Browsing by Author "Bletsas, Konstantinos"
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- Assigning real-time tasks on heterogeneous multiprocessors with two types of processorsPublication . Andersson, Björn; Bletsas, KonstantinosConsider the problem of scheduling a set of implicitdeadline sporadic tasks on a heterogeneous multiprocessor so as to meet all deadlines. Tasks cannot migrate and the platform is restricted in that each processor is either of type-1 or type-2 (with each task characterized by a different speed of execution upon each type of processor). We present an algorithm for this problem with a timecomplexity of O(n·m), where n is the number of tasks and m is the number of processors. It offers the guarantee that if a task set can be scheduled by any non-migrative algorithm to meet deadlines then our algorithm meets deadlines as well if given processors twice as fast. Although this result is proven for only a restricted heterogeneous multiprocessor, we consider it significant for being the first realtime scheduling algorithm to use a low-complexity binpacking approach to schedule tasks on a heterogeneous multiprocessor with provably good performance.
- Assigning real-time tasks on heterogeneous multiprocessors with two unrelated types of processorsPublication . Raravi, Gurulingesh; Andersson, Björn; Bletsas, KonstantinosConsider the problem of partitioned scheduling of an implicit-deadline sporadic task set on heterogeneous multiprocessors to meet all deadlines. Each processor is either of type-1 or type-2. We present a new algorithm, FF-3C, for this problem. FF-3C offers low time-complexity and provably good performance. Specifically, FF-3C offers (i) a time-complexity of O(n⋅max(m,logn)+m⋅logm), where n is the number of tasks and m is the number of processors and (ii) the guarantee that if a task set can be scheduled by an optimal partitioned-scheduling algorithm to meet all deadlines then FF-3C meets all deadlines as well if given processors at most 11−α times as fast (referred to as speed competitive ratio) and tasks are scheduled using EDF; where α is a property of the task set. The parameter α is in the range (0,0.5] and for each task, it holds that its utilization is no greater than α or greater than 1−α on each processor type. Thus, the speed competitive ratio of FF-3C can never exceed 2. We also present several extensions to FF-3C; these offer the same performance guarantee and time-complexity but with improved average-case performance. Via simulations, we compare the performance of our new algorithms and two state-of-the-art algorithms (and variations of the latter). We evaluate algorithms based on (i) running time and (ii) the necessary multiplication factor, i.e., the amount of extra speed of processors that the algorithm needs, for a given task set, so as to succeed, compared to an optimal task assignment algorithm. Overall, we observed that our new algorithms perform significantly better than the state-of-the-art. We also observed that our algorithms perform much better in practice, i.e., the necessary multiplication factor of the algorithms is much smaller than their speed competitive ratio. Finally, we also present a clustered version of the new algorithm.
- Cache-aware Schedulability Analysis of PREM Compliant TasksPublication . Rashid, Syed Aftab; Awan, Muhammad Ali; Souto, Pedro; Bletsas, Konstantinos; Tovar, EduardoThe Predictable Execution Model (PREM) is useful for mitigating inter-core interference due to shared resources such as the main memory. However, it is cache-agnostic, which makes schedulabulity analysis pessimistic, via overestimation of prefetches and write-backs. In response, we present cache-aware schedulability analysis for PREM tasks on fixed-task-priority partitioned multicores, that bounds the number of cache prefetches and write-backs. Our approach identifies memory blocks loaded in the execution of a previous scheduling interval of each task, that remain in the cache until its next scheduling interval. Doing so, greatly reduces the estimated prefetches and write backs. In experimental evaluations, our analysis improves the schedulability of PREM tasks by up to 55 percentage points.
- A conjecture about provably good task assignment on heterogeneous multiprocessor platforms but with a stronger adversaryPublication . Raravi, Gurulingesh; Andersson, Björn; Bletsas, KonstantinosConsider the problem of scheduling a set of implicit-deadline sporadic tasks to meet all deadlines on a heterogeneous multiprocessor platform. We use an algorithm proposed in [1] (we refer to it as LP-EE) from state-of-the-art for assigning tasks to heterogeneous multiprocessor platform and (re-)prove its performance guarantee but for a stronger adversary.We conjecture that if a task set can be scheduled to meet deadlines on a heterogeneous multiprocessor platform by an optimal task assignment scheme that allows task migrations then LP-EE meets deadlines as well with no migrations if given processors twice as fast. We illustrate this with an example.
- Considerations on the Least Upper Bound for Mixed-Criticality Real-Time SystemsPublication . Santos-Jr., J. Augusto; Lima, George; Bletsas, KonstantinosReal-time mixed-criticality systems (MCS) are designed so that tasks with different criticality levels share the same computing platform. Scheduling mechanisms must ensure that high criticality tasks are safe independently of lower criticality tasks’ behaviour. In this paper we provide theoretical schedulability properties for MCS by showing that: (a) the least upper bound on processor utilisation of MCS is in general null for both uniprocessor and multiprocessor platforms; (b) this bound lies in interval [ln 2, 2( √2 − 1)] if higher criticality tasks do not have periods larger than lower criticality ones; and (c) if the task of these uniprocessor systems have harmonic periods, the least upper bound reaches 1.
- CPMD-mindful task assignment for NPS-FPublication . Nelissen, Geoffrey; Bletsas, Konstantinos; Goossens, JoelThe multiprocessor scheduling scheme NPS-F for sporadic tasks has a high utilisation bound and an overall number of preemptions bounded at design time. NPS-F binpacks tasks offline to as many servers as needed. At runtime, the scheduler ensures that each server is mapped to at most one of the m processors, at any instant. When scheduled, servers use EDF to select which of their tasks to run. Yet, unlike the overall number of preemptions, the migrations per se are not tightly bounded. Moreover, we cannot know a priori which task a server will be currently executing at the instant when it migrates. This uncertainty complicates the estimation of cache-related preemption and migration costs (CPMD), potentially resulting in their overestimation. Therefore, to simplify the CPMD estimation, we propose an amended bin-packing scheme for NPS-F allowing us (i) to identify at design time, which task migrates at which instant and (ii) bound a priori the number of migrating tasks, while preserving the utilisation bound of NPS-F.
- Decoupling Criticality and Importance in Mixed-Criticality SchedulingPublication . Bletsas, Konstantinos; Ali Awan, Muhammad; Souto, Pedro; Åkesson, Benny; Burns, Alan; Tovar, EduardoResearch on mixed-criticality scheduling has flourished since Vestal’s seminal 2007 paper, but more efforts are needed in order to make these results more suitable for industrial adoption and robust and versatile enough to influence the evolution of future certification standards in keeping up with the times. With this in mind, we introduce a more refined task model, in line with the fundamental principles of Vestal’s mode-based adaptive mixed-criticality model, which allows a task’s criticality and its importance to be specified independently from each other. A task’s importance is the criterion that determines its presence in different system modes. Meanwhile, the task’s criticality (reflected in its Safety Integrity Level (SIL) and defining the rules for its software development process), prescribes the degree of conservativeness for the task’s estimated WCET during schedulability testing. We indicate how such a task model can help resolve some of the perceived weaknesses of the Vestal model, in terms of how it is interpreted, and demonstrate how the existing scheduling tests for the classic variant’s of Vestal’s model can be mapped to the new task model essentially without changes.
- Efficient schedulability tests for real-time embedded systems with urgent routinesPublication . Santos Jr., J. Augusto; Lima, George; Bletsas, KonstantinosTask scheduling is one of the key mechanisms to ensure timeliness in embedded real-time systems. Such systems have often the need to execute not only application tasks but also some urgent routines (e.g. error-detection actions, consistency checkers, interrupt handlers) with minimum latency. Although fixed-priority schedulers such as Rate-Monotonic (RM) are in line with this need, they usually make a low processor utilization available to the system. Moreover, this availability usually decreases with the number of considered tasks. If dynamic-priority schedulers such as Earliest Deadline First (EDF) are applied instead, high system utilization can be guaranteed but the minimum latency for executing urgent routines may not be ensured. In this paper we describe a scheduling model according to which urgent routines are executed at the highest priority level and all other system tasks are scheduled by EDF. We show that the guaranteed processor utilization for the assumed scheduling model is at least as high as the one provided by RM for two tasks, namely 2(2√−1). Seven polynomial time tests for checking the system timeliness are derived and proved correct. The proposed tests are compared against each other and to an exact but exponential running time test.
- Errata for Three Papers (2004-05) on Fixed-Priority Scheduling with Self-SuspensionsPublication . Bletsas, Konstantinos; Audsley, Neil; Huang, Wen-Hung; Chen, Jian-Jia; Nelissen, GeoffreyThe purpose of this article is to (i) highlight the flaws in three previously published works [Audsley, 2004a; Audsley, 2004b; Bletsas, 2005] on the worst-case response time analysis for tasks with self-suspensions and (ii) provide straightforward fixes for those flaws, hence rendering the analysis safe.
- Hard real-time multiprocessor scheduling resilient to core failuresPublication . Nikolic, Borislav; Bletsas, Konstantinos; M. Petters, StefanMost multiprocessor scheduling theory overlooks the possibility of hardware failures that entirely nullify the computation carried out by a task instance, and potentially also make the respective processor henceforth unusable. Yet, such failures may occur, causing the system to fail. Motivated by this reality, we introduce a new concept of hard real-time schedulability guarantees for critical multiprocessor systems and analysis for their derivation. Namely, all deadlines must be met, even in the event of a core failure. A scheduling approach, based on global fixed priorities, and accompanying analysis, for achieving such guarantees are then formulated.