基于移动实时事务相关图的数据收集

在移动环境下,为了满足实时事务在移动主机断接后数据的可用性,在事务启动前,数据必须收集到移动主机的缓存中。根据实时事务的相关性,给出了实时事务的相关图。介绍了基于时间阈值、空间阈值、数据的有效期阈值的收集算法,有效保证数据的可用性和实时事务的定时限制。     

一种面向混合实时事务调度的并发控制协议

首先给出了一个两层结构的混合实时数据库系统模型,其中支持采用非定期任务调度算法来改进系统的性能．进一步,针对这种模型下混合事务的数据一致性问题,提出了一种新的并发控制协议——MCC-DATI．该协议采用动态优先级驱动的调度算法,通过限制非定期的软实时事务对硬实时事务的阻塞时间,保证硬实时事务的可调度性;同时,采用非定期任务调度算法以及基于时间戳间隔的动态串行化顺序调整机制来减少软实时事务的截止期错失率．仿真实验表明,相对于先前的混合事务的并发控制协议,该协议在不同的系统负载与截止期约束下都能够改进系统的性能。     

移动实时数据库QoS管理和更新事务调度算法

分布式环境下的移动实时数据库服务应用逐渐广泛,但系统负载的不可预测甚至可能的超载使得应用受到限制.系统中事务在低带宽移动环境下竞争有限的系统资源导致重启或夭折,从而给系统带来损失甚至灾难.通过结合时间有效性和值域的有效性提出基于的一种新的衡量系统性能的参数标准,如错过截止期事务的比率、数据新鲜度、CPU利用率去保证QoS,同时提出一种更新事务调度新算法和基于反馈控制的架构去实现.通过仿真实验评估,算法可以从稳定性能和暂态性能保证系统的性能指标不会超过由数据库管理员事先提出的QoS规范.     

实时事务调度及静态可调度性分析

在类似闭环控制的硬实时数据库应用环境,实时事务具有一定的静态可预报性,其中实时事务的可调度性分析是维护实时数据库时间正确性的基础.通过利用抢占阈值,提出了一种新的实时事务处理模型,它集成了CPU调度和数据调度,实现离线并发控制,具有单阻塞的特征与好的静态可预测性,并有利于降低事务系统的负载和改善可调度性.进一步由此建立了实时事务的静态可调度性分析模型以及求最优可行调度的整数规划模型,该模型有利于达到实时事务调度的整体优化.     

实时事务二次调度

实时事务调度的主要目的是调度实时事务使其满足事务的时间约束,它包括针对多个事务的CPU调度以及与之紧密相关的在多个并发事务之间的数据、I/O及内存等资源的调度。关于调度有大量的研究成果,包括:Earliest DeadlineFirst(Liu和Layland,1973;Abbott和Garcia-Molina,1988等),Least Slack First(Abbott Garcia,1992),Weighted Prior-ity(Huang et al.,1989),以及控制优先级倒置的可抢占算法等。无论哪种调度方法,在动态实时环境下满足所有事务的截止期都是很困难的,而提高系统成功率是实时事务调度的主要目标,为此,本文从一个新的角度来研究这个问题,探讨实     

混合事务的ERtTDF调度算法及可调度性分析

在实时数据库及数据处理系统中,针对周期性实时事务,应用经典的EDF等调度算法对其可以得到可行的调度;而对于混合实时事务-事务的时间性质是混合的,经典EDF不太适用。文中扩展EDF为最早实时事务截止期优先-ERtTDF(EarliestReal-timeTransactionDeadlineFirst),它可以有效地调度混合事务。文中给出了其可调度条件和时间需求条件,并把时间需求条件扩展到时限小于周期以及引入资源共享控制等方面,最后给出了集成调度实时、非实时以及混合事务的系统框架。通过性能比较,可以得到ERtTDF算法处理上面事务模型时性能较经典EDF更优。     

基于挖掘具有时态限制数据语义的实时事务调度算法

在实时数据库中,事务对时态数据对象的访问在很多领域的应用日益广泛。目前实时数据库中的事务调度算法大多仅考虑数据的逻辑一致性,而忽略了满足时态的一致性。论文提出了新的实时数据库中具有时态限制的事务调度算法PSBA(PredictionandSimilarity-BasedAlgorithm)。首先提出了临时数据截止期的概念,以定理的形式证明了低成本地预测算法的可行性,尽可能早地终止或推迟无法满足时态一致性要求的事务的执行,节省了计算资源以供其它事务执行。继而通过挖掘数据语义,利用数据相似性,结合预测算法,进一步提高了调度性能。与目前所知的有效算法比较,该算法在性能指标上(事务失败率、浪费的CPU时间)明显优于这些算法。     

更新导出数据对象的实时事务调度算法*

提出两种遍历有向非循环图的更新导出数据对象事务调度算法,并且建立了相应评估系统性能的价值模型用来检验算法执行效率。通过系列仿真实验表明两种算法在一定程度上提高了系统性能。     

运行时间自适应的事务调度算法

实时数据库中是基于事务的不同优先级进行事务的调度,所以事务优先级的确定对实时数据库的效率有着至关重要的作用。本文讨论了一个运行时间自适应的事务调度算法,对算法思想和调度策略进行了深入的研究。该算法事务的估计运行时间与CPU时钟无关,具有自适应的能力,并且可以得到周期性事务的准确运行时间,对于周期性事务比例比较高的应用系统非常适合。     

基于多版本快照的多级安全事务调度算法

现有的各种用于多级事务调度的算法都主要是采用锁机制来解决的,在多级安全系统环境中,正是这个锁机制使得系统在高低安全级事务发生读写冲突时无所适从,从而使得这些算法都只能各自处理某些类多级事务.针对这些缺陷,文中提出了一种有效的、无需对数据上锁的多级安全事务调度算法--多版本快照算法.算法通过消除多版本串行化图(Multiversion Seriarization Graph,MSG)中出现环的方法来达到并行事务的可串行化.文中还证明了算法的正确性,并对算法的安全性进行了分析.     

基于可延迟调度提升实时数据对象时序一致性服务质量算法

针对保证实时数据对象时序一致性调度算法在软实时数据库系统环境下的应用问题,提出了一种基于概率统计的可延迟优化(SDS-OPT)算法。首先,分析和比较了现有算法在可调度性、服务质量(QoS)以及工作负载方面的特征与不足,指出优化现有算法的必要性;然后,利用最速下降法提升作业的执行时间筛选基准值,进而增加实时更新事务可调度的作业数量,以确保实时数据对象的时序一致性服务质量(QoS)最大化;最后,从工作负载和服务质量两个方面对所提算法和现有算法的性能进行对比分析。仿真实验结果表明,相对于已有的针对固定优先级可延迟调度算法(DS-FP)和统计性的非确定性可延迟调度算法(DS-PS),所提算法能够保证实时数据对象的时序一致性,同时降低工作负载,服务质量提升明显。     

移动环境下支持实时事务处理的数据预取

随着移动通信技术的迅速发展,人们提出了新的应用要求:在移动环境下处理实时事务.而移动通信带宽有限性引起较大的数据访问延迟,有时甚至由于网络传输的断接使得事务得不到所需要的数据,数据预取能够很好地解决这个问题.已有的移动环境下数据预取没有考虑到数据的流行性和事务的时间特性.该文分析影响实时事务数据预取的因素,首先考虑数据易变性、活跃性等因素,获得高价值预取数据集合;然后考虑访问预取数据的事务优先级、数据流行性等因素,构造预取数据的选择函数,通过该函数在前面选取的集合中筛选出对满足实时事务截止期更有价值的数据对象进行预取.实验表明,该数据预取策略能降低移动实时事务满足截止期的比率,更好地支持移动实时事务处理.     

基于不精确计算移动实时数据库服务质量管理

移动实时数据库服务应用逐渐广泛,但系统负载不可预测和有限资源常导致事务重启或夭折,给系统带来损失甚至灾难。传统的基于最长执行时间实时调度算法已不能满足性能要求,提出结合不精确计算和反馈控制的新算法。考虑更新数据对象间联系并结合时间和值域有效性,提出性能新参数标准去保证系统性能和服务质量。通过仿真实验表明：算法可从稳定性能和暂态性能保证系统预定的服务质量规范。     

Data access scheduling in firm real-time database systems

A major challenge addressed by conventional database systems has been to efficiently implement the transaction model, which provides the properties of atomicity, serializability, and permanence. Real-time applications have added a complex new dimension to this challenge by placing deadlines on the response time of the database system. In this paper, we examine the problem of real-time data access scheduling, that is, the problem of scheduling the data accesses of real-time transactions in order to meet their deadlines. In particular, we focus on firm deadline real-time database applications, where transactions that miss their deadlines are discarded and the objective of the real-time database system is to minimize the number of missed deadlines. Within this framework, we use a detailed simulation model to compare the performance of several real-time locking protocols and optimistic concurrency control algorithms under a variety of real-time transaction workloads. The results of our study show that in moving from the conventional database system domain to the real-time domain, there are new performance-related forces that come into effect. Our experiments demonstrate that these factors can cause performance recommendations that were valid in a conventional database setting to be significantly altered in the corresponding real-time setting.This work was done while the author was with the Computer Sciences Department, University of Wisconsin-Madison. This research was partially supported by the National Science Foundation under grant IRI-8657323.     

Processing real-time transactions in a replicated database system

A database system supporting a real-time application has to provide real-time information to the executing transactions. Each real-time transaction is associated with a timing constraint, typically in the form of a deadline. It is difficult to satisfy all timing constraints due to the consistency requirements of the underlying database. In scheduling the transactions it is aimed to process as many transactions as possible within their deadlines. Replicated database systems possess desirable features for real-time applications, such as a high level of data availability, and potentially improved response time for queries. On the other hand, multiple copy updates lead to a considerable overhead due to the communication required among the data sites holding the copies. In this paper, we investigate the impact of storing multiple copies of data on satisfying the timing constraints of real-time transactions. A detailed performance model of a distributed database system is employed in evaluating the effects of various workload parameters and design alternatives on the system performance. The performance is expressed in terms of the fraction of satisfied transaction deadlines. A comparison of several real-time concurrency control protocols, which are based on different approaches in involving timing constraints of transactions in scheduling, is also provided in performance experiments. Recommended by: A. ElmagarmidThis work was initiated while the author was at the Computer Science Department, University of Illinois at Urbana-Champaign.     

Improving Predictability of Transaction Execution Times in Real-time Databases

We present a design for multi-versionconcurrency control and recovery in a main memory database, anddescribe logical and physical versioning schemes that allowread-only transactions to execute without obtaining data itemlocks or system latches. Our schemes enable a system to providethe guarantee that updaters will never interfere with read-onlytransactions, and read-only transactions will not be delayeddue to data contention. Consequently, transaction executionsbecome more predictable—this partially alleviates a majorproblem in real-time database system (RTDBS) scheduling, namely,significant unpredictability in transaction execution times.As a result, in addition to a transaction's deadline, a moreaccurate estimate of its execution time can also be taken intoaccount, thus facilitating better scheduling decisions. Our contributionsinclude several space saving techniques for the main-memory implementation,including improved methods for logical aging of data items andthe introduction of physical aging for low-level structures.Some of these schemes have been implemented on a widely-usedsoftware platform within Lucent, and the full scheme is implementedin the Dalí main-memory storage manager.     

Multimedia teleorchestra with independent sources: Part 2 — synchronization algorithms

Based on the time-flow graph models proposed for the multimedia presentation scenarios, a scheduling algorithm and control scheme are presented for the synchronization control which involves multiple sources in a teleorchestra system. In the scheduling, a synchronization dialogue among the independent sources will be conducted to evaluate the presentation scenario. Then the real-time data objects from the different sources can be synchronized in the remote presentation according to the required scenario. The algorithm can be performed in a distributed manner.     

面向数据中心的事务内存框架设计

针对由计算机集群构成的云计算数据中心的特性,提出了一种基于事务内存的分布式编程框架。该框架将云计算任务封装为事务,自动完成所有事务的调度执行、负载均衡和故障恢复;将数据中心的分布式数据封装为事务对象,保证事务访问事务对象时的ACID特性。与同类研究相比,它无需用户关心程序的并行控制,具有简单易用性。该框架已在仿真环境下实现,实验结果表明它具有良好的可扩展性和容错性。