Combining Time-Triggered Plans with Priority Scheduled Task Sets

The final publication is available at Springer via http://dx.doi.org/10.1007/978-3-319-39083-3_13Time-triggered and concurrent priority-based scheduling are the two major approaches in use for real-time and embedded systems. Both approaches have their own advantages and drawbacks. On the one hand, priority-based systems facilitate separation of concerns between functional and timing requirements by relying on an underlying real- time operating system that takes all scheduling decisions at run time. But this is at the cost of indeterminism in the exact timing pattern of execution of activities, namely variable release jitter. On the other hand, time-triggered schedules are more intricate to design since all schedul- ing decisions must be taken beforehand in the design phase, but their advantage is determinism and more chances for minimisation of release jitter. In this paper we propose a software architecture that enables the combined and controlled execution of time-triggered plans and priority- scheduled tasks. We also describe the implementation of an Ada library supporting it. Our aim is to take advantage of the best of both ap- proaches by providing jitter-controlled execution of time-triggered tasks (e.g., control tasks), coexisting with a set of priority-scheduled tasks, with less demanding jitter requirements.This work has been partly supported by the Spanish Government’s project M2C2 (TIN2014-56158-C4-1-P-AR) and the European Commission’s project EMC2 (ARTEMIS-JU Call 2013 AIPP-5, Contract 621429).Real Sáez, JV.; Sáez Barona, S.; Crespo, A. (2016). Combining Time-Triggered Plans with Priority Scheduled Task Sets. En Reliable Software Technologies – Ada-Europe 2016. Springer. 195-212. https://doi.org/10.1007/978-3-319-39083-3_13S195212Liu, C., Layland, J.: Scheduling algorithms for multiprogramming in a hard real-time environment. J. ACM 20(1), 46–61 (1973)Martí, P., Fuertes, J., Fohler, G.: Jitter compensation for real-time control systems. In: Real-Time Systems Symposium (2001)Dobrin, R.: Combining off-line schedule construction and fixed priority scheduling in real-time computer systems. Ph.D. thesis. Mälardalen University (2005)Cervin, A.: Integrated control and real-time scheduling. Ph.D. thesis. Lund Institute of Technology, April 2003Balbastre, P., Ripoll, I., Vidal, J., Crespo, A.: A task model to reduce control delays. Real-Time Syst. 27(3), 215–236 (2004)Hong, S., Hu, X., Lemmon, M.: Reducing delay jitter of real-time control tasks through adaptive deadline adjustments. In: 22nd Euromicro Conference on Real-Time Systems - ECRTS, pp. 229–238. IEEE Computer Society (2010)ISO/IEC-JTC1-SC22-WG9: Ada Reference Manual ISO/IEC 8652:2012(E) (2012). http://www.ada-europe.org/manuals/LRM-2012.pdfBaker, T.P., Shaw, A.: The cyclic executive model and Ada. In: Proceedings IEEE Real Time Systems Symposium 1988, Huntsville, Alabama, pp. 120–129 (1988)Liu, J.W.S.: Real-Time Systems. Prentice-Hall Inc., Upper Saddle River (2000)Pont, M.J.: The Engineering of Reliable Embedded Systems: LPC1769. SafeTTy Systems Limited, Skelmersdale (2014). ISBN: 978-0-9930355-0-0Aldea Rivas, M., González Harbour, M.: MaRTE OS: an Ada kernel for real-time embedded applications. In: Strohmeier, A., Craeynest, D. (eds.) Ada-Europe 2001. LNCS, vol. 2043, pp. 305–316. Springer, Heidelberg (2001)Palencia, J., González-Harbour, M.: Schedulability analysis for tasks with static and dynamic offsets. In: 9th IEEE Real-Time Systems Symposium (1998)Wellings, A.J., Burns, A.: A framework for real-time utilities for Ada 2005. Ada Lett. XXVI XXVII(2), 41–47 (2007)Real, J., Crespo, A.: Incorporating operating modes to an Ada real-time framework. Ada Lett. 30(1), 73–85 (2010)Sáez, S., Terrasa, S., Crespo, A.: A real-time framework for multiprocessor platforms using Ada 2012. In: Romanovsky, A., Vardanega, T. (eds.) Ada-Europe 2011. LNCS, vol. 6652, pp. 46–60. Springer, Heidelberg (2011

Combined Scheduling of Time-Triggered Plans and Priority Scheduled Task Sets

© Owner/Author (2016). This is the author's version of the work. It is posted here for your personal use. Not for redistribution. The definitive Version of Record was published in ACM SIGAda Ada Letters, 36(1), 68-76, http://dx.doi.org/10.1145/10.1145/2971571.2971580.[EN] Preemptive, priority-based scheduling on the one hand, and time-triggered scheduling on the other, are the two major techniques in use for development of real-time and embedded software. Both have their advantages and drawbacks with respect to the other, and are commonly adopted in mutual exclusion. In a previous paper, we proposed a software architecture that enables the combined and controlled execution of time-triggered plans and priority-scheduled tasks. The goal was to take advantage of the best of both approaches by providing deterministic, jitter-controlled execution of time-triggered tasks (e.g., control tasks), coexisting with a set of priority-scheduled tasks, with less demanding jitter requirements. In this paper, we briefly describe the approach, in which the time-triggered plan is executed at the highest priority level, controlled by scheduling decisions taken only at particular points in time, signalled by recurrent timing events. The rest of priority levels are used by a set of concurrent tasks scheduled by static or dynamic priorities. We also discuss several open issues such as schedulability analysis, use of the approach in multiprocessor architectures, usability in mixed-criticality systems and needed changes to make this approach Ravenscar compliant.This work has been partly supported by the Spanish Government’s project M2C2 (TIN2014-56158-C4-1-P-AR) and the European Commission’s project EMC2 (ARTEMIS-JU Call 2013 AIPP-5, Contract 621429).Real Sáez, JV.; Sáez Barona, S.; Crespo Lorente, A. (2016). Combined Scheduling of Time-Triggered Plans and Priority Scheduled Task Sets. Ada Letters. 36(1):68-76. https://doi.org/10.1145/2971571.2971580S6876361T. P. Baker and A. Shaw. The cyclic executive model and Ada. In Proceedings IEEE Real Time Systems Symposium 1988, Huntsville, Alabama, pages 120--129, 1988.P. Balbastre, I. Ripoll, J. Vidal, and A. Crespo. A Task Model to Reduce Control Delays. Real-Time Systems, 27(3):215--236, September 2004.A. Burns and R. Davis. Mixed Criticality Systems - A Review. Technical report, Depatment of Computer Science, University of York, 2013.A. Cervin. Integrated Control and Real-Time Scheduling. PhD thesis, Lund Institute of Technology, April 2003.R. Dobrin. Combining Offline Schedule Construction and Fixed Priority Scheduling in Real-Time Computer Systems. PhD thesis, Mälardalen University, 2005.S. Hong, X. Hu, and M. Lemmon. Reducing Delay Jitter of Real-Time Control Tasks through Adaptive Deadline Adjustments. In IEEE Computer Society, editor, 22nd Euromicro Conference on Real-Time Systems -- ECRTS, pages 229--238, 2010.J. W. S. Liu. Real-Time Systems. Prentice-Hall Inc., 2000.J. Palencia and M. González-Harbour. Schedulability Analysis for Tasks with Static and Dynamic Offsets. In 9th IEEE Real-Time Systems Symposium, 1998.M. J. Pont. The Engineering of Reliable Embedded Systems: LPC1769 edition. Number ISBN: 978-0-9930355-0-0. SafeTTy Systems Limited, 2014.J. Real and A. Crespo. Incorporating Operating Modes to an Ada Real-Time Framework. Ada Letters, 30(1):73--85, April 2010.J. Real, S. Sáez, and A. Crespo. Combining time-triggered plans with priority scheduled task sets. In M. Bertogna and L. M. Pinho, editors, Reliable Software Technologies -- Ada-Europe 2016, volume 9695 of Lecture Notes in Computer Science. Springer, June 2016.S. Sáez, J. Real, and A. Crespo. An integrated framework for multiprocessor, multimoded real-time applications. In M. Brorsson and L. Pinho, editors, Reliable Software Technologies -- Ada-Europe 2012, volume 7308, pages 18--34. Springer-Verlag, June 2012.S. Sáez, J. Real, and A. Crespo. Implementation of Timing-Event Anities in Ada/Linux. Ada Letters, 35(1), April 2015.A. J. Wellings and A. Burns. A Framework for Real-Time Utilities for Ada 2005. Ada Letters, XXVII(2), August 2007

Ravenscar Support for Time-Triggered Scheduling

[EN] This position paper follows from a previous proposal to integrate a time-triggered scheduler in a prioritybased, preemptive scheduler such as that supported by Ada¿s task dispatching policy FIFO Within Priorities . The resulting combined scheduling carries the advantages of both time-triggered and priority-based scheduling, and helps mitigating their drawbacks. The paper presents a system model for the time-triggered subsystem that extends the original proposal, and describes a Ravenscar implementation of the scheduler at the run-time system level, in the form of a new package Ada.Dispatching.TTS. Multiple programming patterns can be implemented on top of this scheduler. With respect to the previously proposed full-Ada implementation, only patterns that implied the use of asynchronous transfer of control have been excluded. On the other hand, the extension of the original model enables new patterns, not supported in our previous proposal, using the new types of continuation and optional slots. We hold that bringing the time-triggered paradigm to Ravenscar is both feasible and convenient for the High-Integrity and Embedded application domains.This work has been partly supported by the Spanish Government’s project M2C2 (TIN2014-56158-C4-1-P-AR) and the European Commission’s projects ENABLE-S3 and AQUAS (ECSEL-JU, Contracts 692455 and 737475)Real Sáez, JV.; Sáez Barona, S.; Crespo, A. (2018). Ravenscar Support for Time-Triggered Scheduling. ACM SIGAda Ada Letters. 38(1):41-54. https://doi.org/10.1145/3241950.3241957S4154381M. Aldea and M. González-Harbour. MaRTE OS: An Ada Kernel for Real-Time Embedded Applications. Reliable Software Technologies - Ada Europe 2001, Lecture Notes in Computer Science, 2043:305-316, 2001.ISO/IEC-JTC1-SC22-WG9. Ada Reference Manual ISO/IEC 8652:2012(E). URL: http://www.ada-europe.org/manuals/LRM-2012.pdf, 2012.J. Leung and J. Whitehead. On the complexity of xed-priority scheduling of periodic, real-time tasks. Performance Evaluation (Netherlands), 2(4):237-250, 1982.C. Liu and J. Layland. Scheduling Algorithms for Multiprogramming in a Hard Real-Time Environment. Journal of the ACM, 20(1):46-61, 1973.J. Real and P. Rogers. Session Summary: Experience. Ada Letters, 36(1):101-102, June 2016.J. Real, S. Sáez, and A. Crespo. Combined scheduling of time-triggeed plans and priority scheduled task sets. Ada Letters, 36(1):68-76, June 2016.J. Real, S. Sáez, and A. Crespo. Combining time-triggered plans with priority scheduled task sets. In M. Bertogna and L. M. Pinho, editors, Reliable Software Technologies - Ada-Europe 2016, volume 9695 of Lecture Notes in Computer Science. Springer, June 2016.S. Sáez and J. Real. TTS Ravenscar runtime. https://doi.org/10.5281/zenodo.1168723, February 2018

Robust plan execution with unexpected observations

In order to ensure the robust actuation of a plan, execution must be adaptable to unexpected situations in the world and to exogenous events. This is critical in domains in which committing to a wrong ordering of actions can cause the plan failure, even when all the actions succeed. We propose an approach to the execution of a task plan that permits some adaptability to unexpected observations of the state while maintaining the validity of the plan through online reasoning. Our approach computes an adaptable, partially-ordered plan from a given totally-ordered plan. The partially-ordered plan is adaptable in that it can exploit beneficial differences between the world and what was expected. The approach is general in that it can be used with any task planner that produces either a totally or a partially-ordered plan. We propose a plan execution algorithm that computes online the complete set of valid totally-ordered plans described by an adaptable partially-ordered plan together with the probability of success for each of them. This set is then used to choose the next action to execute

Olisipo: A Probabilistic Approach to the Adaptable Execution of Deterministic Temporal Plans

The robust execution of a temporal plan in a perturbed environment is a problem that remains to be solved. Perturbed environments, such as the real world, are non-deterministic and filled with uncertainty. Hence, the execution of a temporal plan presents several challenges and the employed solution often consists of replanning when the execution fails. In this paper, we propose a novel algorithm, named Olisipo, which aims to maximise the probability of a successful execution of a temporal plan in perturbed environments. To achieve this, a probabilistic model is used in the execution of the plan, instead of in the building of the plan. This approach enables Olisipo to dynamically adapt the plan to changes in the environment. In addition to this, the execution of the plan is also adapted to the probability of successfully executing each action. Olisipo was compared to a simple dispatcher and it was shown that it consistently had a higher probability of successfully reaching a goal state in uncertain environments, performed fewer replans and also executed fewer actions. Hence, Olisipo offers a substantial improvement in performance for disturbed environments

A hierarchical architecture for time- and event-triggered real-time systems

[EN] This paper proposes an architecture for combining the execution of time- and event-triggered real-time task sets. This makes it possible for the designer to choose the most appropriate mechanism depending on the role and nature of each task in the system. The proposed architecture allows one to choose the priority levels at which time- and event-triggered tasks are executed. This gives the designer an additional degree of freedom to make compromise decisions upon contradicting timing requirements, such as granting reduced jitter and at the same time providing prompt service to non-periodic events, for example. The proposed model is accompanied with a Ravenscar implementation of the time-triggered scheduler and a library of utilities for specifying time-triggered schedules and reusing time-triggered task patterns.This work has been partly supported by Spanish Government and FEDER funds (AEI/FEDER, UE) under grant (TIN2017-86520-C3-1-R) (PRECON-I4); and by European Commission project AQUAS (ECSEL-JU, Contract 737475).Real Sáez, JV.; Sáez Barona, S.; Crespo, A. (2019). A hierarchical architecture for time- and event-triggered real-time systems. Journal of Systems Architecture. 101:1-15. https://doi.org/10.1016/j.sysarc.2019.101652S11510

Robustness envelopes for temporal plans

To achieve practical execution, planners must produce temporal plans with some degree of run-time adaptability. Such plans can be expressed as Simple Temporal Networks (STN), that constrain the timing of action activations, and implicitly represent the space of choices for the plan executor. A first problem is to verify that all the executor choices allowed by the STN plan will be successful, i.e. the plan is valid. An even more important problem is to assess the effect of discrepancies between the model used for planning and the execution environment. We propose an approach to compute the 'robustness envelope' (i.e., alternative action durations or resource consumption rates) of a given STN plan, for which the plan remains valid. Plans can have boolean and numeric variables as well as discrete and continuous change. We leverage Satisfiability Modulo Theories (SMT) to make the approach formal and practical

28th International Symposium on Temporal Representation and Reasoning (TIME 2021)

The 28th International Symposium on Temporal Representation and Reasoning (TIME 2021) was planned to take place in Klagenfurt, Austria, but had to move to an online conference due to the insecurities and restrictions caused by the pandemic. Since its frst edition in 1994, TIME Symposium is quite unique in the panorama of the scientifc conferences as its main goal is to bring together researchers from distinct research areas involving the management and representation of temporal data as well as the reasoning about temporal aspects of information. Moreover, TIME Symposium aims to bridge theoretical and applied research, as well as to serve as an interdisciplinary forum for exchange among researchers from the areas of artifcial intelligence, database management, logic and verifcation, and beyond

Timelines with Temporal Uncertainty

Timelines are a formalism to model planning domains where the temporal aspects are predominant, and have been used in many real-world applications. Despite their practical success, a major limitation is the inability to model temporal uncertainty, i.e. the fact that the plan executor cannot decide the actual duration of some activities. In this paper we make two key contributions. First, we propose a comprehensive, semantically well founded framework that (conservatively) extends with temporal uncertainty the state of the art timeline approach. Second, we focus on the problem of producing time-triggered plans that are robust with respect to temporal uncertainty, under a bounded horizon. In this setting, we present the first complete algorithm, and we show how it can be made practical by leveraging the power of Satisfiability Modulo Theories