Namyun Kim

Visual assessment of a real-time system design: a case study on a CNC controller

We describe our experiments on a real-time system design, focusing on design alternatives such as scheduling jitter, sensor-to-output latency, intertask communication schemes and the system utilization. The prime objective of these experiments was to evaluate a real-time design produced using the period calibration method (Gerber et al., 1995) and thus identify the limitations of the method. We chose a computerized numerical control (CNC) machine as our target real-time system and built a realistic controller and a plant simulator. Our results were extracted from a controlled series of more than a hundred test controllers obtained by varying four test variables. This study unveils many interesting facts: average sensor-to-output latency is one of the most dominating factors in determining control quality; the effect of scheduling jitter appears only when the average sensor-to-output latency is sufficiently small; and loop processing periods are another dominating factor of performance. Based on these results, we propose a new communication scheme and a new objective function for the period calibration method.

Scheduling garbage collector for embedded real-time systems

This paper proposes a new scheduling method for multiple mutators and a garbage collector running on embedded real-time systems with a single processor and no virtual memory. The hard real-time tasks should reserve a certain amount of heap memory to prevent memory starvation and/or deadline miss. Since the memory requirement depends on the worst-case response time of a garbage collector, the traditional approach in which garbage collection is performed in the background demands large memory space. The proposed scheduling algorithm is based on an aperiodic scheduling technique, sporadic server. This paper also presents a modified copying garbage collection algorithm with hardware support. In order to minimize the worst-case response time of a garbage collector thus reducing the memory requirement, the garbage collector runs as the highest priority task with a preset bandwidth. This paper also investigates the schedulability of a garbage collector and mutator tasks as well as the worst-case memory requirement. Performance analysis shows that the proposed algorithm can provide a considerable reduction in the worst-case memory requirement compared with the background policy. Simulation results demonstrate that the proposed algorithm can produce the feasible memory requirement comparable to the complex on-line scheduling algorithm such as slack stealing.

Experimental Assessment of the Period Calibration Method: A Case Study

In this paper we present an experimental evaluation of the period calibration method (PCM) which was developed in Gerber et al. (1994, 1995) as a systematic design methodology for real-time systems. The objective of this experimental study is to assess design alternatives integrated into the method and their performance implication on resultant systems built via the PCM. Such design alternatives include scheduling jitter, sensor-to-output latency, intertask communication schemes, and system utilization. For this study, we have chosen a computerized numerical control (CNC) machine as our target real-time system, and built a realistic controller and a plant simulator. We show the detailed development process of the CNC controller and report its performance. The performance results were extracted from a controlled series of more than hundred test controllers obtained by varying four test variables. This study unveils several weaknesses of the PCM: (1) the communication scheme built into PCM incurs a large latency though average sensor-to-output latency is one of the most dominating factors in determining control quality; (2) scheduling jitter is taken seriously in PCM though its effect appears only when average sensor-to-output latency is sufficiently small; (3) loop processing periods are not properly optimized for control quality though they are another dominating factor of performance; and (4) transient overloads are not considered at all in PCM, even though they can seriously damage the performance of a system. Based on these results, we propose a new communication scheme and a transient overload handling technique for the improved period calibration method.

Burst Mode Bandwidth Allocation for Real-Time Messages in IEEE 802.12 Networks

In IEEE 802.12 network, a repeater arbitrates among transmission requests on a round robin basis. A node can transmit a packet only when it is granted by the repeater. Recently, the IEEE 802.12 Committee approved a burst mode in which a node can transmit multiple packets per grant. In burst mode a bandwidth, or the number of packets per grant should be allocated taking into account the timing constraints of real-time messages. This paper proposes bandwidth allocation algorithms based on the burst mode to guarantee the deadlines of periodic real-time messages. Once the bandwidth of each node is derived from the lengths and deadlines of periodic messages, a node is allowed to transmit packets up to the allocated bandwidth per grant. Experimental results show that the proposed algorithms providemuch higher guarantee ratio and graceful degradation for heavy network load, compared with the existing approaches.

Weight-based Congestion Control Algorithms for H.264/SVC Streaming

Because best-effort Internet provides no guarantees on packet delay and loss, transient network congestion may cause negative effects on H.264/SVC streaming. Thus, the congestion control is required to adjust bit rate by dropping enhancement layers of H.264/SVC streams. This paper differentiates the video streams according to different levels of importance and proposes weighted-based congestion control algorithms to use the rate-distortion characteristics of streams. To maximize the weighted sum of PSNR values of all streams on a bandwidth-constrained node, this paper proposes WNS(Weighted Near-Sighted) and WFS(Weighted Far-Sighted) algorithms to control the number of enhancement layers of streams. Through simulation, this paper shows that weighted-based congestion control algorithm can efficiently adapt streams to network conditions and analyzes the characteristics of congestion control algorithms.

A scheduling technique for real-time systems with end-to-end timing constraints

This paper presents a scheduling technique for guaranteeing end-to-end timing constraints. Applications are structured as a set of tasks which are the sequence of actions to produce an output. Task is decomposed into subtasks and shares subtasks with other tasks. According to the proposed scheduling technique, for each instance of a task, the sequence of subtasks is determined off-line and then called by the task sequentially at runtime. This technique avoids unnecessary execution of shared tasks and satisfies precedence constraints between subtasks even in overload situations.

Personalized Message Syndication As a Smart Media

Since the increasing use of smart devices such as smartphone and tablet PC improves the quality of convergence of broadcasting and telecommunication, many new media have been created. This smart device can provide personal environments based on user`s profile (age, gender, and job) and current location. In addition, it can be regarded as a media platform with two-way communication. This paper proposes personalized message syndication, called PMS, based on smart devices and then analyzes its business and technical issues. The PMS service as a smart media provides consumers customized information and can be used for producer`s marketing tools which can lead consumers` heart and participation.

Message Forwarding System based on User`s Context

The messages on mobile terminal may contain context information such as user`s location, time and preference. This paper proposes message forwarding system based on user`s context. The server stores the predefined target addresses which may be mobile phone number, web server address and email address. On request for message transmission, the server tries to find a target address matching a user`s context. Thus, a sender transmits messages to a representative address without knowing various receiver`s addresses and a receiver can retrieve classified messages according to subject/location.

Dynamic disk scheduling for multimedia storage servers

A multimedia storage server has two different classes of requests: real-time and nonreal-time classes. A real-time class such as a video and audio request requires that timely retrieval of data should be guaranteed whereas a nonreal-time class such as text request requires fast response time. We propose a disk scheduling algorithm satisfying the above two requirements by dynamically controlling the weight which is assigned to each class. The weight of a class is the fraction of bandwidth to be allocated to the class. The performance evaluation of this algorithm demonstrates the improvement of the average response time compared with that of the disk scheduling algorithm with static weight.

Bandwidth Allocation for Transmission of Real-Time Messages in Burst Mode Over IEEE 802.12 Network

Abstract In IEEE 802.12 network. a repeater arbitrates the network access contentions among the nodes on round robin basis. The node having received a grant from the repeater, can transmit a frame. The burst mode, a newly approved feature in IEEE 802.12 network, allows a node to transmit multiple frames per grant. This paper proposes a bandwidth allocation scheme based on the burst mode to guarantee the deadlines of periodic real-time messages. Once the bandwidth of each node is derived from the lengths and deadlines of periodic messages, a node can transmit the frames up to the allocated bandwidth per grant. Simulation study shows that the proposed scheme provides much higher guarantee ratio and graceful degradation for high network load, compared with the existing approaches.