Reinder J. Bril

Controller Area Network (CAN) schedulability analysis: Refuted, revisited and revised

Controller Area Network (CAN) is used extensively in automotive applications, with in excess of 400 million CAN enabled microcontrollers manufactured each year. In 1994 schedulability analysis was developed for CAN, showing how worst-case response times of CAN messages could be calculated and hence guarantees provided that message response times would not exceed their deadlines. This seminal research has been cited in over 200 subsequent papers and transferred to industry in the form of commercial CAN schedulability analysis tools. These tools have been used by a large number of major automotive manufacturers in the design of in-vehicle networks for a wide range of cars, millions of which have been manufactured during the last decade.This paper shows that the original schedulability analysis given for CAN messages is flawed. It may provide guarantees for messages that will in fact miss their deadlines in the worst-case. This paper provides revised analysis resolving the problems with the original approach. Further, it highlights that the priority assignment policy, previously claimed to be optimal for CAN, is not in fact optimal and cites a method of obtaining an optimal priority ordering that is applicable to CAN. The paper discusses the possible impact on commercial CAN systems designed and developed using flawed schedulability analysis and makes recommendations for the revision of CAN schedulability analysis tools.

Worst-case response time analysis of real-time tasks under fixed-priority scheduling with deferred preemption

Fixed-priority scheduling with deferred preemption (FPDS) has been proposed in the literature as a viable alternative to fixed-priority pre-emptive scheduling (FPPS), that obviates the need for non-trivial resource access protocols and reduces the cost of arbitrary preemptions.This paper shows that existing worst-case response time analysis of hard real-time tasks under FPDS, arbitrary phasing and relative deadlines at most equal to periods is pessimistic and/or optimistic. The same problem also arises for fixed-priority non-pre-emptive scheduling (FPNS), being a special case of FPDS. This paper provides a revised analysis, resolving the problems with the existing approaches. The analysis is based on known concepts of critical instant and busy period for FPPS. To accommodate for our scheduling model for FPDS, we need to slightly modify existing definitions of these concepts. The analysis assumes a continuous scheduling model, which is based on a partitioning of the timeline in a set of non-empty, right semi-open intervals. It is shown that the critical instant, longest busy period, and worst-case response time for a task are suprema rather than maxima for all tasks, except for the lowest priority task. Hence, that instant, period, and response time cannot be assumed for any task, except for the lowest priority task. Moreover, it is shown that the analysis is not uniform for all tasks, i.e. the analysis for the lowest priority task differs from the analysis of the other tasks. These anomalies for the lowest priority task are an immediate consequence of the fact that only the lowest priority task cannot be blocked. To build on earlier work, the worst-case response time analysis for FPDS is expressed in terms of known worst-case analysis results for FPPS. The paper includes pessimistic variants of the analysis, which are uniform for all tasks, illustrates the revised analysis for an advanced model for FPDS, where tasks are structured as flow graphs of subjobs rather than sequences, and shows that our analysis is sustainable.

QoS control strategies for high-quality video processing

Video processing in software is often characterized by highly fluctuating, content-dependent processing times, and a limited tolerance for deadline misses. We present an approach that allows close-to-average-case resource allocation to a single video processing task, based on asynchronous, scalable processing, and QoS adaptation. The QoS adaptation balances different QoS parameters that can be tuned by user-perception experiments: picture quality, deadline misses, and quality changes. We model the balancing problem as a discrete stochastic decision problem, and propose two closely related solution strategies, for which the processing-time statistics are determined offline and at run time, respectively. We enhance both strategies with a compensation for structural (non-stochastic) load fluctuations. Finally, we validate our approach by means of simulation experiments, and conclude that both enhanced strategies perform close to the theoretical optimum.

Worst-Case Response Time Analysis of Real-Time Tasks under Fixed-Priority Scheduling with Deferred Preemption Revisited

Fixed-priority scheduling with deferred preemption (FPDS) has been proposed in the literature as a viable alternative to fixed-priority pre-emptive scheduling (FPPS), that obviates the need for non-trivial resource access protocols and reduces the cost of arbitrary preemptions. This paper shows that existing worst-case response time analysis of hard real-time tasks under FPDS, arbitrary phasing and relative deadlines at most equal to periods is pessimistic and/or optimistic. The same problem also arises for fixedpriority non-pre-emptive scheduling (FPNS), being a special case of FPDS. This paper provides a revised analysis, resolving the problems with the existing approaches. The analysis is based on known concepts of critical instant and busy period for FPPS. To accommodate for our scheduling model for FPDS, we need to slightly modify existing definitions of these concepts. The analysis assumes a continuous scheduling model, which is based on a partitioning of the timeline in a set of non-empty, right semi-open intervals. It is shown that the critical instant, longest busy period, and worst-case response time for a task are suprema rather than maxima for all tasks, except for the lowest priority task, i.e. that instant, period, and response time cannot be assumed. Moreover, it is shown that the analysis is not uniform for all tasks, i.e. the analysis for the lowest priority task differs from the analysis of the other tasks. These anomalies for the lowest priority task are an immediate consequence of the fact that only the lowest priority task cannot be blocked. To build on earlier work, the worst-case response time analysis for FPDS is expressed in terms of known worst-case analysis results for FPPS. The paper includes pessimistic variants of the analysis, which are uniform for all tasks.

Multimedia QoS in consumer terminals

Over the past years, there has been a considerable amount of research in the field of QoS support for (distributed) multimedia systems, ie, multimedia processing in, for example, a (networked) workstation environment. QoS for multimedia systems is about media processing in software, using dynamically scalable functions, and trading resources for quality. Unlike QoS for mainstream multimedia systems, QoS support for high volume electronics (HVE) consumer terminals (CT), such as digital TV sets, digitally improved analog TV sets and STB (set-top boxes), has received little attention in the literature. This paper considers multimedia QoS for consumer terminals, with focus on the high-quality video domain.

Generalizing Consistency Checking between Software Views

Inconsistencies between software views are a source of errors for software systems. In this paper we present a general approach that aids in finding inconsistencies between different views. This approach supports both intra phase consistency checking and inter phase consistency checking. The approach is suitable for detecting consistency problems between, for example, multiple diagrams in a UML design as well as between a design and an implementation. The approach is based on verification of rules using relation partition algebra. In this paper, we present two types of rules: obligations and the more commonly used constraints, which can be viewed as lower bounds and upper bounds, respectively. To check consistency between views, rules are derived from one view, the so-called prevailing view, and imposed on another view, the so-called subordinate view. Because our approach does not prescribe which views are prevailing, it can be used in any arbitrary process. Violations to rules can be expressed in terms of either the prevailing view or the subordinate view. Exceptions to rules are easiliy embedded in our general approach to consistency checking.

Integrating hardware limitations in CAN schedulability analysis

The existing schedulability analysis for the Controller Area Network (CAN) does not take into account that a CAN controller has finite buffer space to store outgoing messages and high priority messages may suffer from priority inversion if the buffers are already occupied by low priority messages. This gives rise to an additional delay for high priority messages, which, if not considered, may result in a deadline violation. In this paper, we explain the cause of this additional delay and extend the existing CAN schedulability analysis to integrate it. Finally, we suggest implementation guidelines that minimizes both the run-time CPU overhead and the additional delay due to priority inversion.

Exact response-time analysis for fixed-priority preemption-threshold scheduling

Fixed-priority preemption-threshold scheduling (FPTS) has been proposed as a generalization of fixed-priority preemptive scheduling (FPPS) and fixed-priority non-preemptive scheduling (FPNS) with the aim to improve schedulability and reduce run-time overheads. In this paper, we show that the existing worst-case response time (WCRT) analysis for FPTS is pessimistic and present an exact WCRT analysis. Moreover, we refine the task model for FPTS, making FPTS also a generalization of fixed-priority scheduling with deferred preemption (FPDS). Finally, we present exact analysis for FPTS for this refined task model and an example showing that FPTS can improve on FPPS and FPDS.

Overrun and Skipping in Hierarchically Scheduled Real-Time Systems

Recently, two SRP-based synchronization protocols for hierarchically scheduled real-time systems based on Fixed- Priority Preemptive Scheduling (FPPS) have been presented, i.e., HSRP [9] and SIRAP [4]. Preventing depletion of budget during global resource access, the former implements an overrun mechanism, while the later exploits a skipping mechanism. A theoretical comparison of the performance of these mechanisms revealed that none of them was superior to the other, as their performance is heavily dependent on the system’s parameters. To better understand the relative strengths and weaknesses of these mechanisms, this paper presents a comparative evaluation of the depletion prevention mechanisms overrun (with or without payback) and skipping. These mechanisms are investigated in detail and the corresponding system load imposed by these mechanisms is explored in a simulation study. The mechanisms are evaluated assuming FPPS and a periodic resource model [23]. The periodic resource model is selected as it supports locality of schedulability analysis, allowing for a truthful comparison of the mechanisms. Given system characteristics, guiding the design of hierarchically scheduled real-time systems, the results of this paper indicate when one mechanism is better than the other and how a system should be configured in order to operate efficiently.

Maintaining a legacy: towards support at the architectural level

An organization that develops large, software intensive systems with a long lifetime will encounter major changes in the market requirements, the software development environment, including its platform, and the target platform. In order to meet the challenges associated with these changes, software development has to undergo major changes as well. Especially when these systems are successful, and hence become an asset, particular care shall be taken to maintain this legacy; large systems with a long lifetime tend to become very complex and difficult to understand. Software architecture plays a vital role in the development of large software systems. For the purpose of maintenance, an up-to-date explicit description of the software architecture of a system supports understanding and comprehension of it, amongst other things. However, many large complex systems do not have an up-to-date documented softwarearchitecture. Particularly in cases where these systems have a long lifetime, the (natural) turnover of personnel will make it very likely that many employees contributing to previous generations of the system are no longer available. A need to ‘recover’ the software architecture of the system may become prevalent, facilitating the understanding of the system, providing ways to improve its maintainability and quality and to control architectural changes. This paper gives an overview of an on-going effort to improve the maintainability and quality of a legacy system, and describes the recent introduction of support at the architectural level for program understanding and complexity control. Copyright