Priority Ceiling Protocol Research Articles

Real-time Scheduling and Resource Management for Energy Autonomous Sensors

Energy autonomous sensors are attracting increasing attention because they have potential to provide monitoring for long-time and continuous measurement in many application areas such as transport and health. Energy harvesting can guarantee energy autonomy also called energy neutrality over long period of time if adequate power management strategies are implemented in sensors. As most of them have to execute multiple time critical tasks which access to shared resources in mutual exclusion, classical scheduling strategies should be revisited to provide efficient energy aware solutions. The ED-H scheduler was proved optimal to guarantee energy neutrality of uniprocessor real-time energy harvesting systems when all the tasks are assumed independent. In this paper, we propose to refine it and we introduce a new scheduling scheme, namely ED-H/DPCP which is adapted to DPCP, the well-known Dynamic Priority Ceiling Protocol to support resource access control. The paper provides the schedulability analysis attached to ED-H/DPCP. We also discuss the implementation and we present integration of ED-H/DPCP within a cobotic application.

Resource-Oriented Partitioning for Multiprocessor Systems with Shared Resources

Predictable scheduling and resource sharing primitives are fundamental aspects of real-time systems. To prevent race conditions, access to shared resources must ensure mutual exclusion, e.g., using semaphores. Further, real-time locking protocols are required to avoid un-controlled priority inversions. For uniprocessor systems, the Priority Ceiling Protocol (PCP) has been widely accepted and supported in real-time operating systems. However, it remains arguable as to whether there exists a preferable approach for resource sharing in multiprocessor systems. In this paper, we show that the proposed Resource-Oriented Partitioned (ROP) scheduling with a distributed resource sharing policy, originating from the concept of the Distributed Priority Ceiling Protocol (DPCP), can achieve a non-trivial speedup factor guarantee. Specifically, we prove that the proposed R-PCP-rm-rm algorithm achieves a speedup factor of $11-6/(m+1)$11-6/(m+1) on a platform consisting of $m$m processors, where each job of a task may request at most one shared resource at most one time. Our empirical evaluations show that the proposed algorithm is highly effective in terms of task sets deemed schedulable.

Energy-Efficient Concurrency Control for Dynamic-Priority Real-Time Tasks with Abortable Critical Sections

In this paper, we are interested in energy-efficient concurrency control for real-time tasks on a non-ideal DVS processor. Based on well-known ceiling-based concurrency control protocols (such as priority ceiling protocol (PCP) and stack resource policy (SRP)), researchers have proposed energy-efficient approaches to mange concurrent accesses to shared resources so that the energy consumption can be reduced. However, ceiling-based protocols have a problem of ceiling blocking which imposes a great impact on the performance of real-time systems. In order to achieve sufficient performance, we propose a new protocol, called conditional abortable stack resource policy (CA-SRP), to resolve the ceiling blocking problem for dynamic-priority real-time tasks by incorporating a conditional abort rule into SRP. Based on the schedulability analysis of CA-SRP, we also propose a method, called dynamic speed assignment (DSA), to dynamically calculate and assign proper processor speeds for task execution so that the energy consumption can be reduced further. The capabilities of our proposed CA-SRP and DSA have been evaluated by a series of experiments, for which we have encouraging results.

A misconception in blocking time analyses under multiprocessor synchronization protocols

In multiprocessor systems, synchronization protocols can result in non-trivial (remote) blocking, which can cause timing impacts in real-time systems. To analyze the schedulability or the worst-case response time, it is essential to calculate a safe upper bound on the maximum remote blocking time due to the synchronization protocols. The derivation of the upper bound is sometimes unsafe in the literature when the analyses improperly adopt a misconception that assumes the well-known critical instant theorem. In this paper, we show that the original analyses for the distributed priority ceiling protocol and the multiprocessor priority ceiling protocol are unsafe in the calculation of the blocking time due to this misconception. This results in repetition of unsafe timing analyses in the literature. This paper also provides a simple remedy for such a flaw.

Real-Time Reconfigurations of Embedded Control Systems

This paper deals with the study of the reconfiguration of embedded control systems with safety following component-based approaches from the functional level to the operational level. The authors define the architecture of the Reconfiguration Agent which is modelled by nested state machines to apply local reconfigurations. They propose in this journal paper technical solutions to implement the whole agent-based architecture, by defining UML meta-models for both Control Components and also agents. To guarantee safety reconfigurations of tasks at run-time, they define service and reconfiguration processes for tasks and use the semaphore concept to ensure safety mutual exclusions. As a method to ensure the scheduling between periodic tasks with precedence and mutual exclusion constraints, the authors apply the priority ceiling protocol.

A spiral process of formalization and verification: A case study on verification of the scheduling mechanism of OSEK/VDX

Formalization and verification of a system usually are not one time tasks due to the increasing complexity of software systems. The relation between formalization and verification should not be sequential but iterative in that verification follows formalization and in turn helps validate and refine formalization. The iteration is a spiral process with a formal model being incrementally developed and more properties being verified. In this paper, we present such a spiral process of doing formalization and verification with a concrete case study to demonstrate how we formalize and verify in the spiral manner a scheduling mechanism and Priority Ceiling Protocol (PCP) of an industrial automobile standard called OSEK/VDX. We choose an algebraic formal language called CafeOBJ for its features of modularity and interactive theorem proving functionality. We start with a prototypical model of the scheduling mechanism, validate and refine it based on verification results. By theorem proving, it reinforces our understanding of the specifications and their gap with the specified problem domains. The formal model is refined until all these properties are successfully proved. We incrementally extend it to formalize PCP and verify more properties such as deadlock freedom and priority inversion freedom.

HLC-PCP: A resource synchronization protocol for certifiable mixed criticality scheduling

We propose HLC-PCP (Highest-Locker Criticality, Priority-Ceiling Protocol), which extends the well-known Priority Ceiling Protocol (PCP) to be applicable to AMC (Adaptive Mixed-Criticality), a variant of MCS. We present methods for worst-case blocking time computation with HLC-PCP, used for schedulability analysis of AMC with resource sharing, for both the dual-criticality model and the general multi-criticality model. This helps relax one of the key limiting assumptions of most MCS work, that is, tasks with different levels of criticality do not have common shared resources. Today's safety-critical Cyber-Physical Systems (CPS) often need to integrate multiple diverse applications with varying levels of importance, or criticality. Mixed-Criticality Scheduling (MCS) has been proposed with the objectives of achieving certification at multiple criticality levels and efficient utilization of hardware resources. Current work on MCS typically assumes tasks at different criticality levels are independent and do not share any resources (data). We propose HLC-PCP (Highest-Locker Criticality, Priority-Ceiling Protocol), which extends the well-known Priority Ceiling Protocol (PCP) to be applicable to AMC (Adaptive Mixed-Criticality), a variant of MCS. We present methods for worst-case blocking time computation with HLC-PCP, used for schedulability analysis of AMC with resource sharing, for both the dual-criticality model and the general multi-criticality model.

Efficient Implementation of IPCP and DFP

Most resource control protocols such as IPCP (Immediate Priority Ceiling Protocol) require a kernel system call to implement the necessary control over the shared protected object. This switch can be expensive, involving a potentially slow switch from CPU user-mode to kernel-mode (and back). In this paper we look at two anticipatory schemes (IPCP and DFP - Deadline Floor Protocol) and show how they can be implemented with the minimum number of calls on the kernel. Specifically, no kernel calls are needed when there is no contention, and only one when there is. A standard implementation would need two such calls.

Improved Blocking Time Analysis and Evaluation for the Multiprocessor Priority Ceiling Protocol

The Multiprocessor Priority Ceiling Protocol (MPCP) is a classic suspension-based real-time locking protocol for partitioned fixed-priority (P-FP) scheduling. However, existing blocking time analysis is pessimistic under the P-FP + MPCP scheduling, which negatively impacts the schedulability for real-time tasks. In this paper, we model each task as an alternating sequence of normal and critical sections, and use both the best-case execution time (BCET) and the worst-case execution time (WCET) to describe the execution requirement for each section. Based on this model, a novel analysis is proposed to bound shared resource requests. This analysis uses BCET to derive the lower bound on the inter-arrival time for shared resource requests, and uses WCET to obtain the upper bound on the execution time of a task on critical sections during an arbitrary time interval of Δt. Based on this analysis, improved blocking analysis and its associated worst-case response time (WCRT) analysis are proposed for P-FP + MPCP scheduling. Schedulability experiments indicate that the proposed method outperforms the existing methods and improves the schedulability significantly.

Energy-Efficient Real-Time Scheduling of Tasks with Abortable Critical Sections

In this paper, an energy-efficient scheduling algorithm, called ceiling-based conditional abortable scheduling (CB-CAS) algorithm, is proposed to schedule periodic hard real-time tasks in a non-ideal DVS processor. Based on the schedulability analysis, CB-CAS calculates a proper processor speed for task execution so that the energy consumption can be reduced without violating the timing constraints of tasks. For saving more energy, we also assume that the critical sections of tasks are abortable, which is a strategy originally proposed to reduce priority inversions. In this paper, CB-CAS introduces a conditional abort rule and a dynamic speed adjustment method to work with the rate monotonic scheduling algorithm and the priority ceiling protocol so that the energy consumption could be reduced further. Whenever two tasks are conflicting for the same resource, CB-CAS will examine the cost of blocking the higher-priority task and the cost of aborting the lower-priority task. CB-CAS will abort the lower-priority task and adjust the processor speed dynamically only if it is more energy efficient. The schedulability analysis and the properties of CB-CAS are given in this paper. The capabilities of CB-CAS were also evaluated by a series of experiments, for which we have some encouraging results.

Resolving priority inversions in composable conveyor systems

Abstract The well known problem of priority inversions that occurs in classical real-time systems also manifests in decentralized cyber-physical systems. Using a specific example of composable conveyor systems, we show how priority inversions hinder the transport of entities through the conveyor systems. We present a novel adaptation of the classical priority inheritance protocol for resolving these cyber-physical priority inversions. While the approach resolves cyber-physical priority inversions, the structure and constraints of the conveyor systems cause the jitter associated with the end-to-end latency of the highest priority parts to increase. Further, these constraints also limit the applicability of the classical priority ceiling protocol in this class of cyber-physical systems. Simulation results demonstrate the correctness and reasonable communication overhead of the approach.

HLC-PCP: A Resource Synchronization Protocol for Certifiable Mixed Criticality Scheduling

Today’s safety-critical Cyber-Physical Systems (CPS) often need to integrate multiple diverse applications with varying levels of importance, or criticality. Mixed-criticality scheduling (MCS) has been proposed with the objectives of achieving certification at multiple criticality levels and efficient utilization of hardware resources. Current work on MCS typically assumes tasks at different criticality levels are independent and do not share any resources (data). We propose highest-locker criticality, priority-ceiling protocol (HLC-PCP), which extends the well-known priority ceiling protocol (PCP) to be applicable to adaptive mixed-criticality (AMC), a variant of MCS. We present methods for worst-case blocking time computation with HLC-PCP, used for schedulability analysis of AMC with resource sharing.

A Blocking-Aware Scheduling for Real-Time Task Synchronization Using a Leakage-Controlled Method

Due to the importance of power dissipation in the wireless sensor networks and embedded systems, real-time scheduling has been studied in terms of various optimization problems. Real-time tasks that synchronize to enforce mutually exclusive access to the shared resources could be blocked by lower priority tasks. While dynamic voltage scaling (DVS) is known to reduce dynamic power consumption, it causes increased blocking time due to lower priority tasks that prolong the interval over which a computation is carried out. Additionally, processor slowdown to increase execution time implies greater leakage energy consumption. In this paper, a leakage-controlled method is proposed, which decreases both priority inversion and power consumption. Based on priority ceiling protocol (PCP) and a graph reduction technique, this method can decrease more energy consumption and avoid priority inversion for real-time tasks.

Functional safety of adaptive embedded control systems: new solutions

This paper deals with the functional safety of embedded control system following component-based technologies. The functional safety in a device of the interconnected control components is handled by an agent that controls and adapts the local behaviour at run-time. The interaction between control agents in devices is ensured via the communication protocol based on a coordination agent. We propose technical solutions to implement the whole agent-based architecture, by defining UML meta-models for both control components and also agents. At the operational level, an OS task is assumed to be a set of components having some properties independently from any real-time operating system. To guarantee safety reconfiguration of OS tasks at run-time, we define service and reconfiguration processes for safety. We apply the priority ceiling protocol as a method to ensure the scheduling between periodic tasks with precedence and mutual exclusion constraints.

Exploring the design space of multiprocessor synchronization protocols for real-time systems

The goal of this paper is to explore the design space of protocols for multiprocessor systems with static priority and partitioned scheduling. The design space is defined by a set of characteristics that can vary from one protocol to another. This exploration presents new protocols with different characteristics from existing ones. These new protocols are considered variations of the Multiprocessor Priority Ceiling Protocol (MPCP), but they can also be seen as variations of the Flexible Multiprocessor Locking Protocol (FMLP), since they include features common to both protocols. Schedulability tests are provided for these new variations and they are compared with the original versions of MPCP and FMLP. Such comparisons include an empirical comparison of schedulability and an overhead evaluation of a real implementation. Such comparisons show that these new variations are actually competitive in relation to the existing protocols.

Reduction of Context Switches due to Task Synchronization in Uniprocessor and Multiprocessor Platform

The problem of frequent context switches in multitasking is a real scheduling overhead which wastes extra CPU cycles, memory and causes much delay in scheduling. This paper focuses on reducing the context switches that result due to blocking when jobs are required to synchronize. The Priority Ceiling Protocol (PCP) is used to synchronize the tasks in uniprocessor as well as multiprocessor platforms. The jobs are scheduled using Earliest Deadline First (EDF) policy. The simulation results show that the context switches are reduced by about 20% on an average using our technique of avoiding context switches due to blocking.

Comparative analysis of real time resource access control protocols using UML 2.0

The behaviors of real time software systems do not depend only on the values of input and output signals, but also on the times of their occurrences. Real time systems (RTS) interact with their environments using time constrained input/output signals. The complexity of Real Time Systems is continually increasing which makes their design very challenging. In RTS, the scheduling of tasks with hard deadlines has been an important area of research. Unified Modeling Language (UML), the standard visual object-oriented modeling language, is suitable to deal with this complexity. The main objective of this paper is to compare two protocols using UML 2.0 models. The shortcomings of the existing Priority Inheritance protocol are represented using one UML model. Further, the Stack Based Priority Ceiling protocol is used to overcome this difficulty using an improved model.

Ada 2012

As from Ada 2005, the language has supported different dispatching policies including fixed priority (FP) and earliestdeadline first (EDF). EDF dispatching can be applied across the whole range of priorities or across a restricted range. In this way, EDF scheduling is integrated into a FP framework. Baker's Stack Resource Policy (SRP) is also integrated with Ada's immediate priority ceiling protocol to support resource sharing using protected objects. However, with the introduction of support for global, partitioned and cluster scheduling for multiprocessor systems, the use of protected objects in applications needs to be re-evaluated. This is because Ada does not completely define how protected objects are accessed in a multiprocessor environment and because the SRP cannot be directly applied to multiprocessors. This paper reviews the currently available multiprocessor resource allocation policies and analyzes their applicability to Ada. It proposes some new Ada mechanisms that would facilitate the programming of a variety of protected object access protocols.

Static analysis of interrupt-driven programs synchronized via the priority ceiling protocol

We consider programs for embedded real-time systems which use priority-driven preemptive scheduling with task priorities adjusted dynamically according to the immediate ceiling priority protocol. For these programs, we provide static analyses for detecting data races between tasks running at different priorities as well as methods to guarantee transactional execution of procedures. Beyond that, we demonstrate how general techniques for value analyses can be adapted to this setting by developing a precise analysis of affine equalities.

Real-time synchronization on distributed architecture with Ada-2005

Ada is found in most Avionics processing. It is often the hidden software that makes life safe in systems that fly most passenger/private aircraft. In an effort to promote the use of Ada-2005 in more embedded systems, we will present a Distributed Stack Resource Policy (DSRP) suitable for synchronizing tasks executing on distributed systems, meaning the protocol relies only on message-passing between processors. Accordingly, we will present an empirical evaluation that tests the performance of the DSRP with the Distributed Priority Ceiling Protocol (DPCP) created by Rajkumar. To the best of our knowledge, no empirical distributed-based evaluation of the DPCP has ever been conducted prior to this. We will also discuss the efficacy of Ada and examine the resulting design issues and trade-offs, and the ability of the DSRP to manage multiple shared resources efficiently on distributed processors.