Time-triggered Scheduling Research Articles

Time-triggered scheduling of mixed-critical flows at end-system in asynchronous AFDX avionic network

Avionics Full-DupleX (AFDX) is a switched Ethernet-based network used in modern commercial airplanes for the transmission of command and control avionics flows. These critical flows require deterministic guarantees leading to a lightly loaded network. Aircraft manufacturers envision to carry additional non avionics flows (i.e. video, audio, service) to take advantage of the spare bandwidth. However, it is then compulsory to preserve the real-time guarantees of avionics flows in terms of bounded jitter at the transmitter output and of bounded end-to-end latency. Depending on the type of additional traffic, Quality of Service (QoS) end-to-end guarantees may be offered to the additional flows of lower criticality in terms of reduced delay or bounded jitter for instance. These guarantees can be ensured by scheduling policies at transmitter and switch level. However, an important safety-related constraint is the asynchronous design of avionics distributed systems that prohibits the use of a network-wide synchronization of end systems and switches. Thus, time-triggered networking such as emerging Time-Triggered Ethernet (TTEthernet) or Time-Sensitive Networking (TSN) cannot be leveraged. This paper underlines the benefits of only scheduling flows at the transmitter, which is compatible with the asynchronous safety constraint of avionics systems. We show that it is possible to build a table schedule that carries both critical avionics flows and additional video flows that meet their timing and bandwidth allocation constraints. Therefore, we design scheduling strategies for efficient distribution of flows in the table scheduling whose performance is compared to the optimal schedule minimizing the emission lag of additional flows. Proposed heuristics are constructed such as to favor the network responsiveness for avionics flows thanks to slot over-provisioning. Extensive results on an A350 AFDX industrial configuration show that for a transmitter load of up to 70% of the available bandwidth, the uniform allocation heuristic provides a performance close to optimal in terms of minimum emission lag for additional flows, so offering a practical configuration heuristic to industry.

Sensor Scheduling for Remote State Estimation with Limited Communication Resources: A Time- and Event-Triggered Hybrid Approach.

This paper proposes a time- and event-triggered hybrid scheduling for remote state estimation with limited communication resources. A smart sensor observes a physical process and decides whether to send the local state estimate to a remote estimator via a wireless communication channel; the estimator computes the state estimate of the process according to the received data packets and the known scheduling mechanism. Based on the existing optimal time-triggered scheduling, we employ a stochastic event trigger to save precious communication chances and further improve the estimation performance. The minimum mean-squared error (MMSE) state estimate is derived since the Gaussian property is preserved. The estimation performance upper bound and communication rate are analyzed. The main results are illustrated by numerical examples.

Methods to Realize Preemption in Phased Execution Models

Phased execution models are a good solution to tame the increased complexity and contention of commercial off-the-shelf (COTS) multi-core platforms, e.g., Acquisition-Execution-Restitution (AER) model, PRedictable Execution Model (PREM). Such models separate execution from access to shared resources on the platform to minimize contention. All data and instructions needed during an execution phase are copied into the local memory of the core before starting to execute. Phased execution models are generally used with non-preemptive scheduling to increase predictability. However, the blocking time in non-preemptive systems can reduce schedulability. Therefore, an investigation of preemption methods for phased execution models is warranted. Although, preemption for phased execution models must be carefully designed to retain its execution semantics, i.e., the handling of local memory during preemption becomes non-trivial. This paper investigates different methods to realize preemption in phased execution models while preserving their semantics. To the best of our knowledge, this is the first paper to explore different approaches to implement preemption in phased execution models from the perspective of data management. We introduce two strategies to realize preemption of execution phases based on different methods of handling local data of the preempted task. Heuristics are used to create time-triggered schedules for task sets that follow the proposed preemption methods. Additionally, a schedulability-aware preemption heuristic is proposed to reduce the number of preemptions by allowing preemption only when it is beneficial in terms of schedulability. Evaluations on a large number of synthetic task sets are performed to compare the proposed preemption models against each other and against a non-preemptive version. Furthermore, our schedulability-aware preemption heuristic has higher schedulability with a clear margin in all our experiments compared to the non-preemptive and fully-preemptive versions.

Improving Symbolic System-Level Synthesis by Solver Coordination and Domain-Specific Heuristics

Deciding binding, routing, and scheduling within system synthesis for hard real-time systems can be a challenging task. Symbolic methods leveraging results from the area of satisfiability modulo theories (SMT) solving have shown to be scalable methods for this by splitting the work between a logic solver for routing and binding, and a background theory solver performing schedulability analysis. For these methods, in order to prune the search space of infeasible implementations efficiently, a feedback by the background theory is required. It can be observed that previous approaches might fail here as feedback cannot be derived within a reasonable amount of time. We propose a coordinated synthesis approach that overcomes this issue. Here, we leverage an answer set solver as logic solver that is enhanced with a scheduling-aware binding and routing refinement. Based on the answer set solver’s decisions for binding and routing, a background theory solver then computes time-triggered schedules to resolve resource access conflicts. If no feasible schedule exists, a feedback to the answer set solver can be derived within reasonable time. Our experiments synthesizing massively parallel hardware architectures show that our approach increases the applicability of symbolic synthesis considerably. While more than half of the investigated instances in our experiments cannot be solved in the non-coordinated approach already at small 2-dimensional 3×3 tiled mesh hardware architectures with 60% average computational utilization per tile, the coordinated synthesis approach scales up to 5×5 architectures with average utilization of 70% per tile (2.8× the hardware architecture size than before). Furthermore, we increase the scalability and the robustness of our approach by encoding our domain-knowledge within domain-specific heuristics in our designated answer set solver. Within our experiments, we observe that the domain-specific heuristics enable us to scale up to 6×6 architectures with 70% average utilization per tile.

Networked Motion Control for Smart EV With Multiple-Package Transmissions and Time-Varying Network-Induced Delays

This article aims to address the coupled problem of the multiple-package transmissions (MPT) and time-varying network-induced delays (TND) within the networked motion control systems of smart electric vehicles (EVs). The recent development of advance digital sensors, actuators, and controllers leverage the worldwide upgrade of transportation in terms of electrification and intellectualization. However, for smart EVs, this updating may challenge the in-vehicle network and then the vehicle control performance. In particular, the nonsynchronous MPT and the TND are the key issues in the vehicle's integrated motion control due to the absence of the synchronization mechanism and limitation of communication bandwidth. To deal with the effects of coupled MPT and TND, a hybrid schedule-control framework is developed, where a new MPT scheduler is developed with a flexible time-triggered schedule strategy with fractional-type basic period to eliminate the effect of the MPT and reorganize the TND to a bounded range of delay. Furthermore, an H∞-based linear quadratic regulator control approach is designed to deal with the uncertainties caused by the TND as well as guarantee the system's stability. The effectiveness of the proposed scheme is verified in three cases by the real-time hardware-in-the-loop bed tests.

A fault-tolerant time-triggered scheduling algorithm of mixed-criticality systems

A fault-tolerant time-triggered scheduling algorithm of mixed-criticality systems

Reliability-aware Scheduling and Routing for Messages in Time-sensitive Networking

Time-sensitive Networking (TSN) on Ethernet is a promising communication technology in the automotive and industrial automation industries due to its real-time and high-bandwidth communication capabilities. Time-triggered scheduling and static routing are often adopted in these areas due to high requirements on predictability for safety-critical applications. Deadline-constrained routing and scheduling in TSN have been studied extensively in past research. However, scheduling and routing with reliability requirements in the context of transient faults are not yet studied. In this work, we propose an Satisfiability Modulo Theory-based technique to perform scheduling and routing that takes both reliability constraints and end-to-end deadline constraints into consideration. Heuristics have been applied to improve the scalability of the solution. Extensive experiments have been conducted to demonstrate the efficiency of our proposed technique.

Survey on Periodic Scheduling for Time-triggered Hard Real-time Systems

This survey covers the basic principles and related works addressing the time-triggered scheduling of periodic tasks with deadlines. The wide range of applications and the increasing complexity of modern real-time systems result in the continually growing interest in this topic. However, the articles in this field appear without systematic notation. To address it, we extend the three-field Graham notation to cover periodic scheduling. Moreover, we formally define three example periodic scheduling problems (PSPs) and provide straightforward implementations of these examples in the Satisfiability Modulo Theories formalism with source codes. Then, we present a summary of the complexity results containing existing polynomially solvable PSPs. We also provide an overview of simple state-of-the-art methods and tricks to solve the PSPs efficiently in terms of time. Next, we survey the existing works on PSP according to the resource environment: scheduling on a single resource, on parallel identical resources, and on dedicated resources. In the survey, we indicate which works propose solution methods for more general PSPs. Finally, we present related problems that are not periodic by nature to provide inspiration for the PSP solution.

Towards Fully Jitterless Applications: Periodic Scheduling in Multiprocessor MCSs Using a Table-Driven Approach

In mixed criticality systems (MCSs), the time-triggered scheduling approach focuses on a special case of safety-critical embedded applications which run in a time-triggered environment. Sometimes, for these types of MCSs, perfectly periodical (i.e., jitterless) scheduling for certain critical tasks is needed. In this paper, we propose FENP_MC (Fixed Execution Non-Preemptive Mixed Criticality), a real-time, table-driven, non-preemptive scheduling method specifically adapted to mixed criticality systems which guarantees jitterless execution in a mixed criticality time-triggered environment. We also provide a multiprocessor version, namely, P_FENP_MC (Partitioned Fixed Execution Non-Preemptive Mixed Criticality), using a partitioning heuristic. Feasibility tests are proposed for both uniprocessor and homogenous multiprocessor systems. An analysis of the algorithm performance is presented in terms of success ratio and scheduling jitter by comparing it against a time-triggered and an event-driven method in a non-preemptive context.

Functional test environment for time-triggered control systems in complex MPSoCs

Many safety-critical and especially mixed-criticality computer systems are realized as a time-triggered (TT) system. Such systems execute one or more tasks according to a pre-determined scheduling. For the integration of functionality on such a TT MPSoC, not only the timing, but also the interplay of functional behavior and timing, needs to be validated.In this work we are proposing a new test environment for TT systems. Our proposed framework captures the TT system configuration, performs consistency checks and generates a fast executable simulation model. In the past, functional integration testing has been performed on a prototyping board, sometimes in a Hardware-in-the-Loop (HIL) configuration to support testing against a complex environment model. Compared to a simulation model, a HIL setup requires high effort, has a comparable low observability and is more difficult to debug. For this reason we propose the GALI (Globally Accurate, Locally Inaccurate) simulation model that combines an instruction accurate simulation engine with a global time-triggered scheduler. Thus, combining the low-level view on the implemented system with very good testability and debuggability.The proposed configuration and simulation platform accelerates the design and implementation of future TT systems. We evaluate our presented approach on a safety-relevant multirotor system.

Enhancing Real-Time and Determinacy for Network-Level Schedule in Distributed Mixed-Critical System

Time-triggered Ethernet (TTE) is designed as a deterministic real-time network for mixed-critical real-time systems, such as industrial automation, aerospace, and aviation. Due to asynchrony between tasks and the network in end systems (ESes), time-triggered (TT) messages have to spend uncertain time in waiting for being scheduled after they are generated. The waiting time uncertainly increases end to end delays for TT messages and will degrade the real-time and determinacy of TT messages sequentially. The contribution of this paper is to present a new architecture SDTTE to minimize the end to end delays, so as to enhance real-time and determinacy of TT messages. More specifically, a frame-based network model is built to describe distributed network resources. Under the network model, the end to end delay model with asynchrony between tasks and the network is analyzed. To match the generated time with the triggered time for TT messages, the triggered mechanism of the TT traffic in ESes is regarded as event strategy to detect the generated time of TT messages automatically. Based on this, the software defined TTE (SDTTE) is presented to optimize TT schedule online in switches. Furthermore, a simplified algorithm based on Satisfiability Modulo Theories (SMT) is proposed to satisfy real-time computing requirements. Finally, experiments with three network sizes verify the availability of SDTTE and analyze its performance. The results show that end to end delays for TT messages in SDTTE are decreased by about 95% compared with those in TTE. And the delays for rate-constrained and best-effort messages in SDTTE are almost as well as those in TTE. The processing time is less than 10s. In general, SDTTE provides a method to optimize end to end delays for TT messages in TTE, thus SDTTE has more determinacy and real-time than TTE. Meanwhile, SDTTE makes dynamic reconfiguration possible in practice.

An energy-efficient time-triggered scheduling algorithm for mixed-criticality systems

Real-time safety-critical systems are getting more complicated due to the introduction of mixed-criticality systems. The increasing use of mixed-criticality systems has motivated the real-time systems research community to investigate various non-functional aspects of these systems. Energy consumption minimization is one such aspect which is just beginning to be explored. In this paper, we propose a time-triggered dynamic voltage and frequency scaling (DVFS) algorithm for uniprocessor mixed-criticality systems. We show that our algorithm outperforms the predominant existing algorithm which uses DVFS for mixed-criticality systems with respect to minimization of energy consumption. In addition, ours is the first energy-efficient time-triggered algorithm for mixed-criticality systems. We prove an optimality result for the proposed algorithm with respect to energy consumption. Then we extend our algorithm for tasks with dependency constraints.

A hierarchical architecture for time- and event-triggered real-time systems

Abstract This paper proposes an architecture for combining the execution of time- and event-triggered real-time task sets. This makes it possible for the designer to choose the most appropriate mechanism depending on the role and nature of each task in the system. The proposed architecture allows one to choose the priority levels at which time- and event-triggered tasks are executed. This gives the designer an additional degree of freedom to make compromise decisions upon contradicting timing requirements, such as granting reduced jitter and at the same time providing prompt service to non-periodic events, for example. The proposed model is accompanied with a Ravenscar implementation of the time-triggered scheduler and a library of utilities for specifying time-triggered schedules and reusing time-triggered task patterns.

Methods for Large-Scale Time-Triggered Network Scheduling

Future cyber–physical systems may extend over broad geographical areas, like cities or regions, thus, requiring the deployment of large real-time networks. A strategy to guarantee predictable communication over such networks is to synthesize an offline time-triggered communication schedule. However, this synthesis problem is computationally hard (NP-complete), and existing approaches do not scale satisfactorily to the required network sizes. This article presents a segmented offline synthesis method which substantially reduces this limitation, being able to generate time-triggered schedules for large hybrid (wired and wireless) networks. We also present a series of algorithms and optimizations that increase the performance and compactness of the obtained schedules while solving some of the problems inherent to segmented approaches. We evaluate our approach on a set of realistic large-size multi-hop networks, significantly larger than those considered in the existing literature. The results show that our segmentation reduces the synthesis time by up to two orders of magnitude.

Multi-Variant Scheduling of Critical Time-Triggered Communication in Incremental Development Process: Application to FlexRay

The portfolio of models offered by car manufacturing groups often includes many variants (i.e., different car models and their versions). With such diversity in car models, variant management becomes a formidable task. Thus, there is an effort to keep the variants as close as possible. This simple requirement forms a big challenge in the area of communication protocols. When several vehicle variants use the same signal, it is often required to simultaneously schedule such a signal in all vehicle variants. Furthermore, new vehicle variants are designed incrementally in such a way as to maintain backward compatibility with the older vehicles. Backward compatibility of time-triggered schedules reduces expenses relating to testing and fine-tuning of the components that interact with physical environment (e.g., electromagnetic compatibility issues). As this requirement provides for using the same platform, it simplifies signal traceability and diagnostics, across different vehicle variants, besides simplifying the reuse of components and tools. This paper proposes an efficient and robust heuristic algorithm, which creates the schedules for internal communication of new vehicle variants. The algorithm provides for variant management by ensuring compatibility among the new variants, besides preserving backward compatibility with the preceding vehicle variants. Based on the results of the proposed algorithm, the impact of maintaining compatibility among new variants and of preserving backward compatibility with the preceding variants on the scheduling procedure is examined and discussed. Thanks to the execution time of the algorithm, which is less than one second, the network parameters like the frame length and cycle duration are explored to find their best choice concerning the schedule feasibility.

Time-triggered scheduling table generation method based on schedulability ranking

Time-triggered scheduling table generation method based on schedulability ranking

Ravenscar Support for Time-Triggered Scheduling

This position paper follows from a previous proposal to integrate a time-triggered scheduler in a priority- based, preemptive scheduler such as that supported by Ada's task dispatching policy FIFO Within Priorities . The resulting combined scheduling carries the advantages of both time-triggered and priority-based scheduling, and helps mitigating their drawbacks. The paper presents a system model for the time-triggered subsystem that extends the original proposal, and describes a Ravenscar implementation of the scheduler at the run-time system level, in the form of a new package Ada.Dispatching.TTS. Multiple programming patterns can be implemented on top of this scheduler. With respect to the previously proposed full-Ada implementation, only patterns that implied the use of asynchronous transfer of control have been excluded. On the other hand, the extension of the original model enables new patterns, not supported in our previous proposal, using the new types of continuation and optional slots. We hold that bringing the time-triggered paradigm to Ravenscar is both feasible and convenient for the High-Integrity and Embedded application domains.

Session Summary

This position paper follows from a previous proposal to integrate a time-triggered scheduler in a priority- based, preemptive scheduler such as that supported by Ada's task dispatching policy FIFO Within Priorities . The resulting combined scheduling carries the advantages of both time-triggered and priority-based scheduling, and helps mitigating their drawbacks. The paper presents a system model for the time-triggered subsystem that extends the original proposal, and describes a Ravenscar implementation of the scheduler at the run-time system level, in the form of a new package Ada.Dispatching.TTS. Multiple programming patterns can be implemented on top of this scheduler. With respect to the previously proposed full-Ada implementation, only patterns that implied the use of asynchronous transfer of control have been excluded. On the other hand, the extension of the original model enables new patterns, not supported in our previous proposal, using the new types of continuation and optional slots. We hold that bringing the time-triggered paradigm to Ravenscar is both feasible and convenient for the High-Integrity and Embedded application domains.

Tightening Contention Delays While Scheduling Parallel Applications on Multi-core Architectures

Multi-core systems are increasingly interesting candidates for executing parallel real-time applications, in avionic, space or automotive industries, as they provide both computing capabilities and power efficiency. However, ensuring that timing constraints are met on such platforms is challenging, because some hardware resources are shared between cores. Assuming worst-case contentions when analyzing the schedulability of applications may result in systems mistakenly declared unschedulable, although the worst-case level of contentions can never occur in practice. In this paper, we present two contention-aware scheduling strategies that produce a time-triggered schedule of the application’s tasks. Based on knowledge of the application’s structure, our scheduling strategies precisely estimate the effective contentions, in order to minimize the overall makespan of the schedule. An Integer Linear Programming (ILP) solution of the scheduling problem is presented, as well as a heuristic solution that generates schedules very close to ones of the ILP (5% longer on average), with a much lower time complexity. Our heuristic improves by 19% the overall makespan of the resulting schedules compared to a worst-case contention baseline.

Time-Triggered Scheduling of Mixed-Criticality Systems

Real-time and embedded systems are moving from the traditional design paradigm to integration of multiple functionalities onto a single computing platform. Some of the functionalities are safety critical and subject to certification. The rest of the functionalities are nonsafety critical and do not need to be certified. Designing efficient scheduling algorithms which can be used to meet the certification requirement is challenging. Our research considers the time-triggered approach to scheduling of mixed-criticality jobs with two criticality levels. The first proposed algorithm for the time-triggered approach is based on the OCBP scheduling algorithm which finds a fixed-priority order of jobs. Based on this priority order, the existing algorithm constructs two scheduling tables S LO oc and S HI oc . The scheduler uses these tables to find a scheduling strategy. Another time-triggered algorithm called MCEDF was proposed as an improvement over the OCBP-based algorithm. Here we propose an algorithm which directly constructs two scheduling tables without using a priority order. Furthermore, we show that our algorithm schedules a strict superset of instances which can be scheduled by the OCBP-based algorithm as well as by MCEDF. We show that our algorithm outperforms both the OCBP-based algorithm and MCEDF in terms of the number of instances scheduled in a randomly generated set of instances. We generalize our algorithm for jobs with m criticality levels. Subsequently, we extend our algorithm to find scheduling tables for periodic and dependent jobs. Finally, we show that our algorithm is also applicable to mixed-criticality synchronous programs upon uniprocessor platforms and schedules a bigger set of instances than the existing algorithm.