Georg von der Brüggen

Schedulability and Optimization Analysis for Non-preemptive Static Priority Scheduling Based on Task Utilization and Blocking Factors

For real time task sets, allowing preemption is often considered to be important to ensure the schedulability, as it allows high-priority tasks to be allocated to the processor nearly immediately. However, preemptive scheduling also introduces some additional overhead and may not be allowed for some hardware components, which motivates the needs of non-preemptive or limited-preemptive scheduling. We present a safe sufficient schedulability test for non-preemptive (NP) fixed priority scheduling that can verify the schedulability for Deadline Monotonic (DM-NP) and Rate Monotonic (RM-NP) scheduling in linear time, if task orders according to priority and period are given. This test leads to a better upper bound on the speedup factor for DM-NP and RM-NP in comparison to Earliest Deadline First (EDF-NP) than previously known, closing the gab between lower and upper bound. We improve our test, resulting in interesting properties of the blocking time that allow to determine schedulability by only considering the schedulability of the preemptive case if some conditions are met. Furthermore, we present a utilization bound for RM-NP, based on the ratio γ > 0 of the upper bound of the maximum blocking time to the execution time, significantly improving previous results.

On the Pitfalls of Resource Augmentation Factors and Utilization Bounds in Real-Time Scheduling.

In this paper, we take a careful look at speedup factors, utilization bounds, and capacity augmentation bounds. These three metrics have been widely adopted in real-time scheduling research as the de facto standard theoretical tools for assessing scheduling algorithms and schedulability tests. Despite that, it is not always clear how researchers and designers should interpret or use these metrics. In studying this area, we found a number of surprising results, and related to them, ways in which the metrics may be misinterpreted or misunderstood. In this paper, we provide a perspective on the use of these metrics, guiding researchers on their meaning and interpretation, and helping to avoid pitfalls in their use. Finally, we propose and demonstrate the use of parametric augmentation functions as a means of providing nuanced information that may be more relevant in practical settings.

Uniprocessor Scheduling Strategies for Self-Suspending Task Systems

We study uniprocessor scheduling for hard real-time self-suspending task systems where each task may contain a single self-suspension interval. We focus on improving state-of-the-art fixed-relative-deadline (FRD) scheduling approaches, where an FRD scheduler assigns a separate relative deadline to each computation segment of a task. Then, FRD schedules different computation segments by using the earliest-deadline first (EDF) scheduling policy, based on the assigned deadlines for the computation segments. Our proposed algorithm, Shortest Execution Interval First Deadline Assignment (SEIFDA), greedily assigns the relative deadlines of the computation segments, starting with the task with the smallest execution interval length, i.e., the period minus the self-suspension time. We show that any reasonable deadline assignment under this strategy has a speedup factor of 3. Moreover, we present how to approximate the schedulability test and a generalized mixed integer linear programming (MILP) that can be formulated based on the tolerable loss in the schedulability test defined by the users. We show by both analysis and experiments that through designing smarter relative deadline assignment policies, the resulting FRD scheduling algorithms yield significantly better performance than existing schedulers for such task systems.

Hybrid self-suspension models in real-time embedded systems

1To tackle the unavoidable self-suspension behavior due to I/O-intensive interactions, multi-core processors, computation offloading systems with coprocessors, etc., the dynamic and the segmented self-suspension sporadic task models have been widely used in the literature. We propose new self-suspension models that are hybrids of the dynamic and the segmented models. Those hybrid models are capable of exploiting knowledge about execution paths, potentially reducing modelling pessimism. In addition, we provide the corresponding schedulability analysis under fixed-relative-deadline (FRD) scheduling and explain how the state-of-the-art FRD scheduling strategy can be applied. Empirically, these hybrid approaches are shown to be effective with regards to the number of schedulable task sets.

Exact speedup factors for linear-time schedulability tests for fixed-priority preemptive and non-preemptive scheduling

In this paper, we investigate the quality of several linear-time schedulability tests for preemptive and non-preemptive fixed-priority scheduling of uniprocessor systems. The metric used to assess the quality of these tests is the resource augmentation bound commonly known as the processor speedup factor. The speedup factor of a schedulability test corresponds to the smallest factor by which the processing speed of a uniprocessor needs to be increased such that any task set that is feasible under an optimal preemptive (non-preemptive) work-conserving scheduling algorithm is guaranteed to be schedulable with preemptive (non-preemptive) fixed priority scheduling if this scheduling test is used, assuming an appropriate priority assignment. We show the surprising result that the exact speedup factors for Deadline Monotonic (DM) priority assignment combined with sufficient linear-time schedulability tests for implicit-, constrained-, and arbitrary-deadline task sets are the same as those obtained for optimal priority assignment policies combined with exact schedulability tests. Thus in terms of the speedup-factors required, there is no penalty in using DM priority assignment and simple linear schedulability tests.

Systems with Dynamic Real-Time Guarantees in Uncertain and Faulty Execution Environments

In many practical real-time systems, the physical environment and the system platform can impose uncertain execution behaviour to the system. For example, if transient faults are detected, the execution time of a task instance can be increased due to recovery operations. Such fault recovery routines make the system very vulnerable with respect to meeting hard real-time deadlines. In theory and in practical systems, this problem is often handled by aborting not so important tasks to guarantee the response time of the more important tasks. However, for most systems such faults occur rarely and the results of not so important tasks might still be useful, even if they are a bit late. This implicates to not abort these not so important tasks but keep them running even if faults occur, provided that the more important tasks still meet their hard real time properties. In this paper, we present Systems with Dynamic Real-Time Guarantees to model this behaviour and determine if the system can provide full timing guarantees or limited timing guarantees without any online adaptation after a fault occurred. We present a schedulability test, provide an algorithm for optimal priority assignment, determine the maximum interval length until the system will again provide full timing guarantees and explain how we can monitor the system state online. The approaches presented in this paper can also be applied to mixed criticality systems with dual criticality levels.

State of the art for scheduling and analyzing self-suspending sporadic real-time tasks

In computing systems, a job/process/task/thread may suspend itself when it has to wait for some other internal or external activities, such as computation offloading or memory accesses, to finish before it can continue its execution. In the literature, there are two commonly adopted self-suspending sporadic task models in real-time systems: 1) the dynamic self-suspension model and 2) the segmented self-suspension sporadic task model. A dynamic self-suspending sporadic task is specified with an upper bound on the maximum suspension time for a job (task instance), which allows a job to dynamically suspend itself arbitrary often as long as the suspension time upper bound is not violated. By contrast, a segmented self-suspending sporadic task has a predefined execution and suspension pattern in an interleaving manner. The dynamic self-suspension model is very flexible but inaccurate, whilst the segmented self-suspension model is very restrictive but very accurate. The gap between these two widely-adopted self-suspension task models can be potentially filled by the hybrid self-suspension task model. The investigation of the impact of self-suspension on timing predictability has been started in 1988. This survey paper provides a short summary of the state of the art in the design and analysis of scheduling algorithms and schedulability tests for self-suspending tasks in real-time systems.

Configurations and Optimizations of TDMA Schedules for Periodic Packet Communication on Networks on Chip

In recent years, embedded system design has changed due to an increased demand for computing power and a rising complexity of control tasks. In this paper, we present an approach to create a contention-free communication schedule for a Network on Chip (NoC) using a global Time Division Multiple Access (TDMA) schedule. TDMA can provide an isolation property among interfering tasks and a bandwidth guarantee for different communication paths. The approach allows the derivation of a predictable timing behavior for the communication. The communication schedule is designed using two scheduling strategies: a first-fit greedy strategy and a solver-based approach called rectangular allocation. In the experiments, the approaches are evaluated with synthetic communication task sets on a 2D-meshed NoC-topology.

Reliability Optimization on Multi-Core Systems with Multi-Tasking and Redundant Multi-Threading

Using Redundant Multithreading (RMT) for error detection and recovery is a prominent technique to mitigate soft-error effects in multi-core systems. Simultaneous Redundant Threading (SRT) on the same core or Chip-level Redundant Multithreading (CRT) on different cores can be adopted to implement RMT. However, only a few previously proposed approaches use adaptive CRT managements on the system level and none of them considers both SRT and CRT on the task level. In this paper, we propose to use a combination of SRT and CRT, called Mixed Redundant Threading (MRT), as an additional option on the task level. In our coarse-grained approach, we consider SRT, CRT, and MRT on the system level simultaneously, while the existing results only apply either SRT or CRT on the system level, but not simultaneously. In addition, we consider further fine-grained task level optimizations to improve the system reliability under hard real-time constraints. To optimize the system reliability, we develop several dynamic programming approaches to select the redundancy levels under Federated Scheduling. The simulation results illustrate that our approaches can significantly improve the system reliability compared to the state-of-the-art techniques.

Release enforcement in resource-oriented partitioned scheduling for multiprocessor systems

When partitioned scheduling is used in real-time multiprocessor systems, access to shared resources can jeopardize the schedulability if the task partition is not done carefully. To tackle this problem we change our view angle from focusing on the computing tasks to focusing on the shared resources by applying resource-oriented partitioned scheduling. We use a release enforcement technique to shape the interference from the higher-priority jobs to be sporadic, analyze the schedulability, and provide strategies for partitioning both the critical and the non-critical sections of tasks onto processors individually. Our approaches are shown to be effective, both in the evaluations and from a theoretical point of view by providing a speedup factor of 6, improving previously known results.