网络业务流自相似传播特性研究

对网络业务流自相似特性进行分析研究,证明了业务流自相似在不同会聚业务流之间以及不同网络节点之间均具有传播特性,新增业务流的注入不会“破坏”原有自相似背景流的自相似特性。这些研究结果能较好地解释目前网络业务流自相似现象的普遍存在,并对有效利用网络共享资源、路由选择、负载均衡以及排队性能分析等方面具有一定的参考价值。     

On optimal lateness and tardiness scheduling in real-time systems

We address lateness and tardiness scheduling policies for real-time systems. It is well-known that preemptive Earliest Deadline First (EDF) minimizes the worst lateness and tardiness of a finite set of tasks with known arrival times, service times and deadlines to the finishing time, on a uniprocessor. We extend this result significantly, to include an arbitrary (possibly infinite) number of tasks with arbitrary arrival and service times, and deadlines, and to show thatEDF

1. minimizes the lateness and tardiness of the tasks that are in the system at an arbitrary time.
2. minimizes lateness within a busy interval, for an arbitrary, possibly infinite number of tasks.
3. maximizes the time to the first missed deadline, and
4. minimizes the length of time during which there is at least one missed deadline in the system.

We also show that a combination ofEDF and Shortest Remaining Processing Time First (SRPTF) policy minimizes maximum latenesses in a vector sense (as defined tin the paper) and minimizes the number of tasks that miss their deadline at the time the first missed deadline occurs. For non-preemptive non-idling polices, we establish new, similar results in a stochastic sense. We attempt extending our findings to multiprocessor systems. We demonstrate that under the assumptions of arbitrary distributions of arrival times, service times and deadlines, our results no longer hold true. When a further assumption of unit-length service times and integer-valued arrival times is introduced, we are able to re-establish the results in the multiprocessor case.     

On mixed-level real-time hardware evolutionary systems

A new hardware model is proposed to allow synchronous sequential circuits to evolve their behavior systematically for a prespecified objective. The proposed model is highly modularized, so that a subcircuit of a selected circuit can serve as a building block for superior circuits. The model has generality in the sense that any circuit of this class can be emulated using this model. In addition, the implementation of the model has a regular structure, so that circuits can be reconfigured very quickly through a genetic operation. This work was presented, in part, at the Third International Symposium on Artificial Life and Robotics, Oita, Japan, January 19–21, 1998     

On the complex nature of MDE evolution and its impact on changeability

In model-driven engineering (MDE), a particular MDE setting of employed languages and automated and manual activities has major impact on productivity. Furthermore, it has been observed that such MDE settings evolve over time. However, currently not much is known about this evolution and its impact on the MDE setting’s maturity, i.e., on changeability and other productivity dimensions. Research so far focuses on evolution of separate building blocks, such as (modeling-) languages, tools, or transformation, only. In this article, we address the lack of knowledge about evolution of MDE settings by investigating case studies from different companies. The first results reveal (1) that there is evolution that affects the composition of an MDE setting (structural evolution) and has the potential to strongly impact aspects, such as changeability and (2) that this structural evolution actually occurs in practice. Based on these first results, we investigated (3) whether there are cases in practice, where structural evolution already altered the risks of changeability given by the respective MDE setting. Therefore, we search and identify examples for such evolution steps on MDE settings from practice and collected six case studies on evolution histories in detail. As a result, we show in this paper that structural evolution (a) is not seldom in practice and (b) sometimes leads to the introduction of changeability risks.     

On the impact of soft handoff in cellular systems

We present a model for soft handoff in wireless cellular networks. In such networks, due to overlapping cells, handoffs are not instantaneous and multiple channels may be occupied by a single mobile for a non-zero freeze time period.We provide a mathematical model of wireless cellular networks with soft handoffs. We examine different performance measures and show that freeze time may have a major impact on the system performance if the mobility rate is not negligible. Both exact and approximate formulations are given. Different fixed-point approximation methods are used to reduce the complexity of the exact solution. Various performance measures such as new and handoff blocking and probability of a call dropout are carefully examined.     

On real-time scheduling policies for flexible manufacturing systems

The authors consider a model proposed by J.R. Perkins and P.R. Kumar (ibid., vol.34, no.2, pp. 139-148, Feb. 1989) for real-time control of flexible manufacturing systems. In this model, a machine can process a finite number of part types at specified rates, but only one part type can be processed at any given time. To process multiple part types, the machine uses a feedback rule to switch, from time to time, from one part type to another. Such switches incur a setup time of δ time units during which no parts are processed. By introducing the notion of idling, the authors derive a lower bound on the performance, as measured by average buffer size, of any stable feedback scheduling policy for a single machine     

On controlling prioritized discrete event systems with real-time constraints

We study a class of prioritized Discrete Event Systems (DESs) that involve the control of resources allocated to tasks under real-time constraints. Our work is motivated by applications in communication systems, computing systems, and manufacturing systems where the objective is to minimize energy consumption while guaranteeing that task deadlines are always met. In the off-line setting, we discover several structural properties of the optimal sample path of such DESs. Using the structural properties, we also propose a greedy algorithm which is shown numerically near optimal. For on-line control, we design a Receding Horizon (RH) controller. Using worst-case estimation, the RH control is able to guarantee feasibility (when the off-line problem is feasible) and achieve good performance.     

On the impact of network latency on distributed systems design

Research in distributed database systems to date has assumed a “variable cost” model of network response time. However, network response time has two components: transmission time (variable with message size) and latency (fixed). This research improves on existing models by incorporating a “fixed plus variable cost” model of the network response time. In this research, we: (1) develop a distributed database design approach that incorporates a “fixed plus variable cost”, network response time function; (2) run a set of experiments to create designs using this model, and (3) evaluate the impact the new model had on the design in various types of networks. This revised version was published online in July 2006 with corrections to the Cover Date.     

实时系统中弹性调度策略

弹性调度面向负载可变的实时系统,通过动态调整任务属性以满足系统的灵活性要求,是一种高效的任务调度策略。针对弹性调度研究中的成果及问题,概述了弹性调度的研究背景,从任务模型、调度模型以及调度算法三个方面对弹性调度的国内外研究进展进行综述,探讨当前研究中存在的问题,并对弹性调度未来研究工作进行分析和展望。     

Estimation of the execution time in real-time systems

Estimation of the worst-case execution time (WCET) of programs is an important problem for the development of real-time systems. In particular, the estimation of the WCET is a goal in the verification of aeronautical software specified in DO-178B/C. This is a difficult problem, and its exact solution is often practically impossible. This problem has been studied for many years; as a result, a lot of techniques for various cases have been developed. A survey of the available techniques for estimating the WCET is presented, which can be useful for choosing methods for solving particular problems.     

Deployment of real-time systems in the cloud environment

Interest in real-time systems has grown considerably over recent years, primarily due to significant increase in the use of smart technologies and latency-sensitive applications such as cloud gaming, audio/video streaming, and smart homes. Significant work has been done on resource mapping in the cloud environment, and a number of promising results have been established accordingly where the focus is mainly on resource provisioning. However, the applicability of cloud computing services for real-time systems generated from smart systems is still in its infancy and remains unexplored, relatively. To address this gap, we propose a model for the smart systems that periodically offload computational workload to the cloud environment where virtual machines are allocated according to rate-monotonic scheduling policy to ensure requests are processed within the associated deadlines. Deadlines of tasks have been relaxed to improve server utilization as well as maintain a level of confidence in the timing constrains. Experimental results are discussed to highlight the applicability of static priority assignment for the workload in the context of virtual machines allocation.

     

Achieving scalability in real-time systems

Running real-time applications with a variable-speed processor can result in scheduling anomalies and permanent overloads. A proposed computational model varies task response times continuously with processor speed, enabling the system to predictably scale its performance during voltage changes. Mutually exclusive resources and nonpreemptive code can generate scheduling anomalies in a processor with dynamic voltage scaling, causing tasks to increase their response times when the processor runs at higher speeds. Even worse, decreasing the speed can cause a permanent overload that degrades system performance in an uncontrolled fashion. Such problems can be efficiently handled through a set of kernel mechanisms, including cyclic asynchronous buffers and elastic scheduling that let system designers scale the performance of real-time applications as a function of processor speed. As successfully done in the SHaRK kernel, both CABs and elastic scheduling can be easily implemented on top of any real-time operating system, as a middleware layer, and they should be included in current standards to develop embedded systems with real-time and energy requirements.     

Reliability and safety of real-time systems

Not a day goes by that the general public does not come into contact with a real-time system. As their numbers and importance grow, so do the stakes for software developers. A failure in a critical application may result in great financial loss-or even loss of life. More effort must be expended to analyze the reliability and safety of such systems. Analysis of hardware components in critical applications has matured over the years and commonly followed techniques have emerged. However, methods and techniques for analyzing the reliability and safety of the software part of critical applications are relatively new and still maturing. Yet the vulnerability of the system to software failures is on the rise and may (and in some cases does) exceed hardware failures. Software is not only becoming more prevalent in real-time systems, it is becoming a larger part of real-time systems, in the sense that the amount of effort expended in designing and implementing the software is a larger proportion of the total expended effort     

On the “dimensionality curse” and the“self-similarity blessing”

Spatial queries in high-dimensional spaces have been studied extensively. Among them, nearest neighbor queries are important in many settings, including spatial databases (Find the k closest cities) and multimedia databases (Find the k most similar images). Previous analyses have concluded that nearest-neighbor search is hopeless in high dimensions due to the notorious “curse of dimensionality”. We show that this may be overpessimistic. We show that what determines the search performance (at least for R-tree-like structures) is the intrinsic dimensionality of the data set and not the dimensionality of the address space (referred to as the embedding dimensionality). The typical (and often implicit) assumption in many previous studies is that the data is uniformly distributed, with independence between attributes. However, real data sets overwhelmingly disobey these assumptions; rather, they typically are skewed and exhibit intrinsic (“fractal”) dimensionalities that are much lower than their embedding dimension, e.g. due to subtle dependencies between attributes. We show how the Hausdorff and Correlation fractal dimensions of a data set can yield extremely accurate formulas that can predict the I/O performance to within one standard deviation on multiple real and synthetic data sets     

Representation and self-similarity of shapes

Representing shapes in a compact and informative form is a significant problem for vision systems that must recognize or classify objects. We describe a compact representation model for two-dimensional (2D) shapes by investigating their self-similarities and constructing their shape axis trees (SA-trees). Our approach can be formulated as a variational one (or, equivalently, as MAP estimation of a Markov random field). We start with a 2D shape, its boundary contour, and two different parameterizations for the contour (one parameterization is oriented counterclockwise and the other clockwise). To measure its self-similarity, the two parameterizations are matched to derive the best set of one-to-one point-to-point correspondences along the contour. The cost functional used in the matching may vary and is determined by the adopted self-similarity criteria, e.g., cocircularity, distance variation, parallelism, and region homogeneity. The loci of middle points of the pairing contour points yield the shape axis and they can be grouped into a unique free tree structure, the SA-tree. By implicitly encoding the (local and global) shape information into an SA-tree, a variety of vision tasks, e.g., shape recognition, comparison, and retrieval, can be performed in a more robust and efficient way via various tree-based algorithms. A dynamic programming algorithm gives the optimal solution in O(N/sup 1/), where N is the size of the contour.     

On scheduling garbage collector in dynamic real-time systems with statistical timing assurances

We consider garbage collection (GC) in dynamic real-time systems. We consider the time-based GC approach of running the collector as a separate, concurrent thread, and focus on real-time scheduling to obtain assurances on mutator timing behavior, while ensuring that memory is never exhausted. We present a scheduling algorithm called GCUA. The algorithm considers mutator activities that are subject to time/utility function time constraints, variable execution time demands, the unimodal arbitrary arrival model that allows a strong adversary, and resource overloads. We establish several properties of GCUA including probabilistically-satisfied utility lower bounds for each mutator activity, a lower bound on the system-wide total accrued utility, bounded sensitivity for the assurances to variations in mutator execution time demand estimates, and no memory exhaustion at all times. Our simulation experiments validate our analytical results and confirm the algorithm’s effectiveness and superiority.     

实时视频监控系统的研制

研制低成本、功能强、图像分辨率高的实时视频监控系统。根据要求选择高性价比的CCD视频相机、Matrox的采集卡等主要部件,并利用VC设计监控软件来实现监控系统。研制的系统,达到了预期的效果。本系统成本低、组装简单、易操作,特别适合于工业检测、智能小区等行业。     

Stepwise design of real-time systems

The joint action approach to modeling of reactive systems is presented and augmented with real time. This leads to a stepwise design method where temporal logic of actions can be used for formal reasoning, superposition is the key mechanism for transformations, the advantages of closed-system modularity are utilized, logical properties are addressed before real-time properties, and real-time properties are enforced without any specific assumptions on scheduling. As a result, real-time modeling is made possible already at early stages of specification, and increased insensitivity is achieved with respect to properties imposed by implementation environments     

Task scheduling in distributed real-time systems

An approach to the flow shop scheduling of the computation process in distributed realtime systems is considered. This approach is based on the concept of a solvable class of systems for which simple optimal scheduling algorithms exist.