Kanghee Kim

Stochastic analysis of periodic real-time systems

This paper describes a stochastic analysis method for general periodic real-time systems. The proposed method accurately computes the response time distribution of each task in the system, thus making it possible to determine the deadline miss probability of individual tasks, even for systems with maximum utilization factor greater than one. The method uniformly covers both fixed-priority scheduling (such as rate monotonic) as well as dynamic-priority scheduling (such as earliest deadline first) and can handle arbitrary relative deadlines and execution time distributions. The accuracy of the method is proven by comparing the results from the analysis with those obtained from simulations, as well as other methodologies in the literature.

An exact stochastic analysis of priority-driven periodic real-time systems and its approximations

This paper describes a stochastic analysis framework which computes the response time distribution and the deadline miss probability of individual tasks, even for systems with a maximum utilization greater than one. The framework is uniformly applied to fixed-priority and dynamic-priority systems and can handle, tasks with arbitrary relative deadlines and execution time distributions.

Pessimism in the stochastic analysis of real-time systems: concept and applications

The exact stochastic analysis of most real-time systems is becoming unaffordable in current practice. On one side, the exact calculation of the response time distribution of the tasks is not possible except for simple periodic and independent task sets. On the other side, in practice, tasks introduce complexities like release jitter, blocking in shared resources, stochastic dependencies, etc, which can not be handled by the periodic and independent task set model. This paper introduces the concept of pessimism in the stochastic analysis of real-time systems in the following sense: the exact probability of missing any deadline is always lower than that derived from the pessimistic analysis. Therefore, if real-time constraints are expressed as probabilities of missing deadlines, the pessimistic stochastic analysis provides safe results. Some applications of the pessimism concept are presented. Firstly, the practical problems that arise in the stochastic analysis of periodic and independent task sets are addressed. Secondly, we extend to the stochastic case some well known techniques of the deterministic analysis, such as the blocking in shared resources, and the task priority assignment.

Design and Implementation of a Delay-Guaranteed Motor Drive for Precision Motion Control

This paper proposes a systematic design approach for a precision-guaranteed motion control system. We develop a delay-guaranteed motor drive with our new software implementation and real-time Ethernet, which can be used as a building block to build up a multi-axis motion control system. Our drive software implementation provides a probabilistic guarantee on drive-local processing delays to motor actuation, while real-time Ethernet provides a deterministic guarantee on message communication delays from a motion control host to each drive. In the paper, we address the precision of a motion control system in two terms: host cycle time and simultaneous actuation deviation. The host cycle time is a period with which the host can periodically release motor control messages while the average drive utilization does not exceed 1, and the simultaneous actuation deviation is the difference between the earliest and the latest actuation at different drives in response to the same message. In our approach, the main objective is to minimize the periods of tasks in each drive, using our stochastic analysis, which gives us a minimum possible host cycle time. Together with an existing delay analysis of real-time Ethernet, we analyze the end-to-end delay from message release to motor actuation and in turn the simultaneous actuation deviation. Through experiments, we show that for various requirements on the deadline miss probabilities of the tasks, we can successfully reduce the host cycle time and evaluate the resulting distribution of the simultaneous actuation deviation depending on the number of drives.

Real-Time Program Execution on NAND Flash Memory for Portable Media Players

NAND flash memory has been widely used as a non-volatile storage for storing data. However, it requires a large amount of SRAM for executing program codes stored in it since it only supports page-based reads, not byte-level random reads. This paper proposes a new paging mechanism called RT-PLRU (real-time constrained combination of pinning and LRU) that allows program codes stored in NAND flash memory to be executed satisfying real-time requirements with minimal usage of SRAM. Moreover, the RT-PLRU is optimally configured in a developer-transparent way without giving any burden to the program developer. The developed technique is specifically applied to a media player program targeting a PMP (portable medial player). To the best of our knowledge, this is the first effort to use NAND flash memory as a code storage for storing and executing real-time programs with minimal usage of SRAM.

Performance characterization of prelinking and preloadingfor embedded systems

Application launching times in embedded systems are more crucial than in general-purpose systems since the response times of embedded applications are significantly affected by the launching times. As general-purpose operating systems are increasingly used in embedded systems, reducing appli-cation launching times are one of the most influential factors for performance improvement. In order to reduce the application launching times, three factors should be considered at the same time: relocation time, symbol resolution time, and binary loading time. In this paper, we propose a new application execution model using a combination of prelinking and preloading to reduce the relocation, symbol resolution, and binary load overheads at the same time. Such application execution model is realized using fork and dlopen execution model instead of traditional fork and exec execution model. We evaluate the performance effect of the proposed fork and dlopen application execution model on a Linux-based embedded system using XScale processor. By applying the proposed application execution model using both prelinking and preloading, the application launching times are reduced up to 71% and relocation counts are reduced up to 91% in the benchmark programs we used.

An EtherCAT-based motor drive for high precision motion systems

Industrial Ethernet-based motor drives offer many advantages for motion applications. This paper presents the implementation and analysis of a motor drive with the EtherCAT, an open real-time Ethernet standard, for high-precision motion systems. Considering the characteristics of the multitasked software and the network interface, we analyze the delay in actuating the motor in response to a command from the control host. Based on a response time analysis and the times measured from the developed drive, we discuss performance characteristics of the drive in respect of the end-to-end delay and the maximum achievable throughput for non-real-time traffic when the drive is controlled in position, velocity, or torque mode.

RT-PLRU: A New Paging Scheme for Real--Time Execution of Program Codes on NAND Flash Memory for Portable Media Players

NAND flash memory has been widely used as a nonvolatile storage for storing data. However, it is challenging to execute program codes on NAND flash memory, since NAND flash memory only supports page-based reads, not byte-level random reads. This paper proposes an automated process to find the optimal paging strategy called RT-PLRU (Real-Time constrained combination of Pinning and LRU) that allows program codes stored in NAND flash memory to be executed satisfying real-time requirements with minimal usage of RAM. Moreover, the proposed process optimally configure the RT-PLRU in a developer-transparent way without giving any burden to the program developer. The developed technique is specifically applied to a media player program targeting a portable media player (PMP). To the best of our knowledge, this is the first effort to use NAND flash memory as a code storage for storing and executing real-time programs with minimal usage of RAM.

Time-parameterized sensing task model for real-time tracking

This paper proposes a novel task model in which its physical and temporal parameters are specified as time-parameterized functions and their values are finally determined at the actual dispatch time. This model is clearly differentiated from the classical task model where parameters are fixed at the job release time. The new model better suits sensing tasks in tracking applications, since the sensor parameters such as field-of-view and measurement duration can be properly adjusted at the actual sensing time. The new model, however, creates the cyclic dependency between task parameters and scheduling behavior, that is, the task parameters depend on scheduling behavior and the latter in turn depends on the former. This cyclic dependency makes the schedulability check even more difficult. We handle this difficulty by iterative convergence and probabilistic schedulability envelope, which provides an efficient online schedulability check. The experimental study shows that the new model significantly improves the effective capacity of tracking systems without losing track accuracy

On relaxing task isolation in overrun handling to provide probabilistic guarantees to soft real-time tasks with varying execution times

Task-level or job-level isolation is commonly used in real-time systems to prevent a job that overruns from affecting jobs belonging to other tasks (task-level isolation) or to the same task (job-level isolation). Although such an isolation provides the desired protection and simplifies analysis, it often results in degraded performance (in terms of deadline meeting) because it limits the exploitation of residual processor time resulting from jobs that underruns. We propose a new overrun handling method called randomized dropping that relaxes the isolation to make effective use of such residual processor time. We apply the proposed randomized dropping to the EDF scheduling and give an analysis technique to compute the probability of deadline meeting. Experimental results show that the proposed overrun handling method outperforms previous approaches based task-level or job-level isolation.