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Different task models have been proposed to represent the parallel structure of real-time tasks executing on manycore platforms: fork/join, synchronous parallel, DAG-based, etc. Despite different schedulability tests and resource augmentation bounds are available for these task systems, we experience difficulties in applying such results to real application scenarios, where the execution flow of parallel tasks is characterized by multiple (and nested) conditional structures. When a conditional branch drives the number and size of sub-jobs to spawn, it is hard to decide which execution path to select for modeling the worst-case scenario. To circumvent this problem, we integrate control flow information in the task model, considering conditional parallel tasks (cp-tasks) represented by DAGs composed of both precedence and conditional edges. For this task model, we identify meaningful parameters that characterize the schedulability of the system, and derive efficient algorithms to compute them. A response time analysis based on these parameters is then presented for different scheduling policies. A set of simulations shows that the proposed approach allows efficiently checking the schedulability of the addressed systems, and that it significantly tightens the schedulability analysis of non-conditional (e.g., Classic DAG) tasks over existing approaches.

/pdf/response-time-analysis-of-conditional-dag-tasks-in-3rq9tuzu7k.pdf

Response-Time Analysis of Conditional DAG Tasks in Multiprocessor Systems

Since its official public release, Android has captured the interest from companies, developers and the general audience. From that time up to now, this software platform has been constantly improved either in terms of features or supported hardware and, at the same time, extended to new types of devices different from the originally intended mobile ones. However, there is a feature that has not been explored yet its real-time capabilities. This paper intends to explore this gap and provide a basis for discussion on the suitability of Android in order to be used in Open Real-Time environments. By analysing the software platform, with the main focus on the virtual machine and its underlying operating system environments, we are able to point out its current limitations and, therefore, provide a hint on different perspectives of directions in order to make Android suitable for these environments. It is our position that Android may provide a suitable architecture for real-time embedded systems, but the real-time community should address its limitations in a joint effort at all of the platform layers. Evaluating Android OS for Embedded Real-Time Systems Claudio Maia, Luis Nogueira, Luis Miguel Pinho CISTER Research Centre School of Engineering of the Polytechnic Institute of Porto Porto, Portugal Email:{crrm,lmn,lmp}@isep.ipp.pt Abstract—Since its official public release, Android has captured the interest from companies, developers and the general audience. From that time up to now, this software platform has been constantly improved either in terms of features or supported hardware and, at the same time, extended to new types of devices different from the originally intended mobile ones. However, there is a feature that has not been explored yet its real-time capabilities. This paper intends to explore this gap and provide a basis for discussion on the suitability of Android in order to be used in Open Real-Time environments. By analysing the software platform, with the main focus on the virtual machine and its underlying operating system environments, we are able to point out its current limitations and, therefore, provide a hint on different perspectives of directions in order to make Android suitable for these environments. It is our position that Android may provide a suitable architecture for real-time embedded systems, but the real-time community should address its limitations in a joint effort at all of the platform layers.Since its official public release, Android has captured the interest from companies, developers and the general audience. From that time up to now, this software platform has been constantly improved either in terms of features or supported hardware and, at the same time, extended to new types of devices different from the originally intended mobile ones. However, there is a feature that has not been explored yet its real-time capabilities. This paper intends to explore this gap and provide a basis for discussion on the suitability of Android in order to be used in Open Real-Time environments. By analysing the software platform, with the main focus on the virtual machine and its underlying operating system environments, we are able to point out its current limitations and, therefore, provide a hint on different perspectives of directions in order to make Android suitable for these environments. It is our position that Android may provide a suitable architecture for real-time embedded systems, but the real-time community should address its limitations in a joint effort at all of the platform layers. Keywords-Android, Open Real-Time Systems, Embedded Systems

/pdf/evaluating-android-os-for-embedded-real-time-systems-50w2hw89p6.pdf

Evaluating Android OS for Embedded Real-Time Systems

Federated scheduling is a promising approach to schedule parallel real-time tasks on multi-cores, where each heavy task exclusively executes on a number of dedicated processors, while light tasks are treated as sequential sporadic tasks and share the remaining processors. However, federated scheduling suffers resource waste since a heavy task with processing capacity requirement x+epsilon (where x is an integer and 0 epsilon 1) needs x+1 dedicated processors. In the extreme case, almost half of the processing capacity is wasted. In this paper we propose the semi-federate scheduling approach, which only grants x dedicated processors to a heavy task with processing capacity requirement x+epsilon, and schedules the remaining epsilon part together with light tasks on shared processors. Experiments with randomly generated task sets show the semi-federated scheduling approach significantly outperforms not only federated scheduling, but also all existing approaches for scheduling parallel real-time tasks on multi-cores.

Semi-Federated Scheduling of Parallel Real-Time Tasks on Multiprocessors

OpenMP is increasingly being supported by the newest high-end embedded many-core processors. Despite the lack of any notion of real-time execution, the latest specification of OpenMP (v4.0) introduces a tasking model that resembles the way real-time embedded applications are modeled and designed, i.e., as a set of periodic task graphs. This makes OpenMP4 a convenient candidate to be adopted in future real-time systems. However, OpenMP4 incorporates as well features to guarantee backward compatibility with previous versions that limit its practical usability in real-time systems. The most notable example is the distinction between tied and untied tasks. Tied tasks force all parts of a task to be executed on the same thread that started the execution, whereas a suspended untied task is allowed to resume execution on a different thread. Moreover, tied tasks are forbidden to be scheduled in threads in which other non-descendant tied tasks are suspended. As a result, the execution model of tied tasks, which is the default model in OpenMP to simplify the coexistence with legacy constructs, clearly restricts the performance and has serious implications on the response time analysis of OpenMP4 applications, making difficult to adopt it in real-time environments. In this paper, we revisit OpenMP design choices, introducing timing predictability as a new and key metric of interest. Our first results confirm that even if tied tasks can be timing analyzed, the quality of the analysis is much worse than with untied tasks. We thus reason about the benefits of using untied tasks, deriving a response time analysis for this model, and so allowing OpenMP4 untied model to be applied to real-time systems.

/pdf/timing-characterization-of-openmp4-tasking-model-57dyclajqy.pdf

Timing characterization of OpenMP4 tasking model

Programmers resort to user-level parallel frameworks in order to exploit the parallelism provided by multiprocessor platforms. While such general frameworks do not support the stringent timing requirements of real-time systems, they offer a useful model of computation based on the standard fork/join, for which the analysis of timing properties makes sense. Very few works analyse the schedulability of synchronous parallel real-time tasks, which is a generalisation of the standard fork/join model. This paper proposes to narrow the gap by presenting a model that analyses the response-time of synchronous parallel real-time tasks. The model under consideration targets tasks with fixed priorities, composed of several segments with an arbitrary number of parallel and independent units of execution. We contribute to the state-of-the-art by analysing the response-time behaviour of synchronous parallel tasks. To accomplish this, we take into account concepts previously proposed in the literature and define new concepts such as carry-out decomposition and sliding window technique in order to compute the worst-case workload in a window of interest. Results show that the proposed approach is significantly better than current approaches, improving the state-of-the-art analysis of parallel real-time tasks.

/pdf/response-time-analysis-of-synchronous-parallel-tasks-in-4fssi46byj.pdf

Response-Time Analysis of Synchronous Parallel Tasks in Multiprocessor Systems

Scheduling real-time applications on general purpose multicore platforms is a challenging problem from a timing analysis perspective. Such platforms expose uncontrolled sources of interference whenever concurrent accesses to memory are performed. The non-deterministic bus and memory access behavior complicates the estimations of applications' worst-case execution times (WCET). The 3-phase task model seems a good candidate to circumvent the uncontrolled sources of interference by isolating concurrent memory accesses. A task is divided in three successive phases; first, the task loads its instruction and data in a local memory, then it executes non-preemptively using those pre-loaded instructions and data, and finally, the modified data are pushed back to main memory. Following this execution model, tasks never access the bus during their execution phase. Instead, all the bus accesses are performed during the first and third phases. In this paper, we focus on the global fixed-priority scheduling of the 3-phase task model. A new schedulability test is derived by modelling the interference happening on the bus rather than the interference on the cores as in the state-of-the-art techniques. The effectiveness of the test is evaluated by comparing it against the state-of-the-art.

/pdf/schedulability-analysis-for-global-fixed-priority-scheduling-rqgvosm7iu.pdf

Schedulability analysis for global fixed-priority scheduling of the 3-phase task model

Commercial-of-the-shelf based multi-core systems present timing anomalies that cannot be ignored by the real-time systems community due to their unpredictable behaviour. These timing anomalies, often caused by applications' uncontrolled accesses to shared resources such as the components in the memory hierarchy or in the I/O subsystem, introduce interference that may lead to deadline misses if the problem is neglected.

/pdf/a-closer-look-into-the-aer-model-47ywmzfz4w.pdf

A closer look into the AER Model

This paper explores the behavior of parallel fork-join tasks on multicore platforms by resorting to a semi-partitioned scheduling model. This model offers a promising framework to embedded systems which are subject to stringent timing constraints as it provides these systems with very interesting properties. The proposed approach consists of two stages -- an offline stage and an online stage. During the offline stage, a multi-frame task model is adopted to perform the fork-join task-to-core mapping so as to improve the schedulability and the performance of the system, and during the online stage, work-stealing is exploited among cores to improve the system responsiveness as well as to balance the execution workload. The objective of this work is twofold: (1) to provide an alternative technique that takes advantage of the semi-partitioned scheduling properties by offering the possibility to accommodate fork-join tasks that cannot be scheduled in any pure partitioned environment, and (2) to reduce the migration overhead which has shown to be a traditional major source of non-determinism in global approaches. The simulation results show an improvement of the proposed approach over the state-of-the-art of up to 15% of the average response-time per task set.

/pdf/semi-partitioned-scheduling-of-fork-join-tasks-using-work-2vyn5dljwy.pdf

Cláudio Maia

Papers

Evaluating Android OS for Embedded Real-Time Systems

Response-Time Analysis of Synchronous Parallel Tasks in Multiprocessor Systems

Schedulability analysis for global fixed-priority scheduling of the 3-phase task model

A closer look into the AER Model

Semi-Partitioned Scheduling of Fork-Join Tasks Using Work-Stealing