the sub-optimality of non-preemptive EDF on both uniprocessor and multiprocessor platforms.

VTV - A Voting Strategy for Real-Time Systems

Real-time applications typically have to satisfy high dependability requirements and require fault tolerance in both value and time domains. A widely used approach to ensure fault tolerance in dependable systems is the N-modular redundancy (NMR) which typically uses a majority voting mechanism. However, NMR primarily focuses on producing the correct value, without taking into account the time dimension. In this paper, we propose a new approach, Voting on Time and Value (VTV), applicable to real-time systems, which extends the modular redundancy approach by explicitly considering both value and timing failures, such that correct value is produced at a correct time, under specified assumptions. We illustrate our voting approach by instantiating it in the context of the well-known triple modular redundancy (TMR) approach. Further, we present a generalized version targeting NMR that enables a high degree of customization from the user perspective.

A new error handling algorithm for controller area network in networked control system

An effective error handling mechanism plays an important role to ensure the reliability and robustness of the application of controller area network (CAN) in controlling dynamic systems. This paper addresses a new online error handling approach or named per-sample-error-counting (PSeC) technique that tends to replace native error handling protocol in controller area network (CAN). The mechanism is designed to manage transmission errors of both sensor and control data in networked control system (NCS) used in controlling dynamic system such that the stability of the feedback system is preserved. A new parameter denoted as maximum allowable number of error burst (MAEB) is introduced in which MAEB is selected based on available bandwidth of the CAN network. MAEB serves as the maximum number of attempt of re-transmission of erroneous data per sample which allows the maximum transmission period to be known and guaranteed for time-critical control system. The efficacy of the proposed method is verified by applying the algorithm on the fourth order inverted pendulum system simulated on Matlab/Truetime simulator and the performance is benchmarked with the existing CAN error management protocol. The simulation run under various systems conditions demonstrate that the proposed method results in superior system performance in handling data transmission error as well as meeting control system requirement.

Error Modeling in Dependable Component-Based Systems

Component-based development (CBD) of software, with its successes in enterprise computing, has the promise of being a good development model due to its cost effectiveness and potential for achieving high quality of components by virtue of reuse. However, for systems with dependability concerns, such as real-time systems, a major challenge in using CBD consists of predicting dependability attributes, or providing dependability assertions, based on the individual component properties and architectural aspects. In this paper, we propose a framework which aims to address this challenge. Specifically, we present a revised error classification together with error propagation aspects, and briefly sketch how to compose error models within the context of component-based systems (CBS). The ultimate goal is to perform the analysis on a given CBS, in order to find bottlenecks in achieving dependability requirements and to provide guidelines to the designer on the usage of appropriate error detection and fault tolerance mechanisms.