Soft Aperiodic Tasks Research Articles

Optimal Slack Stealing Servicing for Real-Time Energy Harvesting Systems

AbstractWe consider the problem of real-time scheduling in uniprocessor devices powered by energy harvesters. In particular, we focus on mixed sets of tasks with time and energy constraints: hard deadline periodic tasks and soft aperiodic tasks without deadlines. We present an optimal aperiodic servicing algorithm that minimizes the response times of aperiodic tasks without compromising the schedulability of hard deadline periodic tasks. The server, called Slack Stealing with energy Preserving (SSP), is designed based on a slack stealing mechanism that profits whenever possible from available spare processing time and energy. We analytically establish the optimality of SSP. Our simulation results validate our theoretical analysis.

Energy-efficient thermal-aware multiprocessor scheduling for real-time tasks using TCPN

We present an energy-efficient thermal-aware real-time global scheduler for a set of hard real-time (HRT) tasks running on a multiprocessor system. This global scheduler fulfills the thermal and temporal constraints by handling two independent variables, the task allocation time and the selection of clock frequency. To achieve its goal, the proposed scheduler is split into two stages. An off-line stage, based on a deadline partitioning scheme, computes the cycles that the HRT tasks must run per deadline interval at the minimum clock frequency to save energy while honoring the temporal and thermal constraints, and computes the maximum frequency at which the system can run below the maximum temperature. Then, an on-line, event-driven stage performs global task allocation applying a Fixed-Priority Zero-Laxity policy, reducing the overhead of quantum-based or interval-based global schedulers. The on-line stage embodies an adaptive scheduler that accepts or rejects soft RT aperiodic tasks throttling CPU frequency to the upper lowest available one to minimize power consumption while meeting time and thermal constraints. This approach leverages the best of two worlds: the off-line stage computes an ideal discrete HRT multiprocessor schedule, while the on-line stage manage soft real-time aperiodic tasks with minimum power consumption and maximum CPU utilization.

Energy aware mixed tasks scheduling in real-time systems

We address the problem of scheduling mixed tasks which consist of n hard real time periodic tasks with shared resources and soft aperiodic tasks and consider two conflicting goals: reducing energy consumption and decreasing response time of aperiodic tasks. Firstly, we compute the optimal speed of periodic tasks and present a novel static mixed task scheduling (SMTS) algorithm. It schedules periodic tasks with the optimal speed and aperiodic tasks with maximum processor speed. Secondly, a dynamic mixed task scheduling (DMTS) algorithm which can reclaim dynamic slack time generated from periodic tasks and the constant bandwidth server to reduce energy consumption is proposed. In addition, it combines dynamic voltage scaling and dynamic power management techniques. Finally, we prove that the DMTS algorithm is feasible. The experimental results show that the DMTS algorithm reduces average 7.18% and 53.66% energy consumption × response time compared with SMTS algorithm and baseline algorithm, respectively.

Schedulability Analysis of Fixed Priority Servers for Scheduling Aperiodic Real-Time Tasks – A Review

This paper focus on the schedulability analysis of fixed priority servers. A number of fixed priority servers and their schedulability analysis are reviewed, these results and properties can be used for constructing systems with different timing constraints, where real-time tasks with hard deadlines and the soft aperiodic tasks can be scheduled in the same system. The aperiodic tasks in the fixed priority servers are not preemptable, and they are scheduled in a first-come first-served manner. There is only one server with many periodic or sporadic tasks in the system. The tasks and the server are scheduled by a fixed priority algorithm such as rate monotonic or deadline monotonic.

The Algorithms of Dynamic Servers for Scheduling Aperiodic Real-Time Tasks – A Review

This paper focus on the dynamic server algorithms, and the servers are used for scheduling soft aperiodic tasks. Many types of servers and their schedulability analysis are reviewed, these properties can be used for constructing hierarchical embedded systems, where the soft aperiodic tasks and the hard real-time tasks can be scheduled in the same system. The aperiodic tasks in the server are not preemptable, and they are executed in a first-come first-served (FCFS) manner. If it is not specified, there is only one server in the system, and rest of the processes in the system are ordinary periodic tasks. The servers could be scheduled by fixed priority or dynamic algorithms.

A Mixed Scheduling Algorithm about Hard Periodic and Soft Aperiodic Real-Time Tasks on Heterogeneous Multiprocessor

This paper studies the problem about scheduling composition of periodic real-time tasks and aperiodic soft real-time tasks in heterogeneous multiprocessor environment. It analyzes the response time of each task in periodic real-time task set and the influence factor about the response time of aperiodic soft real-time task. We use a new mixed scheduling algorithm--UEDF and Task-Centric with Slack Defragmentation algorithm (TCSD) to schedule hybrid task set which consist of the periodic real-time tasks and aperiodic soft real-time tasks. It can improve the timeliness of aperiodic soft real-time tasks response, so the ratio of aperiodic tasks to meet soft deadline will increase.

Improving responsiveness of soft aperiodic tasks using proportional slack time

In a real-time system with both hard real-time periodic jobs and soft real-time aperiodic jobs, it is important to guarantee that the deadline of each periodic job is met, as well as to provide a fast response time for each aperiodic job. We propose an algorithm, called Proportional Slack Reserve (PSR), that produces an efficient schedule for such an environment. For every execution unit of a periodic job, the PSR algorithm reserves time which can be used for execution of aperiodic jobs. If reserved time is not available, the algorithm assigns a deadline to an aperiodic job for achieving better responsiveness of aperiodic jobs. The proposed algorithm can fully utilize processing power while meeting all deadlines of periodic jobs. It can also easily reclaim the time unused by the periodic job. We analytically show that for each aperiodic job, the response time in a PSR schedule is no longer than that in a TBS schedule, which is known to be efficient for servicing aperiodic jobs. We also present simulation results in which the response time of PSR is significantly improved over that of TBS, and moreover the performance of PSR compares favorably with TB ( N ) considering scheduling overhead.

Dynamic Real-time Scheduling of Firm Periodic Tasks with Hard and Soft Aperiodic Tasks

In this paper, we address the problem of the dynamic scheduling of skippable periodic task sets (i.e., period tasks allowing occasional skips of instances), together with aperiodic tasks. Schedulin...

Periodic and Aperiodic Task Scheduling in Strongly Partitioned Integrated Real-time Systems

To facilitate the integration of real-time applications in a common platform, temporal and spatial partitioning should be provided. A strongly partitioned integrated real-time system (SPIRIT) is reported in this paper that adopts a two-level hierarchical scheduling mechanism to ensure temporal partitioning. At the lower level, multiple partitions (applications) are dispatched under a cyclic scheduling, whereas, at the higher level, multiple periodic tasks of a partition are scheduled within the partition according to a fixed priority algorithm. The proposed Distance Constraint guaranteed Dynamic Cyclic (DC2( scheduler applies three basic operations, left-sliding, right-putting and compacting, to dynamically schedule aperiodic tasks and, in the meantime, guarantees the distance constraint characteristics of a partition cyclic schedule. In addition, the slack time calculation of these dynamic operations can be applied for scheduling hard aperiodic tasks. With simulation studies, we observe that the DC2 algorithm can result in a significant performance enhancement in terms of the average response time of soft aperiodic tasks and the acceptance rate for hard aperiodic tasks.

Fast scheduling of soft aperiodic tasksin real-time systems

An efficient method for scheduling soft aperiodic tasks in uni-processor real-time systems is presented. Simulation results show that fast average response times can be achieved while the time complexity of the method is O(1).

A Hardware Architecture for Scheduling Complex Real-Time Task Sets

The problem of jointly scheduling both hard deadline periodic tasks and soft aperiodic tasks has been the subject of considerable research in real-time systems. One of the most widely accepted solutions for this problem are slack stealing algorithms. However, these algorithms are rather impractical, since they all imply a considerable scheduler overhead. This paper faces the overhead problem by introducing a complete hardware architecture that implements slack stealing in hardware using an optimal algorithm redesigned to be implemented efficiently in hardware. The proposed solution is a circuit that behaves as a kind of sophisticated interrupt controller taking the task workload and the interrupts as inputs, and providing the highest priority task to be executed in the CPU. From the point of view of hardware design, the algorithm involves two main problems: first, to select the highest priority task at every moment and, second, to locate a set of slack gaps in a real-time computation. Locating slack gaps in a real-time computation is a problem that requires to “look forward in time” into the forecast schedule of a given workload. This paper analyses the different approaches for solving this problem and presents a novel architecture to solve it efficiently using a technique based on an event-driven simulation of the future of a real-time computation. A timing analysis of the proposed design is also presented.

Schedulability analysis of periodic and aperiodic tasks with resource constraints

In this paper, we address the problem of scheduling hybrid task sets consisting of hard periodic and soft aperiodic tasks that may share resources in exclusive mode in a dynamic environment, where tasks are scheduled based on their deadlines. Bounded blocking on exclusive resources is achieved by means of a dynamic resource access protocol which also prevents deadlocks and chained blocking. Aperiodic responsiveness is enhanced by an efficient servicing technique which assigns each aperiodic request a suitable deadline. Feasibility conditions are extended to handle tasks with deadlines different from periods and a reclaiming technique is presented to deal with early completions.

The EDL Server for Scheduling Periodic and Soft AperiodicTasks with Resource Constraints

In this paper, we are concerned with the problem of serving soft aperiodic tasks on a uniprocessor system where periodic tasks are scheduled on a dynamic-priority, preemptive basis and exclusively access to critical sections. Scheduling of tasks is handled by the Dynamic Priority Ceiling Protocol working with an Earliest Deadline scheduler. Our analysis determines the maximum processing time which may be stolen from periodic tasks without jeopardizing both their timing constraints and resource consistency. It provides the basis for an on-line scheduling algorithm, the EDL Server, to deal with the minimization of response times for soft aperiodic tasks.

Optimal deadline assignment for scheduling soft aperiodic tasks in hard real-time environments

We present a novel scheduling approach for servicing soft aperiodic requests in a hard real time environment, where a set of hard periodic tasks is scheduled using the Earliest Deadline First algorithm. The main characteristic of the proposed algorithm is that it achieves full processor utilization and optimal aperiodic responsiveness, still guaranteeing the execution of the periodic tasks. Another interesting feature of the proposed algorithm is that it can easily be tuned to balance performance versus complexity for adapting it to different application requirements. Schedulability issues, performance results, and implementation complexity of the algorithm are discussed and compared with other methods, such as Background, the Total Bandwidth Server, and the Slack Stealer. Resource reclaiming and extensions to more general cases are also considered. Extensive simulations show that a substantial improvement can be achieved with a little increase of complexity, ranging from the performance of the Total Bandwidth Server up to the optimal behavior.

Scheduling Aperiodic Tasks in Fixed-Priority Preemptive Systems

This paper considers the problem of jointly scheduling hard deadline periodic tasks with soft aperiodic tasks in fixed priority preemptive systems. The calculation of slack time available for aperiodic service is based on the notion of maximum acceptable delay at a specified priority level. An efficient slack calculation method which requires less computation and memory space requirement is developed. An algorithm for scheduling soft aperiodic tasks using the available slack time is presented. The objective of the algorithm is to reduce the average response time of soft aperiodic tasks.

The Real-Time Stealer

Slack stealing algorithms are a widely accepted solution for jointly scheduling both hard deadline periodic tasks and soft aperiodic tasks in real-time computations, and they all imply long searches in real-time computations looking for slack gaps. That makes them impractical to implement in software, specially in the dynamic case, since the scheduler overhead would be very important. The advocated solution is to implement them in hardware, but hardware implementations are not straightforward and they often require an algorithm redesign and to face other important design trade-offs to meet time requirements. This paper presents a complete hardware implementation of a dynamic slack stealing algorithm based on a novel algorithm for slack detection that specially fits in a hardware implementation and introduces improvements in response times over previous ones. The algorithm has been implemented in VHDL and built using a FPGA.

An optimal algorithm for scheduling soft aperiodic tasks in dynamic-priority preemptive systems

The paper addresses the problem of jointly scheduling tasks with both hard and soft real time constraints. We present a new analysis applicable to systems scheduled using a priority preemptive dispatcher, with priorities assigned dynamically according to the EDF policy. Further, we present a new efficient online algorithm (the acceptor algorithm) for servicing aperiodic work load. The acceptor transforms a soft aperiodic task into a hard one by assigning a deadline. Once transformed, aperiodic tasks are handled in exactly the same way as periodic tasks with hard deadlines. The proposed algorithm is shown to be optimal in terms of providing the shortest aperiodic response time among fixed and dynamic priority schedulers. It always guarantees the proper execution of periodic hard tasks. The approach is composed of two parts: an offline analysis and a run time scheduler. The offline algorithm runs in pseudopolynomial time O(mn), where n is the number of hard periodic tasks and m is the hyperperiod/min deadline.

Scheduling soft aperiodic tasks in adaptable fixed-priority systems

This paper presents a new type of scheduling algorithm for servicing soft deadline aperiodic tasks in adaptable fixed-priority real-time systems. The major goals of our proposed task scheduling are not only to guarantee all the deadlines of periodic tasks and to obtain fast response time for aperiodic tasks, but also to retain considerable scheduling predictability. To achieve these goals, we have adopted a new aperiodic task scheduling principle in which a normal fixed-priority assignment strategy and the information on a preassignment table built off-line are properly mingled somewhat dynamically according to the aperiodic tasks' arrivals at runtime. The paper also shows some simulation results in terms of the average aperiodic response time, verifying that the new algorithm offers significant performance improvements over other conventional joint scheduling algorithms, especially under a heavy transient overload.

Scheduling of hard aperiodic tasks

In this paper, we present a preemptive joint scheduling of hard deadline periodic and hard deadline aperiodic tasks on a uniprocessor real-time system. The scheduling has extended the Critical Task Indicating (CTI) algorithm [4] of which simulation study shows a considerable performance improvement over the other soft aperiodic task schedulings, especially under a heavy transient overload. Since a hard deadline aperiodic task has its own deadline, the proposed algorithm has a decision making mechanism that performs the on-line acceptance/rejection test upon its arrival. For simplicity and good performance, the algorithm reuses the original CTI table being used in the CTI algorithm as a slack search domain. Moreover, by searching the CTI table similarly to a circular list, it has removed the problem of search space limitation caused by the hyperperiod bound.