LRE-TL: an optimal multiprocessor algorithm for sporadic task sets with unconstrained deadlines

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.

Heterogeneous multiprocessor architectures allow embedded real-time systems to better match computing resources to each application's needs and dynamic workload requirements, thereby providing many opportunities for improved performance with reduced power consumption. Unfortunately, guaranteeing real-time requirements on heterogeneous multiprocessors remains a critical problem due to the lack of appropriate scheduling algorithms and analysis methods. In this paper, we consider EDZL (Earliest Deadline First until Zero-Laxity) for performance asymmetric multiprocessor scheduling. EDZL has been shown to outperform other scheduling policies such as global EDF on identical multiprocessors. We show that EDZL is still effective on performance asymmetric multi-processors, and present an efficient EDZL schedulability test. Experimental results show that EDZL scheduling is able to schedule up to 20% more task sets than global EDF and that our new EDZL schedulability test can accept up to 30% more schedulable task sets than a presently exiting one.

Real-time systems usually consist of a set of periodic and sporadic tasks. Periodic tasks can be divided into two classes: synchronous and asynchronous. The first type does not define the task first release, contrary to the second. Hence, synchronous periodic tasks are assumed to be released at the worst instant: the critical instant. The schedulability test is reduced to check a single execution of the task under analysis. The integration of sporadic tasks is also straightforward: they are treated as a periodic task with maximum arrival frequency. On the other hand, asynchronous periodic tasks require a test for each release in the hyper period and the integration of sporadic tasks is not trivial: the worst release instant is unknown a priori. However, they do not assume that the tasks are released at the worst instant.This paper presents a new schedulability analysis method based on the Response Time Analysis (RTA) to determine the worst response time of both asynchronous periodic and sporadic tasks, scheduled by a fixed-priority preemptive algorithm with general deadlines. It also presents another method that enables the introduction of a user configurable degree of pessimism, reducing the hyper period dependency.

The integration of mixed-critical tasks into a platform is an increasingly important trend in the design of real-time systems due to its efficient resource usage. With a growing variety of activation patterns considered in real-time systems, some of them capture arbitrary activation patterns. As a consequence, the existing scheduling approaches in mixed-criticality systems (MCs), which assume the sporadic tasks with implicit deadlines, have sometimes become inapplicable or are ineffective. In this paper, we extend the sporadically activated task model to the arbitrarily activated task model in MCs with the preemptive fixed-task-priority schedule. By using the event arrival curve to model task activations, we present the necessary and sufficient schedulability tests that are based on the well-established results from Real-Time Calculus. We propose to use the busy-window analysis to do the sufficient test because it has been shown to be tighter than the sufficient test of using Real-Time Calculus. According to our experimental results, for sporadic task sets, our proposed test can achieve the same performance as the state-of-the-art schedulability test. However, compared with the previous schedulability analysis of preemptive fixed-task-priority, our approaches can handle more general tasks with blocking, jitter, and arbitrary deadlines.

We consider the problem of on-line scheduling of a set of hard real-time tasks in an environment where future task parameters are not known a-priori. First, we derive necessary and sufficient conditions to determine whether a given set of tasks with known parameters is feasible using a set of m processors. Next, we consider two sets of tasks T/sub 0/ and T/sub t/ where T/sub 0/ is the set of tasks that need to be scheduled at time 0 such that any future task set T/sub t/, t>0 can be scheduled with or without disturbing the schedule for T/sub 0/. In this context, we derive necessary and sufficient conditions applicable to the schedule obtained for T/sub 0/ at time 0 such that, any future task set T/sub t/ can be scheduled provided the task set T/sub 0//spl cup/T/sub t/ is feasible using m processors.

A polynomial-time algorithm is presented for partitioning a collection of sporadic tasks among the processors of an identical multiprocessor platform. Since the partitioning problem is NP-hard in the strong sense, this algorithm is unlikely to be optimal. A quantitative characterization of its worst-case performance is provided in terms of resource augmentation; it is shown that any set of sporadic tasks that can be partitioned among the processors of an m-processor identical multiprocessor platform will be partitioned by this algorithm on an m-processor platform in which each processor is (4 - 2/m) times as fast

The issue of deadline tardiness is considered under earliest-deadline-first (GEDF) multiprocessor scheduling. New schedulability tests are presented for determining whether a set of sporadic tasks with arbitrary relative deadlines can be scheduled under either preemptive or non-preemptive GEDF so that pre-defined tardiness bounds are met. These tests are of pseudo-polynomial time complexity, and can be used in hard real-time, soft real-time, and mixed contexts.

(m, k)-firm constraints have been used to schedule tasks in soft/firm real-time systems under overloaded conditions. In general, they are provided by application designers to guarantee the minimum levels of quality of service (QoS). Many problems concentrating in task schedulability under these constraints were investigated. However, little work has been done in combining the optimization of the QoS and task schedulability subject to these (m, k)-firm constraints. In this paper, we consider the problem of maximizing the guaranteed performance while maintaining a schedulable task set in periodic firm real-time systems. To quantify the performance, we propose a granularity-related metric called granularity of quality of service-reward (GQoS-reward). We then show that maximizing the total GQoS-reward is an NP-hard problem and a heuristic method to solve the problem is studied. In addition to the improvement to the GQoS, positive effects on other main performance metrics for soft/firm real time systems, such as effective processor utilization (EPU), total accumulated reward and instability, are also supported by the simulation results using our optimization strategy

In this paper, we tackle the problem of minimizing the power consumption of multiple-processors-core system using multiple variable supply voltages. Given a set of tasks and their dependency, and a set of target processors, we propose a method which carries out task allocation on processors, task scheduling and voltage assignment for each task concurrently. The problem is formulated as a mixed integer nonlinear programming (MINLP) optimization problem and optimal solution can be obtained. The MINLP problem is solved by the branch and bound algorithm. To reduce the run time, we propose two modifications of the formulation. Experimental results show that significant power reduction is achieved using the proposed approach.

Allocation problem has always been one of the fundamental issues of building the applications in real-time computing systems. For real-time applications, the allocation problem should directly address the issues of task and communication scheduling. In this context, the allocation of tasks has to fully utilize the available processors and the scheduling of tasks has to meet the specified timing constraints. Clearly, the execution of tasks under the allocation and schedule has to satisfy the precedence, resources, and other synchronization constraints among them. Recently, the timing requirements of the real-time systems emerge that the relative timing constraints are imposed on the consecutive executions of each task and the inter-task temporal relationships are specified across task periods. In this paper we consider the allocation and scheduling problem of the periodic tasks with such timing requirements. Given a set of periodic tasks, we consider the least common multiple (LCM) of the task periods. Each task is extended to several instances within the LCM. The scheduling window for each task instance is derived to satisfy the timing constraints. We develop a simulated annealing algorithm as the overall control algorithm. An example problem of the sanitized version of the Boeing 777 Aircraft Information Management System is solved by the algorithm. Experimental results show that the algorithm solves the problem in a reasonable time complexity.

The problem of fixed priority scheduling of periodic tasks where each task's execution priority may vary is considered. Periodic tasks are decomposed into serially executed subtasks, where each subtask is characterized by an execution time and a fixed priority and is permitted to have a deadline. A method for determining the schedulability of each task is presented along with its theoretical underpinnings. This method can be used to analyze the schedulability of complex task sets which involve interrupts, certain synchronization protocols, nonpreemptible sections and, in general, any mechanism that contributes to a complex priority structure. The authors introduce a simple but realistic real-time robotics application and illustrate how one uses the schedulability equations presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Prior work has identified several optimal algorithms for scheduling independent, implicit-deadline sporadic (or periodic) real-time tasks on identical multiprocessors. These algorithms, however, are subject to high conceptual complexity and typically incur considerable runtime overheads. This paper establishes that, empirically, near-optimal schedulability can also be achieved with a far simpler approach that combines three well-known techniques (reservations, semi-partitioned scheduling, and period transformation) with some novel task-placement heuristics.In large-scale schedulability experiments, the proposed approach is shown to achieve near-optimal hard real-time schedulability (99+% schedulable utilization) across a wide range of processor and task counts. With an implementation in LITMUSRT, the proposed approach is shown to be practical and to incur only low runtime overheads, comparable to a conventional partitioned scheduler. It is further shown that basic slack management techniques can help to avoid more than 50% of all migrations of semi-partitioned reservations if tasks execute on average for less than their provisioned worst-case execution time.Two main conclusions are drawn: pragmatically speaking, global scheduling is not required to support static workloads of independent, implicit-deadline sporadic (or periodic) tasks; and since such simple workloads are well supported, future research on multiprocessor real-time scheduling should consider more challenging workloads (e.g., adaptive workloads, dynamic task arrivals or mode changes, shared resources, precedence constraints, etc.).

As semiconductor technology continues to evolve, the chip temperature increases rapidly due to the exponentially growing power consumption. In the meantime, the high chip temperature increases the leakage power, which is becoming the dominate part in the overall power consumption for sub-micron IC circuits. A power/thermal-aware computing technique becomes ineffective if this temperature/leakage relation is not properly addressed in the sub-micron domain. In this paper, we study the feasibility problem for scheduling a hard real-time periodic task set under the peak temperature constraint, with the interaction between temperature and leakage being taken into consideration. Three analysis techniques are developed to guarantee the schedulability of periodic real-time task sets under the maximal temperature constraint. Our experiments, based on technical parameters from a processor using the 65 nm technology, show that the feasibility analysis without considering the interactions between temperature and leakage can be significantly overoptimistic.

A majority of multi-rate real-time systems are constrained by a multitude of timing requirements, in addition to the traditional deadlines on well-studied response times. This means, the timing predictability of these systems not only depends on the schedulability of certain task sets but also on the timely propagation of data through the chains of tasks from sensors to actuators. In the automotive industry, four different timing constraints corresponding to various data propagation delays are commonly specified on the systems. This paper identifies and addresses the source of pessimism as well as optimism in the calculations for one such delay, namely the reaction delay, in the state-of-the-art analysis that is already implemented in several industrial tools. Furthermore, a generic framework is proposed to compute all the four end-to-end data propagation delays, complying with the established delay semantics, in a scheduler and hardware-agnostic manner. This allows analysis of the system models already at early development phases, where limited system information is present. The paper further introduces mechanisms to generate job-level dependencies, a partial ordering of jobs, which need to be satisfied by any execution platform in order to meet the data propagation timing requirements. The job-level dependencies are first added to all task chains of the system and then reduced to its minimum required set such that the job order is not affected. Moreover, a necessary schedulability test is provided, allowing for varying the number of CPUs. The experimental evaluations demonstrate the tightness in the reaction delay with the proposed framework as compared to the existing state-of-the-art and practice solutions.

A Feedback Control method of Deadline Assignment (FC-DA) is presented to reduce output jitter of control tasks. The prevailing PID control law is employed, whose parameters can be adaptively adjusted online according to CPU load and jitter. The schedulability of whole task set is guaranteed dynamically. A simulation experiment shows the improvement when compared with proportional-only law.