Mmhp Martijn van den Heuvel

Generalized Fixed-Priority Scheduling with Limited Preemptions

Fixed-priority scheduling with deferred preemption(FPDS) and fixed-priority scheduling with preemption thresholds(FPTS) have been proposed in the literature as viable alternatives to fixed-priority preemptive scheduling (FPPS), that reduce memory requirements, reduce the cost of arbitrary preemptions, and may improve the feasibility of a task set even when preemption overheads are neglected. This paper aims at advancing the relative strength of limited preemptive schedulers by combining FPDS and FPTS. In particular, we present a refinement of FPDS with preemption thresholds for both jobs and sub-jobs, termed FPGS. We provide an exact schedulability analysis for FPGS, and show how to maximize the feasibility of a set of sporadic tasks under FPGS for given priorities, computation times, periods, and deadlines of tasks. We evaluate the effectiveness of FPGS by comparing the feasibility of task sets under FPGS with other fixed-priority scheduling algorithms by means of a simulation. Our experiments show that FPGS allows an increase of the number of task sets that are schedulable under fixed-priority scheduling.

Protocol-transparent resource sharing in hierarchically scheduled real-time systems

Hierarchical scheduling frameworks (HSFs) provide means for composing complex real-time systems from well-defined, independently analyzed subsystems. To support resource sharing within two-level HSFs, three synchronization protocols based on the stack resource policy (SRP) have recently been presented, i.e. HSRP [1], SIRAP [2] and BROE [3]. This paper describes the first implementation presenting these three SRP-based synchronization protocols side-by-side in a HSF-enabled real-time operating system. We base our implementations on the commercially available real-time operating system μC/OS-II, extended with proprietary support for periodic tasks, idling periodic servers and two-level preemptive scheduling. Moreover, we investigate the system overhead of the synchronization primitives of each protocol. Transparent interfaces allow a protocol to be selected during integration time based on its relative strengths1.

Constant-bandwidth supply for priority processing

Todays consumer electronic devices feature multiple applications which have to share scarcely available resources. We consider a priority-processing-based video application, which comprises multiple scalable video algorithms (SVAs) that are executed on a shared, virtual platform. This application is given a guaranteed processor share by means of a constant-bandwidth server (CBS), which in addition efficiently reclaims all spare processor time. A decision scheduler distributes the assigned processor share among the SVAs, with the aim to maximize their overall output quality. To correctly distribute this processor share we introduce the concept of a virtual timer. This timer only advances when its associated virtual platform is executing.

Dependable Resource Sharing for Compositional Real-Time Systems

Hierarchical scheduling frameworks (HSFs) have been extensively investigated as a paradigm for facilitating temporal isolation between components that need to be integrated on a single shared processor. In the presence of shared resources, however, temporal isolation may break when one of the accessing components executes longer than specified during global resource access. The ability to confine such temporal faults makes the HSF more dependable. As a solution we propose a stack-resource-policy (SRP)-based synchronization protocol for HSFs, named Hierarchical Synchronization protocol with Temporal Protection (HSTP). When a component exceeds its specified critical-section length, HSTP enforces a component to self-donate its own budget to accelerate the resource release. In addition, a component that blocks on a locked resource may donate budget. The schedulability of those components that are independent of the locked resource is unaffected. HSTP efficiently limits the propagation of temporal faults to resource-sharing components by disabling local preemptions in a component during resource access. We finally show that HSTP is SRP-compliant and applies to existing synchronization protocols for HSFs.

Improved feasibility of fixed-priority scheduling with deferred preemption using preemption thresholds for preemption points

This paper aims at advancing the relative strength of limited-preemptive schedulers by improving the feasibility of a task set and simultaneously limiting, or even precluding, arbitrary preemptions. In particular, we present a refinement of existing limited-preemptive fixed-priority scheduling (FPS) schemes with preemption thresholds for preemption points next to preemption thresholds for sub-jobs, termed fixed-priority scheduling with varying preemption thresholds (FPVS). We derive exact schedulability analysis for FPVS and we develop algorithms to maximize the schedulability of a set of sporadic tasks for given priorities. Since FPVS generalizes existing FPS schemes, we apply our algorithms to those schemes to compare the ratio of schedulable systems. Our experiments show that FPVS can achieve the same schedulability ratio with limited-preemptive sub-jobs as with entirely non-preemptive sub-jobs.

Exploiting harmonic periods to improve linearly approximated response-time upper bounds

Hard real-time embedded systems need to guarantee that tasks always meet their deadlines. Exact schedulability tests can guarantee this for fixed-priority, preemptively scheduled systems even under the tightest resource constraints. However, these tests are pseudo-polynomial in complexity. This can become a limiting factor in open systems where it might be necessary to have run-time admission tests. A linear-time sufficient test [1] has therefore been developed to estimate response-time upper bounds. In line with utilization-based sufficient tests, we propose to improve this test for task sets with harmonically related task periods. Moreover, we make it possible to reuse this test in the context of hierarchically scheduled (partitioned) resources. In such systems several applications are given a virtual share (budget) of the processor. By modeling the unavailability of processor resources to an application as two fictive tasks, we can also use a budgets period to improve response-time bounds1.

Fixed priority scheduling with pre-emption thresholds and cache-related pre-emption delays: integrated analysis and evaluation

Commercial off-the-shelf programmable platforms for real-time systems typically contain a cache to bridge the gap between the processor speed and main memory speed. Because cache-related pre-emption delays (CRPD) can have a significant influence on the computation times of tasks, CRPD have been integrated in the response time analysis for fixed-priority pre-emptive scheduling (FPPS). This paper presents CRPD aware response-time analysis of sporadic tasks with arbitrary deadlines for fixed-priority pre-emption threshold scheduling (FPTS), generalizing earlier work. The analysis is complemented by an optimal (pre-emption) threshold assignment algorithm, assuming the priorities of tasks are given. We further improve upon these results by presenting an algorithm that searches for a layout of tasks in memory that makes a task set schedulable. The paper includes an extensive comparative evaluation of the schedulability ratios of FPPS and FPTS, taking CRPD into account. The practical relevance of our work stems from FPTS support in AUTOSAR, a standardized development model for the automotive industry. [(This paper forms an extended version ofxa0Bril et al. (in Proceedings of 35th IEEE real-time systems symposium (RTSS), 2014). The main extensions are described in Sect.xa01.2.]

Mode-based resource-usage prediction for runtime resource reservations of video components

Scalability and reusability of software components enable consumer electronics products to become more open and flexible. Some important features in many of those products are provided via video components. Since these components have highly fluctuating resource usage, dynamic re-allocation of resources is more cost-effective than static allocation. Dynamic resource allocation, however, requires strategies and mechanisms for predicting the requested resources of a component. In this paper, a novel mechanism to detect resource-usage modes for video components is proposed. First, these resource-usage modes are defined and experimentally identified for two example video components. Second, a modedetection algorithm and heuristics for choosing proper parameters for mode detection are introduced. Finally, the run-time properties of a mode-prediction mechanism are evaluated.

An engineering approach to synchronization based on overrun for compositional real-time systems

Hierarchical scheduling frameworks (HSFs) provide means for composing complex real-time systems from well-defined independently developed and analyzed subsystems. To support shared logical resources requiring mutual exclusive access in two-level HSFs, overrun without payback has been proposed as a mechanism to prevent budget depletion during resource access arbitrated by the stack resource policy (SRP). In this paper, we revisit the global schedulability analysis of synchronization protocols based on SRP and overrun without payback for fixed-priority scheduled HSFs. We derive a new global schedulability analysis based on the observation that the overrun budget is merely meant to prevent budget depletion during global resource access. The deadline of a subsystem therefore only needs to hold for its normal budget rather than the sum of the normal and overrun budget. Our novel analysis is considerably simpler than an earlier, initially improved analysis, which improved both the original local and global schedulability analyses. We evaluate the new analysis based on an extensive simulation study and compare the results with the existing analysis. Our simplified analysis does not significantly affect schedulability compared to the initially improved analysis. It is therefore proposed as a preferable engineering approach to synchronization protocols for compositional real-time systems. We accordingly present the implementation of our improvement in an OSEK-compliant real-time operating system to sketch its applicability in todays industrial automotive standards. Both implementation and run-time overheads are discussed providing measured results1.

Extending ExSched with Mixed Criticality Support — An Experience Report

This paper describes our experience with extending ExSched, an operating system independent external CPU scheduler framework for real-time systems, with support for mixed criticality. We used the so-called adaptive mixed criticality (AMC) scheme as a starting point for mixed criticality. We extended that scheme from two to more than two criticality levels and complemented it with specified behavior for criticality level changes.