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Resource sharing using the rollback mechanism in hierarchically scheduled real-time open systems
Mälardalen University, School of Innovation, Design and Engineering.ORCID iD: 0000-0001-6157-5199
Mälardalen University, School of Innovation, Design and Engineering.ORCID iD: 0000-0001-6132-7945
Mälardalen University, School of Innovation, Design and Engineering.ORCID iD: 0000-0002-1687-930X
2013 (English)In: Real-Time Technology and Applications - Proceedings, 2013, 129-140 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this paper we present a new synchronization protocol called RRP (Rollback Resource Policy) which is compatible with hierarchically scheduled open systems and specialized for resources that can be aborted and rolled back. We conduct an extensive event-based simulation and compare RRP against all equivalent existing protocols in hierarchical fixed priority preemptive scheduling; SIRAP (Subsystem Integration and Resource Allocation Policy), OPEN-HSRPnP (open systems version of Hierarchical Stack Resource Policy no Payback) and OPEN-HSRPwP (open systems version of Hierarchical Stack Resource Policy with Payback). Our simulation study shows that RRP has better average-case response-times than the state-of-the-art protocol in open systems, i.e., SIRAP, and that it performs better than OPEN-HSRPnP/OPEN-HSRPwP in terms of schedulability of randomly generated systems. The simulations consider both resources that are compatible with rollback as well as resources incompatible with rollback (only abort), such that the resource-rollback overhead can be evaluated. We also measure CPU overhead costs (in VxWorks) related to the rollback mechanism of tasks and resources. We use the eXtremeDB (embedded real-time) database to measure the resource-rollback overhead.

Place, publisher, year, edition, pages
2013. 129-140 p.
Series
Real-Time Technology and Applications - Proceedings
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:mdh:diva-20920DOI: 10.1109/RTAS.2013.6531086Scopus ID: 2-s2.0-84881110752OAI: oai:DiVA.org:mdh-20920DiVA: diva2:645828
Conference
013 IEEE 19th Real-Time and Embedded Technology and Applications Symposium, RTAS 2013; Philadelphia, PA; United States; 9 April 2013 through 11 April 2013
Available from: 2013-09-05 Created: 2013-08-16 Last updated: 2014-01-07Bibliographically approved
In thesis
1. Synthesis and Synchronization Support for Hierarchically Scheduled Real-Time Systems
Open this publication in new window or tab >>Synthesis and Synchronization Support for Hierarchically Scheduled Real-Time Systems
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A piece of software, that we define as a software system, can consist of anything from a few lines of program code or the entire software stack in a vehicle. Software systems can be divided into smaller and partially independent parts called subsystems/partitions (we use the words partition and subsystem interchangeably). The non-functional isolation of subsystems, that appears when the software system is hierarchically divided, has great advantages when it comes to preventing fault propagation between subsystems. The hierarchical division, that we refer to as hierarchical scheduling, has other advantages as well. It facilitates re-usability and it makes timing analysis of software systems easier. Hierarchical scheduling has been shown to be a useful tool in counteracting the verification challenges that comes from the growing complexity in software. For example, the avionics-specification ARINC653 and the safety-critical operating systems seL4 and PikeOS safely divide resources for independent safety-critical applications by using hierarchical scheduling.

Hierarchical scheduling can be implemented in many different ways, depending on what resource that is supposed to be shared among applications. The resource could be the CPU, memory, network etc. The work in this thesis is focused on the practical aspects of timing isolation among subsystems, i.e., sharing of the CPU resource. Hence, this work elaborates on how to adapt and extend the operating-system task-scheduler to support hierarchical scheduling. We have focused on both independent and semi-dependent subsystems. Independent subsystems only share general resources such as the CPU and memory. Semi-independent subsystems share not only the general resources, but also other logical resources that can only be accessed in a mutually exclusive way, i.e., by one subsystem at a time. An example of such a resource could be a shared memory-space, e.g., a database, a memory-mapped device etc.

This thesis has two main parts related to hierarchical scheduling: scheduler synthesis, and synchronization.

Scheduler synthesis is related to implementation and design strategies when adding support for hierarchical scheduling in an operating system. We have focused on various operating systems that were lacking the feature of hierarchical scheduling. The two most interesting operating systems that we worked on was Linux and seL4. These two operating systems represent two extremes, where Linux is more focused towards soft real-time systems and seL4 towards pure hard real-time (safety-critical) systems. Linux-based systems have in general less strict demands on correctness and more requirements on usability. Usability implies less installation efforts and less limitations in the usage of the available Linux functionality. The usability aspect is especially important for Linux systems since kernel updates occur much more frequently compared to any other operating system. Hence, extending/modifying the functionality of Linux must be done in a way that does not require any modifications to the kernel. seL4 on the other hand has strict requirements on safety, i.e., functional and non-functional correctness, but also performance efficiency. Guaranteeing correctness implies a potential loss of performance due to the added overhead that the verified software can bring. The correctness aspect includes strategies on how to verify hierarchical schedulers, but also how to minimize the scheduler overhead and achieve as good run-time performance as possible. Conclusively, there are many challenges when it comes to scheduler synthesis. There are requirements on performance, usability, correctness etc. The contribution in the synthesis part includes a scheduler framework called ExSched (External Scheduler). We have also contributed with a novel approach to verify hierarchical schedulers, and a code generator called TAtoC (Timed Automata to C) which contributes to the effective run-time performance of synthesized timed-automata models.

The second part of this thesis, synchronization, is an important general aspect of hierarchically scheduled systems since the isolation of subsystems makes resource sharing among subsystems more challenging. We have advanced the state-of-the-art in this research area by introducing a new synchronization protocol called RRP (Rollback Resource Policy) that improves on the robustness and run-time performance compared to the existing protocols. We have also conducted a large scale experimental evaluation of all existing protocols that we have implemented in the widely used real-time operating system VxWorks.

Place, publisher, year, edition, pages
Västerås: Mälardalen University, 2014. 266 p.
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 149
National Category
Computer Engineering
Research subject
Computer Science
Identifiers
urn:nbn:se:mdh:diva-23462 (URN)978-91-7485-131-1 (ISBN)
Public defence
2014-01-31, Gamma, Västerås, 10:00 (English)
Opponent
Supervisors
Available from: 2014-01-07 Created: 2013-12-12 Last updated: 2014-01-21Bibliographically approved

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