The Experts below are selected from a list of 150 Experts worldwide ranked by ideXlab platform
Albert M. K. Cheng - One of the best experts on this subject based on the ideXlab platform.
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Poster Abstract: Memory-Aware Response Time Analysis for P-FRP Tasks
2016 IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS), 2016Co-Authors: Albert M. K. ChengAbstract:Summary form only given. Functional Reactive Programming (FRP) is playing and potentially going to play a more important role in real-time systems. Priority-based (preemptive) FRP (P-FRP), a variant of FRP with more real-time characteristics, demands more research in its scheduling and timing analysis. In a P-FRP system, similar to a classic preemptive system, a higher Priority Task can preempt a Lower-Priority one and make the latter abort. The Lower-Priority Task will restart after the higher Priority Tasks complete their execution. However, unlike the classic preemptive model, when a Task aborts, all the changes made by the Task are discarded (Abort and Restart). In previous studies, the value of Worst Case Execution Time (WCET) of a Task is used for all its restarted Tasks. However, in practice restarted Tasks likely consume less time than WCET when considering the memory effect such as cache-hit in loading code and data. Here we consider a typical Task life cycle without being interrupted (cold started Task): (1) code is loaded from hard drive and data is loaded from main memory; (2) computation is done by processor(s); (3) results are committed to main memory. In the P-FRP model, the time spent in phase (2) and (3) is wasted when a Task is aborted, however, since the existence of memory hierarchy, the time spent in phase (1) can be less when a Task is restarted, for example, the Task code is still in cache and does not need to be read from slow main memory again. This memory effect is not considered in previous studies of P-FRP systems. In this paper, we present our preliminary memory-aware P-FRP Task response time analysis and experimental results. Our ongoing research is to present more theoretical response time analysis and Priority assignment research in the memory-aware P-FRP Task scheduling. And since the execution time difference is likely related to data placement/locality, we will address this difference in our multi-core P-FRP Task scheduling research too.
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P-FRP Task scheduling: A survey
2016 1st CPSWeek Workshop on Declarative Cyber-Physical Systems (DCPS), 2016Co-Authors: Albert M. K. Cheng, Yu JiangAbstract:Functional Reactive Programming (FRP) is a declarative approach for modeling and building reactive systems. The FRP has been shown to be an expressive formalism for building graphics, robotic, and vision applications. The Priority-based FRP (P-FRP) is a formalism of FRP that allows preemption of execution and guarantees real-time response. Since functional programs cannot maintain state and mutable data, changes made by programs that are preempted have to be rolled back, and the work done by the preempted programs has to be discarded. Hence in the P-FRP model, a preempted Lower Priority Task will have to restart after higher Priority Tasks have completed execution. Current real-time research mainly focuses on the classic preemptive or non-preemptive models and plenty methods have been developed to analyze the real-time guarantees of these models. Unfortunately, due to its transactional nature where preempted Tasks are aborted and have to restart, the execution semantics of the P-FRP model does not fit into the standard definitions of classic preemptive or non-preemptive execution. In this survey paper, we review existing researches on the P-FRP Task scheduling, and present a few research areas for future work.
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DCPS - P-FRP Task scheduling: A survey
2016 1st CPSWeek Workshop on Declarative Cyber-Physical Systems (DCPS), 2016Co-Authors: Albert M. K. Cheng, Yu JiangAbstract:Functional Reactive Programming (FRP) is a declarative approach for modeling and building reactive systems. The FRP has been shown to be an expressive formalism for building graphics, robotic, and vision applications. The Priority-based FRP (P-FRP) is a formalism of FRP that allows preemption of execution and guarantees real-time response. Since functional programs cannot maintain state and mutable data, changes made by programs that are preempted have to be rolled back, and the work done by the preempted programs has to be discarded. Hence in the P-FRP model, a preempted Lower Priority Task will have to restart after higher Priority Tasks have completed execution. Current real-time research mainly focuses on the classic preemptive or non-preemptive models and plenty methods have been developed to analyze the real-time guarantees of these models. Unfortunately, due to its transactional nature where preempted Tasks are aborted and have to restart, the execution semantics of the P-FRP model does not fit into the standard definitions of classic preemptive or non-preemptive execution. In this survey paper, we review existing researches on the P-FRP Task scheduling, and present a few research areas for future work.
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A Scratchpad Memory-Based Execution Platform for Functional Reactive Systems and Its Static Timing Analysis
2016 IEEE 22nd International Conference on Embedded and Real-Time Computing Systems and Applications (RTCSA), 2016Co-Authors: Zeinab Kazemi, Albert M. K. ChengAbstract:Priority-based Functional Reactive Programming (P-FRP) is a new variant of FRP to model reactive applications in real-time systems. In P-FRP, when the currently running Task is preempted by an arriving higher-Priority Task, the Lower-Priority running Task is aborted and the higher-Priority Task will execute. The Lower-Priority Task restarts when the higher-Priority one completes. However, unlike the preemptive model, when a Task aborts, all the changes made by this Task are discarded. That is to say, when an aborted ask restarts, it should execute from the beginning. In order to provide a realistic Worst-Case Response Time (WCRT) of the Tasks in P-FRP, it is therefore mandatory to derive a realistic Worst Case Execution Time (WCET) of each Task. Previous studies have ignored memory latency in the derivation of the WCRT, making the resulting estimate inaccurate and unrealistic. Furthermore, these studies have also assumed that the WCET of each Task is known a priori. In this paper, we introduce a scratchpad memory (SPM)-based platform for executing P-FRP Tasks and an approach to determine the WCET of the Tasks by considering the memory cost of the aborted Tasks. We first compute the WCET of a Task in a P-FRP system, and then calculate the memory penalty caused by preemption. In the next step, we derive the WCRT of the Task sets in P-FRP by considering memory latency in the proposed platform. Experimental results from the derivations of the WCET and WCRT using Task sets from the SNU real-time benchmarks and randomly generated Tasks are presented to validate this approach.
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Feasibility interval for the transactional event handlers of P-FRP
Journal of Computer and System Sciences, 2013Co-Authors: Chaitanya Belwal, Albert M. K. ChengAbstract:Functional Reactive Programming (FRP) is a resource aware declarative approach for modeling and building safety-critical embedded systems. Recently, Priority-based FRP (P-FRP) was introduced as a formalism that guarantees real-time response. Due to the state-less nature of execution of functional programs, P-FRP implements a transactional nature of execution where preempted Lower-Priority Tasks are aborted. This makes the response time of a Lower-Priority Task completely dependent on the execution pattern of higher Priority Tasks. The feasibility interval in the classical preemptive model11In this paper the classical preemptive model refers to a real-time system in which Tasks can be preempted by higher Priority Tasks and can resume execution from the point they were preempted. of real-time systems is known and is dependent on the least common multiple (LCM) of Task periods. However, since the abort nature of preemption can induce side-effects on the execution of Lower-Priority Tasks, it has been unknown to date if the feasibility in P-FRP is also dependent on the LCM. In this paper, we rigorously prove that these side-effects of preemption are bounded within the LCM and formally derive a value of the feasibility interval in P-FRP. This value of feasibility interval is vital for more robust schedulability analysis of the P-FRP execution model. Highlights� We rigorously prove that Task abortions in P-FRP are bounded within the Least Common Multiple (LCM) of Task periods. � We formally derive a value of the feasibility interval in P-FRP. � We address both synchronous and asynchronous releases of P-FRP Tasks. � Feasibility intervals of synchronous releases in P-FRP are proved to be the same as those in the preemptive model. � A game-board method is proposed to compute the actual response time.
Bjorn B. Brandenburg - One of the best experts on this subject based on the ideXlab platform.
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TimerShield: Protecting High-Priority Tasks from Low-Priority Timer Interference (Outstanding Paper)
2017 IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS), 2017Co-Authors: Pratyush Patel, Manohar Vanga, Bjorn B. BrandenburgAbstract:Timer interference arises when a high-Priority realtime Task is delayed by a timer interrupt that is intended for a Lower-Priority Task. We demonstrate that high-resolution timers, as exposed for instance by Linux's hrtimer API, can cause substantial timer interference, which manifests as significantly increased response times and Lowered throughput. To eliminate this source of unpredictability, we propose TimerShield, a Priority-aware highresolution timer subsystem that selectively delays the servicing of Lower-Priority timer interrupts while a high-Priority Task is executing. We present the design and implementation of a fully functional TimerShield prototype in Linux PREEMPT RT and compare it against Linux's stock hrtimer subsystem on two different platforms (x86 and ARM). Our results show that TimerShield adds only little overhead, while completely eliminating the timing unpredictability and throughput degradation caused by unnecessary interrupts.
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RTAS - TimerShield: Protecting High-Priority Tasks from Low-Priority Timer Interference (Outstanding Paper)
2017 IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS), 2017Co-Authors: Pratyush Patel, Manohar Vanga, Bjorn B. BrandenburgAbstract:Timer interference arises when a high-Priority realtime Task is delayed by a timer interrupt that is intended for a Lower-Priority Task. We demonstrate that high-resolution timers, as exposed for instance by Linux's hrtimer API, can cause substantial timer interference, which manifests as significantly increased response times and Lowered throughput. To eliminate this source of unpredictability, we propose TimerShield, a Priority-aware highresolution timer subsystem that selectively delays the servicing of Lower-Priority timer interrupts while a high-Priority Task is executing. We present the design and implementation of a fully functional TimerShield prototype in Linux PREEMPT RT and compare it against Linux's stock hrtimer subsystem on two different platforms (x86 and ARM). Our results show that TimerShield adds only little overhead, while completely eliminating the timing unpredictability and throughput degradation caused by unnecessary interrupts.
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TimerShield: Protecting High-Priority Tasks from Low-Priority Timer Interference
2017Co-Authors: Pratyush Patel, Manohar Vanga, Bjorn B. BrandenburgAbstract:Timer interference arises when a high-Priority realtime Task is delayed by a timer interrupt that is intended for a Lower-Priority Task. We demonstrate that high-resolution timers, as exposed for instance by Linux's hrtimer API, can cause substantial timer interference, which manifests as significantly increased response times and Lowered throughput. To eliminate this source of unpredictability, we propose TimerShield, a Priority-aware highresolution timer subsystem that selectively delays the servicing of Lower-Priority timer interrupts while a high-Priority Task is executing. We present the design and implementation of a fully functional TimerShield prototype in Linux PREEMPT RT and compare it against Linux's stock hrtimer subsystem on two different platforms (x86 and ARM). Our results show that TimerShield adds only little overhead, while completely eliminating the timing unpredictability and throughput degradation caused by unnecessary interrupts.
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An asymptotically optimal real-time locking protocol for clustered scheduling under suspension-aware analysis
ACM SIGBED Review, 2013Co-Authors: Bjorn B. BrandenburgAbstract:The purpose of real-time locking protocols is to limit Priority inversions [5], which, intuitively, occur when a high-Priority Task is delayed by a Lower-Priority Task. Such locking-related delay, also called Priority inversion blocking (pi-blocking), is problematic in real-time systems because it can result in deadline misses. However, some pi-blocking is unavoidable when using locks and thus must be bounded and accounted for during schedulability analysis.
Rodrigo S. Jamisola - One of the best experts on this subject based on the ideXlab platform.
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Relative Task prioritization for dual-arm with multiple, conflicting Tasks: Derivation and experiments
2013 IEEE International Conference on Robotics and Automation, 2013Co-Authors: Pyung Hun Chang, Rodrigo S. JamisolaAbstract:This paper presents new formulations in Task-prioritization for dual-arms with multiple, conflicting Tasks and experimental validations. An essential part of the proposed method is the use of relative Jacobian that treats the dual-arm as an equivalent single arm. As a result, three formulations are derived. The first formulation, called relative Task prioritization, expresses a Task prioritization at the acceleration level for a dual-arm, with multiple Tasks, that is controlled as a single manipulator. The second formulation is an impedance control equation that allows direct control of the relative motion and impedance between two end-effectors. Our third formulation is a control law that combines relative Task prioritization, impedance control, and time-delay estimation, which contributes to the ease of implementation of our proposed method. In the physical implementation, one arm draws a circle on a plate attached to the other arm in parallel with three subTasks. Then, intentional conflict among subTasks is induced. The experimental results show that when such conflict occurs, the higher Priority Task is guaranteed an immediate execution without influence from the Lower Priority Task.
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ICRA - Relative Task prioritization for dual-arm with multiple, conflicting Tasks: Derivation and experiments
2013 IEEE International Conference on Robotics and Automation, 2013Co-Authors: Pyung Hun Chang, Rodrigo S. JamisolaAbstract:This paper presents new formulations in Task-prioritization for dual-arms with multiple, conflicting Tasks and experimental validations. An essential part of the proposed method is the use of relative Jacobian that treats the dual-arm as an equivalent single arm. As a result, three formulations are derived. The first formulation, called relative Task prioritization, expresses a Task prioritization at the acceleration level for a dual-arm, with multiple Tasks, that is controlled as a single manipulator. The second formulation is an impedance control equation that allows direct control of the relative motion and impedance between two end-effectors. Our third formulation is a control law that combines relative Task prioritization, impedance control, and time-delay estimation, which contributes to the ease of implementation of our proposed method. In the physical implementation, one arm draws a circle on a plate attached to the other arm in parallel with three subTasks. Then, intentional conflict among subTasks is induced. The experimental results show that when such conflict occurs, the higher Priority Task is guaranteed an immediate execution without influence from the Lower Priority Task.
Pyung Hun Chang - One of the best experts on this subject based on the ideXlab platform.
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Relative Task prioritization for dual-arm with multiple, conflicting Tasks: Derivation and experiments
2013 IEEE International Conference on Robotics and Automation, 2013Co-Authors: Pyung Hun Chang, Rodrigo S. JamisolaAbstract:This paper presents new formulations in Task-prioritization for dual-arms with multiple, conflicting Tasks and experimental validations. An essential part of the proposed method is the use of relative Jacobian that treats the dual-arm as an equivalent single arm. As a result, three formulations are derived. The first formulation, called relative Task prioritization, expresses a Task prioritization at the acceleration level for a dual-arm, with multiple Tasks, that is controlled as a single manipulator. The second formulation is an impedance control equation that allows direct control of the relative motion and impedance between two end-effectors. Our third formulation is a control law that combines relative Task prioritization, impedance control, and time-delay estimation, which contributes to the ease of implementation of our proposed method. In the physical implementation, one arm draws a circle on a plate attached to the other arm in parallel with three subTasks. Then, intentional conflict among subTasks is induced. The experimental results show that when such conflict occurs, the higher Priority Task is guaranteed an immediate execution without influence from the Lower Priority Task.
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ICRA - Relative Task prioritization for dual-arm with multiple, conflicting Tasks: Derivation and experiments
2013 IEEE International Conference on Robotics and Automation, 2013Co-Authors: Pyung Hun Chang, Rodrigo S. JamisolaAbstract:This paper presents new formulations in Task-prioritization for dual-arms with multiple, conflicting Tasks and experimental validations. An essential part of the proposed method is the use of relative Jacobian that treats the dual-arm as an equivalent single arm. As a result, three formulations are derived. The first formulation, called relative Task prioritization, expresses a Task prioritization at the acceleration level for a dual-arm, with multiple Tasks, that is controlled as a single manipulator. The second formulation is an impedance control equation that allows direct control of the relative motion and impedance between two end-effectors. Our third formulation is a control law that combines relative Task prioritization, impedance control, and time-delay estimation, which contributes to the ease of implementation of our proposed method. In the physical implementation, one arm draws a circle on a plate attached to the other arm in parallel with three subTasks. Then, intentional conflict among subTasks is induced. The experimental results show that when such conflict occurs, the higher Priority Task is guaranteed an immediate execution without influence from the Lower Priority Task.
Pratyush Patel - One of the best experts on this subject based on the ideXlab platform.
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TimerShield: Protecting High-Priority Tasks from Low-Priority Timer Interference (Outstanding Paper)
2017 IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS), 2017Co-Authors: Pratyush Patel, Manohar Vanga, Bjorn B. BrandenburgAbstract:Timer interference arises when a high-Priority realtime Task is delayed by a timer interrupt that is intended for a Lower-Priority Task. We demonstrate that high-resolution timers, as exposed for instance by Linux's hrtimer API, can cause substantial timer interference, which manifests as significantly increased response times and Lowered throughput. To eliminate this source of unpredictability, we propose TimerShield, a Priority-aware highresolution timer subsystem that selectively delays the servicing of Lower-Priority timer interrupts while a high-Priority Task is executing. We present the design and implementation of a fully functional TimerShield prototype in Linux PREEMPT RT and compare it against Linux's stock hrtimer subsystem on two different platforms (x86 and ARM). Our results show that TimerShield adds only little overhead, while completely eliminating the timing unpredictability and throughput degradation caused by unnecessary interrupts.
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RTAS - TimerShield: Protecting High-Priority Tasks from Low-Priority Timer Interference (Outstanding Paper)
2017 IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS), 2017Co-Authors: Pratyush Patel, Manohar Vanga, Bjorn B. BrandenburgAbstract:Timer interference arises when a high-Priority realtime Task is delayed by a timer interrupt that is intended for a Lower-Priority Task. We demonstrate that high-resolution timers, as exposed for instance by Linux's hrtimer API, can cause substantial timer interference, which manifests as significantly increased response times and Lowered throughput. To eliminate this source of unpredictability, we propose TimerShield, a Priority-aware highresolution timer subsystem that selectively delays the servicing of Lower-Priority timer interrupts while a high-Priority Task is executing. We present the design and implementation of a fully functional TimerShield prototype in Linux PREEMPT RT and compare it against Linux's stock hrtimer subsystem on two different platforms (x86 and ARM). Our results show that TimerShield adds only little overhead, while completely eliminating the timing unpredictability and throughput degradation caused by unnecessary interrupts.
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TimerShield: Protecting High-Priority Tasks from Low-Priority Timer Interference
2017Co-Authors: Pratyush Patel, Manohar Vanga, Bjorn B. BrandenburgAbstract:Timer interference arises when a high-Priority realtime Task is delayed by a timer interrupt that is intended for a Lower-Priority Task. We demonstrate that high-resolution timers, as exposed for instance by Linux's hrtimer API, can cause substantial timer interference, which manifests as significantly increased response times and Lowered throughput. To eliminate this source of unpredictability, we propose TimerShield, a Priority-aware highresolution timer subsystem that selectively delays the servicing of Lower-Priority timer interrupts while a high-Priority Task is executing. We present the design and implementation of a fully functional TimerShield prototype in Linux PREEMPT RT and compare it against Linux's stock hrtimer subsystem on two different platforms (x86 and ARM). Our results show that TimerShield adds only little overhead, while completely eliminating the timing unpredictability and throughput degradation caused by unnecessary interrupts.
