Soft Real-time Tasks Research Articles

An Algorithm to Reduce the Time Complexity of Earliest Deadline First Scheduling Algorithm in Real-Time System

To this paper we have study to Reduce the time Complexity of Earliest Deadline First (EDF), a global scheduling scheme for Earliest Deadline First in Real Time System tasks on a Multiprocessors system. Several admission control algorithms for earliest deadline first are presented, both for hard and soft real-time tasks. The average performance of these admission control algorithms is compared with the performance of known partitioning schemes. We have applied some modification to the global earliest deadline first algorithms to decrease the number of task migration and also to add predictability to its behavior. The Aim of this work is to provide a sensitivity analysis for task deadline context of multiprocessor system by using a new approach of EFDF (Earliest Feasible Deadline First) algorithm. In order to decrease the number of migrations we prevent a job from moving one processor to another processor if it is among the m higher priority jobs. Therefore, a job will continue its execution on the same processor if possible (processor affinity). The result of these comparisons outlines some situations where one scheme is preferable over the other. Partitioning schemes are better suited for hard real-time systems, while a global scheme is preferable for soft real-time systems.

Pareto Front Based Realistic Soft Real-Time Task Scheduling with Multi-objective Genetic Algorithm on Arbitrary Heterogeneous Multiprocessor System

Task scheduling is an essential aspect of parallel processing system. This problem assumes fully connected processors and ignores contention on the communication links. However, as arbitrary processor network (APN), communication contention has a strong influence on the execution time of a parallel application. In this paper, we propose a multi-objective genetic algorithm to solve task scheduling problems with time constraints on arbitrary heterogeneous processors to find the scheduling with minimum makespan and total tardiness. To optimize objectives, we use Pareto front based technique and vector based method. In this problem, just like tasks, we schedule messages on suitable links during the minimization of the makespan and total tardiness. To find a path to transfer a message between processors we use a classic routing algorithm. We compare our method with bubble scheduling and allocation (BSA) method that is a well known algorithm. Experimental results show our method is better than BSA and yields better makespan and total tardiness.

Partition oriented frame based fair scheduler

Proportionate fair schedulers provide an effective methodology for scheduling recurrent real-time tasks on multiprocessors. However, a drawback in these schedulers is that they ignore a task’s affinity towards the processor where it was executed last, causing frequent inter-processor task migrations which ultimately results in increased execution times. This paper presents Partition Oriented Frame Based Fair Scheduler (POFBFS), an efficient proportional fair scheduler for periodic firm and soft real-time tasks that ensures a bounded number of task migrations. Experimental results reveal that POFBFS can achieve 3 to 100 times reduction in the number of migrations suffered with respect to the General-ERfair algorithm (for a set of 25 to 100 tasks running on 2 to 8 processors) while simultaneously maintaining high fairness accuracy.

Supporting soft real-time tasks in the xen hypervisor

Virtualization technology enables server consolidation and has given an impetus to low-cost green data centers. However, current hypervisors do not provide adequate support for real-time applications, and this has limited the adoption of virtualization in some domains. Soft real-time applications, such as media-based ones, are impeded by components of virtualization including low-performance virtualization I/O, increased scheduling latency, and shared-cache contention. The virtual machine scheduler is central to all these issues. The goal in this paper is to adapt the virtual machine scheduler to be more soft-real-time friendly. We improve two aspects of the VMM scheduler -- managing scheduling latency as a first-class resource and managing shared caches. We use enterprise IP telephony as an illustrative soft real-time workload and design a scheduler S that incorporates the knowledge of soft real-time applications in all aspects of the scheduler to support responsiveness. For this we first define a laxity value that can be interpreted as the target scheduling latency that the workload desires. The load balancer is also designed to minimize the latency for real-time tasks. For cache management, we take cache-affinity into account for real time tasks and load-balance accordingly to prevent cache thrashing. We measured cache misses and demonstrated that cache management is essential for soft real time tasks. Although our scheduler S employs a different design philosophy, interestingly enough it can be implemented with simple modifications to the Xen hypervisor's credit scheduler. Our experiments demonstrate that the Xen scheduler with our modifications can support soft real-time guests well, without penalizing non-real-time domains.

A hierarchical multiprocessor bandwidth reservation scheme with timing guarantees

A multiprocessor scheduling scheme is presented for supporting hierarchical containers that encapsulate sporadic soft and hard real-time tasks. In this scheme, each container is allocated a specified bandwidth, which it uses to schedule its children (some of which may also be containers). This scheme is novel in that, with only soft real-time tasks, no utilization loss is incurred when provisioning containers, even in arbitrarily deep hierarchies. Presented experiments show that the proposed scheme performs well compared to conventional real-time scheduling techniques that do not provide container isolation.

EFFICIENT COMBINED SCHEDULING OF HARD AND SOFT REAL-TIME TASKS IN MULTIPROCESSOR SYSTEMS UNDER A PROCESSING POWER-SHARE STRATEGY

Increasingly, real-time systems are being used in applications that contain tasks that have deadlines and require predictable performance. Many complex real-time applications require modern operating systems capable of scheduling multiple classes of tasks in an integrated way. These applications require scheduling that result in high utilization of available processing power to accommodate as many tasks as possible while satisfying the required deadlines of each task. In this paper, we propose a combined heuristic approach to schedule a set of independent soft and hard real-time tasks in multiprocessor computing systems. Each of these tasks is characterized by its arrival time, deadline and required processing power. The proposed approach distributes the total available processing power of any processor, if it is needed and possible, among more than one task, while ensuring that hard real-time tasks are given higher priority and enough processing power to meet deadlines. This strategy can be used as a tool to efficiently guide scheduling processes. In addition, it can help to optimize processor utilization and maintain higher success ratios by maximizing the schedulability of soft tasks without jeopardizing the schedulability of hard tasks.

A New Slack Reclaiming Algorithm for Real-time Systems

Real-time applications are ubiquitous in generalpurpose computing environments, while the real-time systems are growing in complexity. Thus in these hybrid real-time systems, schedulers must guarantee that all hard real-time jobs be completed before their deadlines and improve QoS of soft real-time tasks as much as possible. Towards this goal we have proposed a new slack reclaiming algorithm for server-based real-time systems, and have also implemented it in a real time emulator (RTSIM). This algorithm, named HBASH, which enhances the Constant Bandwidth Server (CBS) by slack reclaiming, allocates slack generated from the running process to the task that needs the slack most, and then this selected task will be scheduled immediately. Hence the algorithm is able to make full use of slack and reduce the response time of soft realtime tasks as much as possible. In this paper, we proved that our algorithm does not violate the schedulability of tasks, and we also evaluated the performance of this algorithm. The experimental results demonstrate that HBASH outperforms other slack reclaiming algorithms and improves soft real-time performance significantly.

A Real-Time Scheduling Algorithm for Soft Periodic Tasks

Since now, some scheduling algorithms have been presented to guarantee the time constraints of soft real-time tasks, but most of them have some drawbacks. The first drawback of them is that scheduling decisions are based on parameters which are assumed to be crisp; however, in many environments the values of these parameters are uncertain. The second one is the high time complexity of these methods. In this paper, a new soft real time scheduling algorithm, called Maximum Miss First (MMF), is proposed to overcome some of these disadvantages. MMF determines priority of tasks according to their miss ratios. The miss ratios of all the system tasks are approximately equal when MMF is used. Because miss ratio affects other QoS parameters, using MMF is suitable for systems that need similar QoS for its tasks even in overrun and overload conditions. To evaluate MMF, its performance was compared with the EDF algorithm as well as three recent works in this context (gEDF, RLP and RD-EDF) by simulation. Simulation results showed that MMF generally outperformed the EDF, RD-EDF, RLP, and gEDF in terms of system utilization, output jitter, and fairness, but gEDF had higher success ratio because it tends to service smaller jobs. The time complexity of proposed method is near to EDF.

Real-time task scheduling by multiobjective genetic algorithm

Real-time tasks are characterized by computational activities with timing constraints and classified into two categories: a hard real-time task and a soft real-time task. In hard real-time tasks, tardiness can be catastrophic. The goal of hard real-time tasks scheduling algorithms is to meet all tasks’ deadlines, in other words, to keep the feasibility of scheduling through admission control. However, in the case of soft real-time tasks, slight violation of deadlines is not so critical. In this paper, we propose a new scheduling algorithm for soft real-time tasks using multiobjective genetic algorithm (moGA) on multiprocessors system. It is assumed that tasks have precedence relations among them and are executed on homogeneous multiprocessor environment. The objective of the proposed scheduling algorithm is to minimize the total tardiness and total number of processors used. For these objectives, this paper combines adaptive weight approach (AWA) that utilizes some useful information from the current population to readjust weights for obtaining a search pressure toward a positive ideal point. The effectiveness of the proposed algorithm is shown through simulation studies.

A BASELINE MODEL FOR SCHEDULING SOFT REAL TIME TASKS ON UNI-PROCESSOR SYSTEMS

In this paper, we consider the different performance parameters that influence the scheduling of soft real time tasks on uni-processor systems. This environment is rich of parameters that affect the rejection ratio of the system. This work models such stochastic environment and proposes a baseline model that estimates the rejection ratio of the system before applying any special scheduling algorithm. The model presents many cases where the system produces a small rejection ratio without the need for any special scheduling algorithm. We validate our analytical model with simulations that represent the real computing environment. The results of our simulations show that our mathematical model can predict the expected percentage of tasks that miss their deadline accurately.

An $\mathsf{EDF}$ -based restricted-migration scheduling algorithm for multiprocessor soft real-time systems

There has been much recent interest in the use of the earliest-deadline-first ( $\mathsf{EDF}$ ) algorithm for scheduling soft real-time sporadic task systems on identical multiprocessors. In hard real-time systems, a significant disparity exists between $\mathsf{EDF}$ -based schemes and Pfair scheduling: on M processors, the worst-case schedulable utilization for all known $\mathsf{EDF}$ variants is approximately M/2, whereas it is M for optimal Pfair algorithms. This is unfortunate because $\mathsf{EDF}$ -based algorithms entail lower scheduling and task-migration overheads. However, such a disparity in schedulability can be alleviated by easing the requirement that all deadlines be met, which may be sufficient for soft real-time systems. In particular, in recent work, we have shown that if task migrations are not restricted, then $\mathsf{EDF}$ (i.e. , global $\mathsf{EDF}$ ) can ensure bounded tardiness for a sporadic task system with no restrictions on total utilization. Unrestricted task migrations in global $\mathsf{EDF}$ may be unappealing for some systems, but if migrations are forbidden entirely, then bounded tardiness cannot be guaranteed. In this paper, we address the issue of striking a balance between task migrations and system utilization by proposing an algorithm called $\mathsf{EDF-fm}$ , which is based upon $\mathsf{EDF}$ and treads a middle path, by restricting, but not eliminating, task migrations. Specifically, under $\mathsf{EDF-fm}$ , the ability to migrate is required for at most M?1 tasks, and it is sufficient that every such task migrate between two processors and at job boundaries only. $\mathsf{EDF-fm}$ , like global $\mathsf{EDF}$ , can ensure bounded tardiness to a sporadic task system as long as the available processing capacity is not exceeded, but, unlike global $\mathsf{EDF}$ , may require that per-task utilizations be capped. The required cap is quite liberal, hence, $\mathsf{EDF-fm}$ should enable a wide range of soft real-time applications to be scheduled with no constraints on total utilization.

Time synchronization of hierarchical real-time networked CNC system based on ethernet/internet

With the development of Ethernet/Internet, using Ethernet/Internet technology to implement the communication of computer numerical control (CNC) system is a new research field of networked CNC system. To support the synchronization control of the CNC system based on Ethernet/Internet, a time synchronization solution for the hierarchical real-time networked CNC system based on Ethernet/Internet is designed in this paper. To evaluate the performance, some experiments were conducted and the results were analyzed. The results show that the field level time synchronization based on the hard real-time Ethernet time synchronization protocol IEEE 1588 meets the requirement of the hard real-time tasks, and the time synchronization between the NC server and the NC core computer based on global positioning system (GPS) and network time protocol (NTP) also meets the requirement of the soft real-time tasks. The results testify that the time synchronization solution based on Ethernet/Internet for the CNC system is realizable.

실시간처리 운영체계 환경에서 Hybrid 방식을 이용한 디지털 DBS 위성수신기 성능개선

A Digital Broadcasting Satellite (DBS) receiver converts digital A/V streams received from a satellite to analog NTSC A/V signals in real-time. Multi-tasking is an efficient way to improve the utilization of the processor core in real-time applications. In this paper, we propose a hybrid approach with a balanced trade-off between hardware kernel and multi-tasking programming to increase a system throughput. First, the schedulability of the critical hard real-time tasks in the DBS receiver is verified by using a simple feasibility test. Then, several soft real-time tasks are thoughtfully programmed to satisfy functional requirements of the system.

Hierarchical real-time networked CNC system based on the transparent model of industrial Ethernet

Given the rising importance of industrial Ethernet in the market, a networked CNC (Computer Numerical Control) system based on the transparent model of industrial Ethernet is urgently needed. To this end, a hierarchical real-time network based on Ethernet/Internet, called RNH-CNC, is designed in this paper. To evaluate the performance, some experiments were conducted and the results were analyzed. The results show that the field level communication based on the real-time Ethernet meet the requirement of hard real-time tasks, and the communication between the NC server computer and the NC core computer based on the switched Ethernet also meet the requirement of soft real-time tasks. The results testify that the CNC system based on the transparent model of industrial Ethernet is realizable.

A Hybrid Real-Time Scheduling Algorithm Based on Rigorously Proportional Dispatching of Serving

In hybrid real-time systems, schedulers must guarantee that all of hard real-time jobs are finished by their deadlines and the QoS of soft real-time tasks and non real-time tasks are improved as greatly as possible. This paper presents RPDS (rigorously proportional dispatching server) algorithm, and constructs a hierarchical scheduling framework based on that. RPDS partitions CPU time flow into continuous segments, and in each segment RPDS will forcibly assign one time slice to non-hard real-time tasks. Experimental results show that RPDS can allocate processor time to various application classes reasonably and reduce the deadline miss ratio of real-time

A Technique for Adaptive Scheduling of Soft Real-Time Tasks

A number of multimedia and process control applications can take advantage from the ability to adapt soft real-time load to available computational capacity. This capability is required, for example, to react to changed operating conditions as well as to ensure graceful degradation of an application under transient overloads. In this paper, we illustrate a novel adaptive scheduling technique based on rate modulation of a set of periodic tasks in a range of admissible rates. By casting constraints on rate ranges in a linear programming formulation, several adaptation policies can be considered, along with additional constraints reflecting various application requirements. The paper investigates the effectiveness of rate modulation strategies both on simulated task sets and on real experiments.

Highly predictable execution support for critical applications with HARETICK kernel

In this paper, the problem of providing a fully predictable execution environment for critical and hard real-time applications on embedded and DSP-based platforms is studied from the viewpoint of system architecture and operation. We introduce a set of homogenous models for time, signals and tasks, which will further serve as a basis for describing the architecture and operation of a particular hard real-time kernel – “HARETICK”. The kernel provides support for concurrent operation of hard real-time tasks (the HRT execution environment), using non-preemptive scheduling algorithms, along with soft real-time tasks (the SRT environment), using classical, preemptive, priority-based scheduling algorithms. A set of applications has been developed to test the correct operation of the HARETICK kernel according to the theoretical models and to evaluate its abilities to provide high predictability of execution for critical applications. Some of the main testing results are also discussed in the paper.

An adaptive scheme for fault-tolerant scheduling of soft real-time tasks in multiprocessor systems

The scheduling of real-time tasks with primary-backup-based fault-tolerant requirements has been an important problem for several years. Most of the known scheduling schemes are non-adaptive in nature meaning that they do not adapt to the dynamics of faults and task's parameters in the system. In this paper, we propose an adaptive fault-tolerant scheduling scheme that has a mechanism to control the overlap interval between the primary and backup versions of tasks such that the overall performance of the system is improved. The overlap interval is determined based on the observed fault rate and task's soft laxity. We also propose a new performance index, called SR index, that integrates schedulability ( S) and reliability ( R) into a single metric. To evaluate the proposed scheme, we have conducted analytical and simulation studies under different fault and deadline scenarios, and found that the proposed adaptive scheme adapts to system dynamics and offers better SR index than that of the non-adaptive schemes.

Bounded energy allocation and scheduling for real-time Embedded Systems

For energy-constrained real-time embedded systems, the power-delay tradeoff property of Volta ge-Clock Scaling (VCS) needs to be carefully considered in scheduling real-time tasks for meeting strict timing requirements. In addition, non-real-time tasks require minimized response times. Thus, a bounded energy allocation model should be adopted to solve a combination of optimization objectives in systems having mixed hard and soft real-time tasks. In this paper, we propose an energy sharing model that allocates the energy budget among hard and soft real-time tasks, exploiting the interplay between utilization and energy consumption in VCS-based earliest-deadline-first scheduling (VCS-EDF). Also a dynamic scheduling is presented which is designed to reduce energy consumption by switching between two scheduling policies and utilizing an explicit pattern of event occurrences at run-time. Through simulations, we show that this Dual-Policy Dynamic Scheduling can outperform in the reduction of energy consumption while introducing small delay in average response time of non-real-time tasks.

Hard Real-Time Systems with Stochastic Execution Times: Deterministic and Probabilistic Guarantees

This article presents a method to calculate a lower bound on the probabilistic guarantee tha t soft real-time tasks of a soft/hard system meet their deadlines in successive instantiations. It is based on a sufficient, although not necessary, schedulability condition relating the work function at each deadline and the deadline itself. The method shows an acceptable tradeoff between exactness of results and simplicity of calculations.