Sara Afshar

Support for hierarchical scheduling in FreeRTOS

This paper presents the implementation of a Hierarchical Scheduling Framework (HSF) on an open source real-time operating system (FreeRTOS) to support the temporal isolation between a number of applications, on a single processor. The goal is to achieve predictable integration and reusability of independently developed components or applications. We present the initial results of the HSF implementation by running it on an AVR 32-bit board EVK1100. The paper addresses the fixed-priority preemptive scheduling at both global and local scheduling levels. It describes the detailed design of HSF with the emphasis of doing minimal changes to the underlying FreeRTOS kernel and keeping its API intact. Finally it provides (and compares) the results for the performance measures of idling and deferrable servers with respect to the overhead of the implementation.

Resource sharing among prioritized real-time applications on multiprocessors

In this paper, we propose a new protocol for handling resource sharing among prioritized real-time applications composed on a multiprocessor platform. We propose an optimal priority assignment algorithm which assigns unique priorities to the applications based on information in their interfaces. We have performed experimental evaluations to compare the proposed protocol (called MSOS-Priority) to the current state of the art locking protocols under multiprocessor partitioned scheduling, i.e., MPCP, MSRP, FMLP, MSOS, and OMLP. The evaluations show that MSOS-Priority mostly performs significantly better than alternative approaches.

Resource Sharing under Multiprocessor Semi-partitioned Scheduling

Semi-partitioned scheduling has become the subject of recent interest for multiprocessors due to better utilization results, compared to conventional global and partitioned scheduling algorithms. Under semi-partitioned scheduling, a major group of tasks are assigned to fixed processors while a low number of tasks are allocated to more than one processor. Various task assigning techniques have recently been proposed in a semi-partitioned environment. However, a synchronization protocol for resource sharing among tasks in semi-partitioned scheduling has not yet been investigated. In this paper we propose and evaluate two protocols for handling resource sharing under semi-partitioned scheduling in multiprocessor platforms. The main challenge addressed in this paper is to serve the resource requests of tasks that are assigned to different processors.

Flexible spin-lock model for resource sharing in multiprocessor real-time systems

Various approaches can be utilized upon resource locking for mutually exclusive resource access in multiprocessor platforms. So far two conventional approaches exist for dealing with tasks that are blocked on a global resource in a multi-processor platform. Either the blocked task performs a busy wait, i.e. spins, at the highest priority level until the resource is released, or it is suspended. Although both approaches provide mutually exclusive access to resources, they can introduce long blocking delays to tasks, which may be unacceptable for many industrial applications. In this paper, we propose a general spin-based model for resource sharing in multiprocessor platforms in which the priority of the blocked tasks during spinning can be selected arbitrarily. Moreover, we provide the analysis for two selected spin-lock priorities and we show by means of a general comparison as well as specific examples that these solutions may provide a better performance for higher priority tasks.

Towards resource sharing under multiprocessor semi-partitioned scheduling

Semi-partitioned scheduling has been the subject of recent interest, compared with conventional global and partitioned scheduling algorithms for multiprocessors, due to better utilization results. In semi-partitioned scheduling most tasks are assigned to fixed processors while a low number of tasks are split up and allocated to different processors. Various techniques have recently been proposed to assign tasks in a semi-partitioned environment. However, an appropriate resource sharing mechanism for handling the resource requests between tasks in semi-partitioned scheduling has not yet been investigated. In this paper we propose two methods for handling resource sharing under semi-partitioned scheduling in multiprocessor platforms. The main challenge is to handle the resource requests of tasks that are split over multiple processors.

Optimal priority and threshold assignment for fixed-priority preemption threshold scheduling

Fixed-priority preemption-threshold scheduling (FPTS) is a generalization of fixed-priority preemptive scheduling (FPPS) and fixed-priority non-preemptive scheduling (FPNS). Since FPPS and FPNS are incomparable in terms of potential schedulability, FPTS has the advantage that it can schedule any task set schedulable by FPPS or FPNS and some that are not schedulable by either. FPTS is based on the idea that each task is assigned a priority and a preemption threshold. While tasks are admitted into the system according to their priorities, they can only be preempted by tasks that have priority higher than the preemption threshold. This paper presents a new optimal priority and preemption threshold assignment (OPTA) algorithm for FPTS which in general outperforms the existing algorithms in terms of the size of the explored state-space and the total number of worst case response time calculations performed. The algorithm is based on back-tracking, i.e. it traverses the space of potential priorities and preemption thresholds, while pruning infeasible paths, and returns the first assignment deemed schedulable. We present the evaluation results where we compare the complexity of the new algorithm with the existing one. We show that the new algorithm significantly reduces the time needed to find a solution. Through a comparative evaluation, we show the improvements that can be achieved in terms of schedulability ratio by our OPTA compared to a deadline monotonic priority assignment.

Resource sharing under global scheduling with partial processor bandwidth

Resource efficient approaches are of great importance for resource constrained embedded systems. In this paper, we present an approach targeting systems where tasks of a critical application are partitioned on a multi-core platform and by using resource reservation techniques, the remaining bandwidth capacity on each core is utilized for one or a set of non-critical application(s). To provide a resource efficient solution and to exploit the potential parallelism of the extra applications on the multi-core processor, global scheduling is used to schedule the tasks of the non-critical applications. Recently a specific instantiation of such a system has been studied where tasks do not share resources other than the processor. In this paper, we enable semaphore-based resource sharing among tasks within critical and non-critical applications using a suspension-based synchronization protocol. Tasks of non-critical applications have partial access to the processor bandwidth. The paper provides the systems schedulability analysis where blocking due to resource sharing is bounded. Further, we perform experimental evaluations under balanced and unbalanced allocation of tasks of a critical application to cores.

Agent-Centred Approach for Assuring Ethics in Dependable Service Systems

As the world enters the information era, more and more dependable services controlling and even making our decisions are moved to the ubiquitous smart devices. While various standards are in place to impose the societal ethical norms on decision-making of those devices, the rights of the individuals to satisfy their own moral norms are not addressed with the same scrutiny. Hence, the right of the individuals to reason on their own and evaluate morality of certain decisions is at stake. In this work we propose an agent-centred approach for assuring ethics in dependable technological service systems. We build upon assurance of safety and security and propose the notion of ethics assurance case as a way to assure that individual users have been made aware of all the ethically challenging decisions that might be performed or enabled by the service provider. We propose a framework for identifying and categorising ethically challenging decisions, and documenting the ethics assurance case. We apply the framework on an illustrative example.

An optimal spin-lock priority assignment algorithm for real-time multi-core systems

Support for exclusive access to shared (global) resources is instrumental in the context of embedded real-time multi-core systems, and mechanisms for achieving such access must be deterministic and efficient. There exist two traditional approaches for multiprocessors when a task requests a global resource that is locked by a task on a remote core: a spin-based approach, i.e. non-preemptive busy waiting for the resource to become available, and a suspension-based approach, i.e. the task relinquishes the processor. A suspension-based approach can be viewed as a spin-based approach where the lowest priority on a core is used during spinning, similar to a non-preemptive spin-based approach where the highest priority on a core is used. By taking such a view, we previously provided a general model for spinning, where any arbitrary priority can be used for spinning, i.e. from the lowest to the highest priority on a core. Targeting partitioned fixed-priority preemptive scheduled multiprocessors and spin-based approaches that use a fixed priority for spinning per core for all tasks, we aim at increasing the schedulability of multiprocessor systems by using the spin-lock priority per core as parameter. In this paper, we present (i) a generalization of the traditional worst-case response-time analysis for non-preemptive spin-based approaches addressing an arbitrary but fixed spin-lock priority per core, (ii) an optimal spin-lock priority assignment (OSPA) algorithm per core, i.e. an algorithm that will find a fixed spin-lock priority per core that will make the system schedulable, whenever such an assignment exists and, (iii) comparative evaluations of the OSPA algorithm with the spin-based and suspension-based approaches where OSPA showed up to 38% improvement compared to both approaches.1

Per processor spin-based protocols for multiprocessor real-time systems

Two traditional approaches exist for a task that is blocked on a global resource; a task either performs a non-preemptive busy wait, i.e., spins, or suspends and releases the processor. Previously, ...