Global Earliest Deadline First Research Articles

LAG-based schedulability analysis for preemptive global EDF scheduling with dynamic cache allocation

In recent years, the maturation of modern multicore processor technology and its increasing adoption in critical industrial domains have posed significant challenges for real-time systems, primarily due to contention for shared cache resources and the resulting uncertainty. To address this issue, contemporary processors employ cache partitioning techniques, enhancing temporal predictability by isolating cache access among processor cores. However, this isolation technique may lead to real-time tasks missing their deadlines due to an insufficient number of cache partitions. Consequently, this paper investigates the schedulability of preemptive global Earliest Deadline First (EDF) real-time scheduling algorithms that support dynamic cache allocation. We propose an innovative LAG-based schedulability analysis method for these algorithms and present a utilization-based schedulability condition that reduces analysis time complexity while improving analysis accuracy. Building upon this foundation, and incorporating task characteristics into our analysis, this paper further introduces an optimization technique aimed at minimizing the pessimism inherent in the schedulability test. Lastly, the performance and efficiency of the proposed schedulability determination method are validated through simulation experiments with randomly generated tasks.

Schedulability Analysis for Coscheduling Real-Time Tasks on Multiprocessors

The real-time coscheduling problem, where tasks may have multiple phases executing on different types of processors, is known to be hard. The (already hard) self-suspending task scheduling simplifies the coscheduling problem by assuming that the latency a task may experience on the other type of processors is naturally bounded, which is unfortunately not true in practice. In this article, we present a novel analysis technique, namely, the vertical view analysis, for analyzing the schedulability of coscheduling sporadic tasks under global earliest-deadline-first (GEDF) on a heterogeneous multiprocessor consisting of two types of processors. We derive both hard (no deadline miss) and soft (bounded response times) real-time utilization-based tests. To the best of our knowledge, these results are the first-of-its-kind for the coscheduling problem and may allow real-time schedulability analysis to be carried out on more practical scenarios under heterogeneous computing.

First steps towards an IEEE 802.1AS clock for EDF scheduling in distributed real-time systems

The EDF (Earliest Deadline First) scheduling policy can take advantage of the existence of a synchronization clock mechanism to reach higher utilizations in distributed real-time systems, as indicated by theoretical research. In this work, we give the first steps to enable the usage of EDF and a global clock in a distributed system. To this end, our proposal relies on Linux to provide the global clock and on a Real-Time Operating System (RTOS) to provide the global EDF policy. The objective is the evaluation of global EDF to verify to what extent the theoretical results are met.

Towards Hybrid Energy-Efficient Power Management in Wireless Sensor Networks.

Wireless Sensor Networks (WSNs) are prone to highly constrained resources, as a result ensuring the proper functioning of the network is a requirement. Therefore, an effective WSN management system has to be integrated for the network efficiency. Our objective is to model, design, and propose a homogeneous WSN hybrid architecture. This work features a dedicated power utilization optimization strategy specifically for WSNs application. It is entitled Hybrid Energy-Efficient Power manager Scheduling (HEEPS). The pillars of this strategy are based on the one hand on time-out Dynamic Power Management (DPM) Intertask and on the other hand on Dynamic Voltage and Frequency Scaling (DVFS). All tasks are scheduled under Global Earliest Deadline First (GEDF) with new scheduling tests to overcome the Dhall effect. To minimize the energy consumption, the HEEPS predicts, defines and models the behavior adapted to each sensor node, as well as the associated energy management mechanism. HEEPS’s performance evaluation and analysis are performed using the STORM simulator. A comparison to the results obtained with the various state of the art approaches is presented. Results show that the power manager proposed effectively schedules tasks to use dynamically the available energy estimated gain up to 50%.

Suspension-Aware Earliest-Deadline-First Scheduling Analysis

While the earliest-deadline-first (EDF) scheduling algorithm has extensively been utilized in real-time systems, there is almost no literature considering EDF for task sets with dynamic self-suspension behavior. To be precise, there is no specialized result for uniprocessor systems, besides the trivial suspension-oblivious approach. The work by Liu and Anderson (in ECRTS 2013) and Dong and Liu (in RTSS 2016) for suspension-aware multiprocessor global EDF can also be applied to uniprocessor systems and therefore be considered the state-of-the-art. In this work, two novel schedulability analyses (one for sporadic and one for periodic task sets) for suspension-aware EDF on uniprocessor systems are proposed, which outperform the state-of-the-art on such systems in empirical and theoretical comparison. We further show that the analysis by Dong and Liu is in fact not suspension-aware for uniprocessor systems.

Limited Busy Periods in Response Time Analysis for Tasks Under Global EDF Scheduling

Response time analysis (RTA) is a kind of effective methods to test the schedulability of real-time tasks under the global earliest deadline first (G-EDF) scheduling. The main idea of the RTA methods is to judge the schedulability of the task set by comparing the worst-case response time (WCRT) and deadline of each task. For obtaining a safe WCRT, existing RTA methods assume that the response times of carry-in jobs are equal to the WCRTs of the tasks generating these jobs. This assumption causes the low accuracy of existing RTA methods, since the busy period lengths of carry-in jobs are usually limited and the response times of these jobs thus cannot always reach the assumed WCRTs. This article proposes a new RTA method that has a higher accuracy than existing RTAs. The main idea of the new method is to limit the busy period lengths of carry-in jobs in the WCRT calculation. We also propose an efficiency improvement scheme for the new method. A set of experiments is conducted to evaluate the performances of the new method and the efficiency improvement scheme. The experimental results show that the new method has a higher acceptance ratio than the state-of-the-art RTA methods and the efficiency improvement scheme can significantly reduce the execution time of the new method.

Real-Time Scheduling of DAG Tasks with Arbitrary Deadlines

Real-time and embedded systems are shifting from single-core to multi-core processors, on which the software must be parallelized to fully utilize the computation capacity of the hardware. Recently, much work has been done on real-time scheduling of parallel tasks modeled as directed acyclic graphs (DAG). However, most of these studies assume tasks to have implicit or constrained deadlines. Much less work considered the general case of arbitrary deadlines (i.e., the relative deadline is allowed to be larger than the period), which is more difficult to analyze due to intra-task interference among jobs. In this article, we study the analysis of Global Earliest Deadline First (GEDF) scheduling for DAG parallel tasks with arbitrary deadlines. We develop new analysis techniques for GEDF scheduling of a single DAG task and this new analysis techniques can guarantee a better capacity augmentation bound 2.41 (the best known result is 2.5) in the case of a single task. Furthermore, the proposed analysis techniques are also extended to the case of multiple DAG tasks under GEDF and federated scheduling. Finally, through empirical evaluation, we justify the out-performance of our schedulability tests compared to the state-of-the-art in general.

Response Time Analysis for Tasks with Fixed Preemption Points under Global Scheduling

As an effective method for detecting the schedulability of real-time tasks on multiprocessor platforms, Response time analysis (RTA) has been deeply researched in recent decades. Most of the existing RTA methods are designed for tasks that can be preempted at any time. However, in some real-time systems, a task may have some fixed preemption points (FPPs) that divide its execution into a series of non-preemptive regions (NPRs). In such environments, the task can only be preempted at its FPPs, which makes existing RTA methods for arbitrary preemption tasks not applicable. In this article, we study the schedulability analysis on tasks with FPPs under both global fixed-priority (G-FP) scheduling and global earliest deadline first (G-EDF) scheduling. First, based on the idea of limiting the time interval between two consecutive executions of an NPR, a novel RTA method for tasks with FPPs under G-FP scheduling is proposed. Second, we propose an effective RTA method for tasks with FPPs under G-EDF scheduling. Finally, extensive simulations are conducted and the results validate the effectiveness of the proposed methods.

Execution-Efficient Response Time Analysis on Global Multiprocessor Platforms

Response time analysis (RTA) is an important and fundamental tool for analyzing the schedulability of real-time tasks on multiprocessor platforms, and many promising techniques have been developed during the past few years. However, most of the existing researches focus on improving the analysis precision, while less has been done on enhancing the execution efficiency. In this paper, we take, to our best knowledge, the first effort towards improving the efficiency of the state-of-the-art RTA methods for sporadic tasks under both global fixed-priority (G-FP) and global earliest deadline first (G-EDF) scheduling. Specifically, three factors that impact the efficiency of existing global RTA tests are identified: 1) pessimistic initial value when computing the worst-case response time (WCRT) of tasks under both G-FP and G-EDF; 2) conservative interference upper bound under both G-FP and G-EDF; and 3) unnecessary recalculation of WCRTs when there is update of any task WCRT under G-EDF. By addressing these three limitations, we propose two efficient RTA methods for G-FP and G-EDF scheduling, respectively, which achieve better run-time performance but without sacrificing any analysis precision. Experimental evaluations with randomly generated task sets show that the proposed methods exhibit remarkable performance improvements and can save on average 60 and 61 percent run time, as compared to the state-of-the-art technologies under G-FP and G-EDF scheduling, respectively.

A scheduling based energy-aware core switching technique to avoid thermal threshold values in multi-core processing systems

The increasing on-chip temperature has become a critical issue in the multiprocessor system on chip (MPSoC). It does not only disturb the reliability and performance of the system but it also increases the power consumption. Load balancing is a common scheduling technique to address the thermal issues where the load is transferred to relatively less used cores. However, during the transfer of workload, the destination cores may be in sleep mode and thus take some time to switch to the running mode. It results in wastage of some processing time. This paper presents a thermal-aware load balancing technique that avoids thermal emergencies while eliminating the switching delays. The proposed technique estimates the time taken by a task set to reach the temperature threshold values with the help of offline recorded thermal profiles of datasets. Moreover, cores are selected in a round-robin manner to equilibrate thermal gradients. The technique is based on Global Earliest Deadline First (GEDF) scheduling algorithm and considers ambient temperature, thermal cycles and energy consumption. The proposed technique is tested in a simulation environment comprising of scheduling and thermal simulation model. The results show that the technique reduces the average temperature up to 17%, thermal cycles up to 15%, energy consumption up to 20% and lowers the temporal and spatial gradients as compared to the commonly used predictive thermal-aware and thermal balancing techniques.

Stretching algorithm for global scheduling of real-time DAG tasks

Parallelism is becoming more important nowadays due to the increasing use of multiprocessor systems. A Directed Acyclic Graph (DAG) is a general model of parallel tasks with inter-subtask parallelism. It consists of a collection of dependent subtasks under precedence constraints. In this paper, we study the problem of scheduling n periodic parallel real-time DAG tasks on m homogeneous multiprocessor systems. The dependencies between subtasks make scheduling process more challenging. We propose a stretching algorithm to be applied on each DAG task prior to scheduling process. Thus, DAGs are transformed into a set of independent sequential threads with intermediate offsets and deadlines. The threads obtained due to the transformation are of two types, (i) fully-stretched master threads with utilization equal to 1 and (ii) independent constrained-deadline threads. We propose a scheduling method over RTOS to ensure the execution of fully-stretched master threads on dedicated processors while the remaining processors $$\overline{m} \le m$$ , are used to schedule the independent constrained-deadline threads using any multiprocessor scheduling algorithm. In this work, we analyze the stretching algorithm while considering two global preemptive scheduling algorithms from different priority assignment families; the Global Earliest Deadline First (GEDF) from the fixed job priority family, and the Global Deadline Monotonic (GDM) from the fixed task priority family. We prove that GEDF scheduling of stretched threads has a resource augmentation bound equal to $$\frac{3+\sqrt{5}}{2}$$ for all task sets with $$n < \varphi \times \overline{m}$$ , where n is the number of tasks in the set and $$\varphi $$ is the golden ratio (the value of the golden ratio is $$\frac{1+\sqrt{5}}{2}$$ ). While GDM has a resource augmentation bound equal to $$2+\sqrt{3}$$ for all task sets with $$n < \frac{1+\sqrt{3}}{2} \times \overline{m}$$ .

On the ineffectiveness of 1/m-based interference bounds in the analysis of global EDF and FIFO scheduling

Enormous efforts have been spent in the derivation of sufficient schedulability tests for popular global schedulers such as global fixed-priority (G-FP) and global earliest-deadline first (G-EDF). Among all the proposals, response-time analysis techniques are established as the most popular ones and have been widely adopted by several authors in subsequent researches. Such tests are based on interference bounds that have been derived by exploiting the work-conserving property of such schedulers, and are typically expressed with a 1 / m multiplier in the interference equation (with m being the number of available processors). This paper shows that the popular response-time analysis for G-EDF is ineffective with respect to partitioned EDF (P-EDF) scheduling. That is, it is proven that every task set that is deemed schedulable by such an analysis is also schedulable under P-EDF by adopting a trivial partitioning algorithm. Furthermore, an analogous result is proven for global first-in first-out (G-FIFO) scheduling when analyzed with a 1 / m-based interference bound. Finally, experimental results are presented to compare sufficient tests for G-EDF and G-FIFO with the corresponding partitioned schemes under a vast collection of partitioning heuristics. The results show that the considered tests for global scheduling are always outperformed by those for partitioned scheduling exhibiting a significant performance gap.

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).

Improved Carry-in Workload Estimation for Global Multiprocessor Scheduling

As an important and fundamental tool for analyzing the schedulability of a real-time task set on the multiprocessor platform, response time analysis (RTA) has been researched for several years on both Global Fixed Priority (G-FP) and Global Earliest Deadline First (G-EDF) scheduling. This paper proposes a new analysis that improves over current state-of-the-art RTA methods for both G-FP and G-EDF scheduling, by reducing their pessimism. The key observation is that when estimating the carry-in workload, all the existing RTA techniques depend on the worst case scenario in which the carry-in job should execute as late as possible and just finishes execution before its worst case response time (WCRT). But the carry-in workload calculated under this assumption may be over-estimated, and thus the accuracy of the response time analysis may be impacted. To address this problem, we first propose a new method to estimate the carry-in workload more precisely. The proposed method does not depend on any specific scheduling algorithm and can be used for both G-FP and G-EDF scheduling. We then propose a general RTA algorithm that can improve most existing RTA tests by incorporating our carry-in estimation method. To further improve the execution efficiency, we also introduce an optimization technique for our RTA tests. Experiments with randomly generated task sets are conducted and the results show that, compared with the state-of-the-art technologies, the proposed tests exhibit considerable performance improvements, up to 9 and 7.8 percent under G-FP and G-EDF scheduling respectively, in terms of schedulability test precision.

Assessment of nested-parallel task model under real-time scheduling on multi-core processors

Real-time applications contain numerous time-bound parallel tasks with enormous computations. Parallel models, not the sequential models, have the capability to handle intra-task parallelism and accomplish such tasks in a specific time or before. Previous researchers presented the task models for parallel tasks, but not for the nested-parallel tasks. This paper deals with the real-time scheduling of periodic nested-parallel tasks having an implicit deadline on multi-core processors. Initially, an nested-parallel task model is developed. Next, a novel task disintegration technique is studied where the MAM's ratio is defined to categorise the segments. It is theoretically proved that the discussed disintegration technique achieved a speedup factor of 4.30 and 3.40 when the tasks, after disintegration, are scheduled under partitioned deadline monotonic (DM) and global earliest deadline first (EDF) scheduling, respectively. Further, considering the overhead factor (β) for non-preemptive global EDF scheduling, disintegration technique is analysed and achieved a speedup factor of 3.73 (for β = 1). The proposed disintegration technique is assessed through the simulations thereby indicating the adequacy of derived speedup factors.

New response time analysis for global EDF on a multiprocessor platform

Time-predictability is the most important requirement for a real-time system, and researchers have therefore paid attention to the duration between the arrival and completion of a real-time task, called response time. RTA (Response Time Analysis) studies, however, rely on the same technique, yielding room for further improvement, especially regarding multiprocessor platforms. For this paper, we investigated the properties of an existing utilization-based schedulability analysis for global EDF (Earliest Deadline First) on a multiprocessor platform, and developed a new RTA technique based on the corresponding properties, which calculates the response times of tasks in task sets deemed schedulable by the existing analysis. We demonstrated through simulations that our proposed RTA technique not only calculates response times that are less pessimistic than those of the existing approach, but also successfully derives response times that cannot be obtained by the existing approach.

Global EDF scheduling for parallel real-time tasks

As multicore processors become ever more prevalent, it is important for real-time programs to take advantage of intra-task parallelism in order to support computation-intensive applications with tight deadlines. In this paper, we consider the global earliest deadline first (GEDF) scheduling policy for task sets consisting of parallel tasks. Each task can be represented by a directed acyclic graph (DAG) where nodes represent computational work and edges represent dependences between nodes. In this model, we prove that GEDF provides a capacity augmentation bound of $$4-\frac{2}{m}$$ and a resource augmentation bound of $$2-\frac{1}{m}$$ . The capacity augmentation bound acts as a linear-time schedulability test since it guarantees that any task set with total utilization of at most $$m/(4-\frac{2}{m})$$ where each task’s critical-path length is at most $$1/(4-\frac{2}{m})$$ of its deadline is schedulable on $$m$$ cores under GEDF. In addition, we present a pseudo-polynomial time fixed-point schedulability test for GEDF; this test uses a carry-in work calculation based on the proof for the capacity bound. Finally, we present and evaluate a prototype platform—called PGEDF—for scheduling parallel tasks using global earliest deadline first (GEDF). PGEDF is built by combining the GNU OpenMP runtime system and the $$\text {LITMUS}^\text {RT}$$ operating system. This platform allows programmers to write parallel OpenMP tasks and specify real-time parameters such as deadlines for tasks. We perform two kinds of experiments to evaluate the performance of GEDF for parallel tasks. (1) We run numerical simulations for DAG tasks. (2) We execute randomly generated tasks using PGEDF. Both sets of experiments indicate that GEDF performs surprisingly well and outperforms an existing scheduling techniques that involves task decomposition.

Using an adversary simulator to evaluate global EDF scheduling of sporadic task sets on multiprocessors

Schedulability analysis of real-time multiprocessor systems is usually based on sufficient but not necessary tests that produce pessimistic results. One difficulty in evaluating the effectiveness of sufficient schedulability tests has been distinguishing the cause of a task set failing the test, i.e., finding out whether the task set is in fact not schedulable or it is actually schedulable but the test itself is too pessimistic. Necessary schedulability tests help to distinguish between these two situations, since if a task set fails in the test then it is guaranteed to be unschedulable. An adversary simulator is a scheduling simulator that uses the non-determinism of the task model to generate scenarios that will stress a specific scheduling algorithm, improving the odds of a deadline miss. In this paper we describe a new adversary simulator algorithm for sporadic task sets executed on multiprocessors scheduled by Global Earliest Deadline First (G-EDF). It is shown that this new adversary simulator is more effective as a necessary test than existing approaches. We also estimate the uncertainty regarding G-EDF by applying to the same task sets a well-known sufficient schedulability test from the literature and the necessary schedulability test based on the adversary simulator.

Fair lateness scheduling: reducing maximum lateness in G-EDF-like scheduling

In prior work on soft real-time (SRT) multiprocessor scheduling, tardiness bounds have been derived for a variety of scheduling algorithms, most notably, the global earliest-deadline-first (G-EDF) algorithm. In this paper, we devise G-EDF-like (GEL) schedulers, which have identical implementations to G-EDF and therefore the same overheads, but that provide better tardiness bounds. We discuss how to analyze these schedulers and propose methods to determine scheduler parameters to meet several different tardiness bound criteria. We employ linear programs to adjust such parameters to optimize arbitrary tardiness criteria, and to analyze lateness bounds (lateness is related to tardiness). We also propose a particular scheduling algorithm, namely the global fair lateness (G-FL) algorithm, to minimize maximum absolute lateness bounds. Unlike the other schedulers described in this paper, G-FL only requires linear programming for analysis. We argue that our proposed schedulers, such as G-FL, should replace G-EDF for SRT applications.

A unified analysis of global EDF and fixed-task-priority schedulability of sporadic task systems on multiprocessors

Fixed-task-priority (FTP) scheduling and earliest-deadline-first (EDF) scheduling policies are alike in fixing the priority of each job of a task at the time the job is released. This common feature of FTP and EDF scheduling permits a unified analysis of scheduling failures, to derive new sufficient tests for meeting all deadlines of a set of independent sporadic tasks under a global preemptive FTP or EDF scheduling policy. The performance of the new tests has been evaluated in comparison to prior schedulability tests, by simulation. The new tests are able to verify some schedulable task systems that could not be verified by prior tests, but also fail to verify some systems that can be verified by prior analysis techniques. The biggest gain appears to be for fixed-task-priority scheduling, especially with post-period deadlines.