Fixed Priority Research Articles

A Unified Blocking Analysis for Parallel Tasks With Spin Locks Under Global Fixed Priority Scheduling

Spin locks are widely used in embedded systems to coordinate mutually exclusive accesses to shared resources from different tasks. Although the design and analysis of locking protocols have been intensively studied for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sequential</i> real-time tasks, there have been few works on this topic for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">parallel</i> real-time tasks. In this paper, we study the analysis of parallel real-time tasks modeled by directed acyclic graphs (DAGs) under global fixed priority scheduling using both preemptable and non-preemptable spin locks to protect accesses to shared resources in three commonly used request serving orders (unordered, FIFO-order and priority-order). In particular, we develop a general schedulability analysis framework where the blocking time caused by resource contention is formally defined, so that the blocking analysis can be performed independently and easy to combine with the traditional interference analysis techniques. Moreover, we present a unified blocking analysis technique where the blocking time is analyzed in a scalable manner based on a linear-programming (LP) approach, making our method flexible and extendable. We conduct comprehensive experiments to evaluate our method with other the-state-of-the-art approaches for scheduling real-time parallel tasks using semaphores and spin locks.

An exact comparison of global, partitioned, and semi-partitioned fixed-priority real-time multiprocessor schedulers

We evaluate the performance of global, partitioned, and semi-partitioned fixed-priority ( FP ) multiprocessor schedulers for sporadic real-time tasks. By conducting a range of exhaustive experiments, we support a prior observation of Baruah regarding the incomparability of global and partitioned fixed-priority schedulers, by demonstrating the existence of multiple task sets, which are schedulable by either one or another approach, but not both simultaneously. In addition, we demonstrate that a suboptimal semi-partitioning heuristic, named Deadline Monotonic with Priority Migration ( DM - PM ), in certain cases outperforms both global and partitioned FP schedulers. Unlike other works, our evaluation is based on an exact schedulability test for global scheduling, as well as relies on optimal task priorities for global FP , and an optimal partitioning of tasks to processors. However, due to the exponential computational complexity of the available exact schedulability test, our evaluation is constrained to a limited number of tasks in a set. On average, partitioned scheduling performs better for a lower number of heavy tasks in a task set, while global scheduling is the opposite. The schedulability gain of one approach over another might exceed 50%, for both cases. To make such an evaluation possible, we derive a method to compute optimal task priorities for global FP by using a set of pruning rules. We demonstrate that the gain of such optimal priorities over other widely used priority assignment heuristics, including Deadline Monotonic ( DM ), can be significant, in certain cases. Moreover, we evaluate the performance of an optimal task partitioning over the available suboptimal partition heuristics, namely first-fit decreasing (FFD) and worst-fit decreasing (WFD), and confirm no significant performance difference between them, for considered task settings. Software tools used in our evaluation are implemented using C++ libraries, and are publicly available.

Excluding Parallel Execution to Improve Global Fixed Priority Response Time Analysis

Response Time Analysis (RTA) is an effective method for testing the schedulability of real-time tasks on multiprocessor platforms. Existing RTAs for global fixed priority scheduling calculate the upper bound of the worst case response time of each task. Given a target task, existing RTAs first calculate the workload upper bound of each higher priority task (than the target task), and then calculate the interference on the target task by each higher priority task according to the obtained workload upper bounds. The workload of a task consists of three parts: carry-in, body and carry-out. The interference from all these three parts may be overestimated in existing RTAs. However, although the overestimation of the interference from body is the major factor that causes the low accuracy of existing RTAs, all existing work only focuses on how to reduce the overestimation of the interference from carry-in, and there is no method to reduce the overestimation of the interference from body or carry-out. In this work, we propose a method to calculate the lower bound of the accumulative time in which the target task and higher priority tasks are executed in parallel. By excluding the parallel execution time from the interference, we derive a new RTA test that can reduce the overestimation of the interference from all three parts of the workload. Extensive experiments are conducted to verify the superior performance of the proposed RTA test.

Schedulability Analysis of Global Scheduling for Multicore Systems With Shared Caches

Shared caches in multicore processors introduce serious difficulties in providing guarantees on the real-time properties of embedded software due to the interaction and the resulting contention in the shared caches. To address this problem, we develop a new schedulability analysis for real-time multicore systems with shared caches, globally scheduled by Earliest Deadline First (EDF) and Fixed Priority (FP) algorithms. We construct an integer programming formulation, which can be transformed to an integer linear programming formulation, to calculate an upper bound on cache interference exhibited by a task within a given execution window. Using the integer programming formulation, an iterative algorithm is presented to obtain the upper bound on cache interference a task may exhibit during one job execution. The upper bound on cache interference is subsequently integrated into the schedulability analysis to derive a new schedulability condition. A range of experiments is performed to investigate how the schedulability is degraded by shared cache interference. We also evaluate the schedulability performance of EDF against FP scheduling over randomly generated tasksets. Our empirical evaluations show that EDF is better than FP scheduling in terms of the number of task sets deemed schedulable.

Real-time scheduling under heterogeneous routing for industrial Internet of Things

Industrial Internet of Things (IIoT) provide a promising opportunity for building efficient industrial wireless systems by leveraging the growing ubiquity of sensor nodes. However, IIoT exhibits strict reliability and has a high requirement for real-time communications. Furthermore, different criticality level flows are coexisted in many IIoT. The system must guarantee the reliability and real-time performance of high-criticality flows to avoid disasters. To address this issue, this paper first proposes a heterogeneous routing model in which both source and graph routing coexist. Then, we propose Relative-execution Deadline First (RDF) scheduling and prove it has a better performance than the fixed priority (FP) and earliest deadline first (EDF) methods. By extending RDF in mixed-criticality IIoT, a Mixed-Criticality Relative-execution Deadline scheduling algorithm (MCRD) has proposed, which can further improve system schedulability by making a trade-off between reliability and real-time performance. Simulation and experiment results show that our approach outperforms the existing methods.

Real-Time Schedulability Analysis and Enhancement of Transiently Powered Processors With NVMs

Recent Internet-of-Things or Wireless Sensor Network devices are often operated with energy harvesters. As there are no energy storages in those devices, power is not consistently provided to the devices at all times. In such transiently powered systems, in order to keep the system reliable without losing any execution contexts, non-volatile memories (NVMs) are typically used for swift backup/restoration of execution contexts. In this article, we perform a real-time schedulability analysis of the transiently powered processors with NVMs. We first quantitatively characterize the charging and discharging behaviors of the energy harvester and extract the compute capability of the system in time interval domain. Then, based on Real-Time Calculus, we determine whether the given multi-task workload is schedulable or not with respect to the earliest deadline first (EDF) or fixed-priority (FP) scheduling policies. In addition, we study how the choice of the threshold voltage parameter affects the schedulability, then propose a feasible threshold selection algorithm to enhance schedulability. We verify the effectiveness of the proposed technique with extensive simulations. Compared to the naive selection method, the proposed technique always shows improvements in schedulability in various workloads.

Response-Time Analysis of Multipath Flows in Hierarchically-Scheduled Time-Partitioned Distributed Real-Time Systems

Modern industrial cyberphisical systems exhibit increasingly complex execution patterns like multipath end-to-end flows, that force the real-time community to extend the schedulability analysis methods to include these patterns. Only then it is possible to ensure that applications meet their deadlines even in the worst-case scenario. As a driving motivation, we present a real industrial application with safety requirements, that needs to be re-factored in order to leverage the features of new execution paradigms such as time partitioning. In this context we develop a new response-time analysis technique that provides the capacity of obtaining the worst-case response time of multipath flows in time-partitioned hierarchical schedulers and also in general fixed-priority (FP) real-time systems. We show that the results obtained with the new analysis reduce the pessimism of the currently used holistic analysis approach.

An N-Modular Redundancy Framework Incorporating Response-Time Analysis on Multiprocessor Platforms

A timing constraint and a high level of reliability are the fundamental requirements for designing hard real-time systems. To support both requirements, the N modular redundancy (NMR) technique as a fault-tolerant real-time scheduling has been proposed, which executes identical copies for each task simultaneously on multiprocessor platforms, and a single correct one is voted on, if any. However, this technique can compromise the schedulability of the target system during improving reliability because it produces N identical copies of each job that execute in parallel on multiprocessor platforms, and some tasks may miss their deadlines due to the enlarged computing power required for completing their executions. In this paper, we propose task-level N modular redundancy (TL-NMR), which improves the system reliability of the target system of which tasks are scheduled by any fixed-priority (FP) scheduling without schedulability loss. Based on experimental results, we demonstrate that TL-NMR maintains the schedulability, while significantly improving average system safety compared to the existing NMR.

An improved formalization analysis approach to determine schedulability of global multiprocessor scheduling based on symbolic safety analysis and statistical model checking in smartphone systems

Modern smart phones are typically equipped with multicore processors. In this paper, we address the problem of determining schedulability of a real-time periodic taskset with fixed release offsets and possible release jitters on a multicore processor with global Fixed-Priority (FP) or EDF (Earliest Deadline First) scheduling algorithms, using the model-checker UPPAAL for Timed Automata. In addition to yes/no schedulability analysis for hard real-time systems, we also apply statistical model checking to estimate the probability of unschedulability for soft real-time systems, described with two statistical parameters (confidence and accuracy).

Real-Time Scheduling on Uni- and Multiprocessors based on Priority Promotions

This paper addresses the problem of real-time scheduling of a set of sporadic tasks on uni- and multiprocessor platform based on priority promotion. A new preemptive scheduling algorithm, called Fixed-Priority with Priority Promotion (FPP), is proposed. In FPP scheduling, tasks are executed similar to traditional fixed-priority (FP) scheduling but the priority of some tasks are promoted at fixed time interval (called, promotion point) relative to the release time of each job. A policy called Increase Priority at Deadline Difference (IPDD) to compute the promotion points and promoted priorities for each task is proposed. FPP scheduling prioritizes jobs according to Earliest-Deadline-First (EDF) priority when all tasks' priorities follow IPDD policy. It is known that managing (i.e., inserting and removing) jobs in the ready queue of traditional EDF scheduler is more complex than that of FP scheduler. To avoid such problem in FPP scheduling, a simple data structure and efficient operations to manage jobs in the ready queue are proposed. In addition, techniques for implementing priority promotions with and without the use of a hardware timer are proposed. Finally, an effective scheme to reduce the average number of priority promotions is proposed: if a task set is not schedulable using traditional FP scheduling, then promotion points are assigned only to those tasks that need them to meet the deadlines; otherwise, tasks are assigned traditional fixed priorities without any priority promotion. Empirical investigation shows the effectiveness of the proposed scheme in reducing overhead on uniprocessor and in accepting larger number of task sets in comparison to that of using state-of-the-art global schedulability tests for multiprocessors.

Energy-Efficient Scheduling for Embedded Real-Time Systems Using Threshold Work-Demand Analysis

In this paper, we study the problem of reducing the energy consumption for hard real-time systems scheduled according to either fixed-priority (FP) or earliest-deadline-first (EDF) scheme. To balance the static and dynamic energy consumptions, the concept of critical speed was proposed in previous research. Moreover, when combined with the processor idle/shutdown state, the critical speed was widely used as the lower bound for voltage scaling in literature. In this paper, we show that this strategy might not always be more energy efficient than the traditional DVS strategy and there exists a dynamic tradeoff between these two strategies depending on the job’s work-demand to be finished within certain intervals. To effectively address this issue, we propose a unified approach that combines these two strategies to achieve better overall energy saving performance. Our approach determines the energy-efficient speeds for real-time jobs in their corresponding feasible intervals based on the threshold work-demand analysis. Our experimental results demonstrate that the proposed techniques significantly outperform previous approaches in the overall energy saving performance.

A new sufficient schedulability analysis for hybrid scheduling

Abstract Earliest deadline first (EDF) and fixed priority (FP) are the most commonly used and studied scheduling algorithms for real-time systems. This paper focuses on combining the EDF and FP strategies in one system. We provide a new sufficient schedulability analysis for real-time hybrid task systems which are scheduled by EDF and FP. The proposed analysis has a polynomial time complexity and no restrictions on task parameters, where the relative deadline of each task could be less than, equal to, or greater than its period. By extensive experiments, we show that our proposed analysis significantly improves the acceptance ratio compared with the existing results of the sufficient schedulability test for hybrid scheduling systems.

A pre-order relation for exact schedulability test of sporadic tasks on multiprocessor Global Fixed-Priority scheduling

In this paper we present an exact schedulability test for sporadic real-time tasks scheduled by the Global Fixed Priority Fully Preemptive Scheduler on a multiprocessor system. The analysis consists in modeling the system as a Linear Hybrid Automaton, and in performing a reachability analysis for states representing deadline miss conditions. To mitigate the problem of state space explosion, we propose a pre-order relationship over the symbolic states of the model: states that are simulated by others can be safely eliminated from the state space. We also formulate the concept of decidability interval with respect to a set of constrained-deadline sporadic tasks on multiprocessor. The decidability interval is a bounded time interval such that, if a deadline miss occurs in the schedule, then it is possible to find a configuration of arrival times for the tasks such that the deadline miss happens within the bounded interval. Vice versa, if no configuration of arrival times produces a deadline miss in the bounded interval, then no deadline miss is ever possible in the schedule. Hence we prove that the schedulability analysis problem is decidable, and we provide a formula for computing the decidability interval. To our knowledge, this is the first time such a time interval is proposed for sporadic tasks running on multiprocessor. The proposed schedulability analysis has been implemented in a software tool. For the first time we assess the pessimism of the state-of-the-art approximate schedulability test through experiments. Moreover, we show that the use of the proposed model permits to analyse tasks with more general parameter values than other exact algorithms in the literature. Nevertheless, even with our approach the complexity remains too high for analysing practical task sets with more than seven tasks.

Exact comparison of fixed priority and EDF scheduling based on speedup factors for both pre-emptive and non-pre-emptive paradigms

This paper investigates the relative effectiveness of fixed priority (FP) scheduling in a uniprocessor system compared to earliest deadline first (EDF) scheduling. The quantitative metric used in this comparison is the processor speedup factor, defined as the factor by which processor speed needs to increase to ensure that any task set that is schedulable according to EDF can be scheduled using fixed priorities. In the pre-emptive case, exact speedup factors are known for sporadic task sets with implicit or constrained deadlines. In this paper, we derive exact speedup factors for both pre-emptive and non-pre-emptive fixed priority scheduling of arbitrary deadline sporadic task sets. We also show that the exact speedup factor for the pre-emptive case holds when tasks share resources according to the stack resource policy/deadline floor protocol.

An Asynchronous Adaptive Priority Round‐Robin Arbiter Based on Four‐Phase Dual‐rail Protocol

An asynchronous Adaptive priority round- robin arbiter (APRA) based on four-phase dual-rail proto- col is proposed. Combining the advantages of synchronous and asynchronous circuits, it provides the required band- width allocation on basis of arbitration fairness and works in synchronous SoC. In the Nonidling and nonpreemptive (NINP) model, simulations and verifications are made. The results show that the speed of the proposed arbiter is im- proved by 18%-50.4% and the dynamic and static power is reduced by 8.4%-46.2% and 81.8%-90.9% respectively compared with the commonly-used Fixed priority (FP), Round-Robin (RR) and Lottery arbiters. The proposed arbiter is better in output bandwidth allocation and it also has advantages in speed and power. Furthermore, it is easy in reconfiguration, strong in practicability and low in complexity of system integration and suits various extreme communication traffics.

Earliest-Deadline-Based Scheduling to Reduce Urban Traffic Congestion

One of the major problems, caused by traffic congestion, owes its existence to the unwanted delay experienced by the priority vehicles. The evaluation of two scheduling algorithms as adaptive traffic control algorithms has been proposed here to reduce this unwanted delay. One of these algorithms is the earliest deadline first (EDF) algorithm, whereas the other is the fixed priority (FP) algorithm. The performance of both algorithms as adaptive traffic light control algorithms is evaluated for isolated traffic intersections. A comparative study is performed here, where the performance of these algorithms is compared against a fixed static traffic light controller. Moreover, their performance is also compared against each other. Conclusive results from the simulation of the algorithms reveal that the number of stops, average delay, and mean trip time of the priority vehicles is significantly reduced by the implementation of these algorithms. Furthermore, it has been shown that the overall performance of EDF is much better than FP in terms of improvement of different performance measures for congestion reduction of priority vehicles.

Improvement of Real-Time Multi-CoreSchedulability with Forced Non-Preemption

While tasks may be preemptive or non-preemptive (due to their transactional operations), deadline guarantees in multi-core systems have been made only for those task sets in each of which all tasks are preemptive or non-preemptive, not a mixture thereof,i.e., fully preemptive or fully non-preemptive. In this paper, we first develop a schedulability analysis framework that guarantees the timing requirements of a given task set in which a task can be either preemptive or non-preemptive in multi-core systems. We then apply this framework to the prioritization polices of EDF (earliest deadline first) and FP (fixed priority), yielding schedulability tests of mpn-EDF (Mixed Preemptive/Non-preemptive EDF) and mpn-FP, which are generalizations of corresponding fully-preemptive and non-preemptive algorithms, i.e., fp-EDF and np-EDF, and fp-FP and np-FP. In addition to their timing guarantees for any task set that consists of a mixture of preemptive and non-preemptive tasks, the tests outperform the existing schedulability tests of np-EDF andnp-FP (i.e., special cases of mpn-EDF and mpn-FP). Using these tests, we also improve schedulability by developing an algorithm that optimally disallows preemption of a preemptive task under a certain assumption. We demonstrate via simulation that the algorithm finds up to 47.6 percent additional task sets that are schedulable with mpn-FP (likewise mpn-EDF), but not with fp-FP and np-FP (likewisefp-EDF and np-EDF).

A Comparison between Fixed Priority and EDF Scheduling accounting for Cache Related Pre-emption Delays

In multitasking real-time systems, the choice of scheduling algorithm is an important factor to ensure that response time requirements are met while maximising limited system resources. Two popular scheduling algorithms include fixed priority (FP) and earliest deadline first (EDF). While they have been studied in great detail before, they have not been compared when taking into account cache related pre-emption delays (CRPD). Memory and cache are split into a number of blocks containing instructions and data. During a pre-emption, cache blocks from the pre-empting task can evict those of the pre-empted task. When the pre-empted task is resumed, if it then has to re-load the evicted blocks, CRPD are introduced which then affect the schedulability of the task. In this paper we compare FP and EDF scheduling algorithms in the presence of CRPD using the state-of-the-art CRPD analysis. We find that when CRPD is accounted for, the performance gains offered by EDF over FP, while still notable, are diminished. Furthermore, we find that under scenarios that cause relatively high CRPD, task layout optimisation techniques can be applied to allow FP to schedule tasksets at a similar processor utilisation to EDF. Thus making the choice of the task layout in memory as important as the choice of scheduling algorithm. This is very relevant for industry, as it is much cheaper and simpler to adjust the task layout through the linker than it is to switch the scheduling algorithm.

A model-checking approach to schedulability analysis of global multiprocessor scheduling with fixed offsets

It is an active research topic to determine schedulability of a real-time sporadic or periodic taskset on a multicore processor with global scheduling policies such as global fixed-priority (FP) or earliest deadline first (EDF) algorithms. Analytical techniques such as utilisation bound tests and response time analysis algorithms generally suffer from excessive pessimism, and may cause low system utilisation. In this paper, we apply model-checking to address the restricted task model of periodic tasks with fixed release offsets and possible release jitter. We believe that this restricted task model is more realistic for current industry practice than the more general sporadic task model, since it can achieve higher CPU utilisation and better predicability. We present an approach to schedulability analysis of this restricted task model using the timed automata model-checker UPPAAL. This modelling framework is flexible and expressive, and can achieve reasonable scalability.

Interference-aware fixed-priority schedulability analysis on multiprocessors

This paper presents new schedulability tests for preemptive global fixed-priority (FP) scheduling of sporadic tasks on identical multiprocessor platform. One of the main challenges in deriving a schedulability test for global FP scheduling is identifying the worst-case runtime behavior, i.e., the critical instant, at which the release of a job suffers the maximum interference from the jobs of its higher priority tasks. Unfortunately, the critical instant is not yet known for sporadic tasks under global FP scheduling. To overcome this limitation, pessimism is introduced during the schedulability analysis to safely approximate the worst-case. The endeavor in this paper is to reduce such pessimism by proposing three new schedulability tests for global FP scheduling. Another challenge for global FP scheduling is the problem of assigning the fixed priorities to the tasks because no efficient method to find the optimal priority ordering in such case is currently known. Each of the schedulability tests proposed in this paper can be used to determine the priority of each task based on Audsley's approach. It is shown that the proposed tests not only theoretically dominate but also empirically perform better than the state-of-the-art schedulability test for global FP scheduling of sporadic tasks.