Real-Time Virtual Resources

实时虚拟资源随着嵌入式系统变得越来越复杂，一个典型的嵌入式系统可能会包含共享同一嵌入式运行时平台的软实时和硬实时应用程序的混合。一般来说，一个应用程序可能包含一个或多个任务。例如，在手持式传感器计算机上，用户可能希望根据目标地标确定自己的位置，同时还希望收到传感器计算机功耗状态的警告。虽然功耗警告任务可能具有非常松散的定时约束，但定位固定任务具有相当严格的定时和其他QOS（服务质量）要求。理想情况下，应用程序开发人员应该能够像在专用计算机上运行一样编写应用程序，而不必担心来自其他应用程序的干扰。如果操作系统调度器可以访问所有应用程序的QOS需求的全局知识，那么可以执行全局可调度性分析以确保每个任务满足其QOS需求，则可以维持这种错觉。然而，这在“开放系统”的环境中是不可能的，因为操作系统可能不知道所有应用程序任务的时间需求，就像大多数商业嵌入式和实时内核的情况一样。一种简单的解决方案是为时间关键型应用程序分配更高的优先级，并让非实时应用程序只在时间关键型应用程序完成时运行。这种方法只适用于最简单的嵌入式系统，其中只有一个实时任务，并且实时任务和非实时任务之间没有交互。如果所有应用程序都在不同程度上对时间至关重要（硬或软），那么实时和非实时应用程序之间的区别就会变得模糊。在这个传感器-计算机的例子中，如果功耗警告任务被分配了一个低优先级，因为它的时间约束较松，那么当高优先级的定位任务独占CPU时，可能会耗尽电源。这可能会导致传感器复位，从而影响位置固定结果的有效性。该项目的目标是实现在开放系统环境中运行的健壮的嵌入式实时系统的设计。我们正在研究的创新是一种优雅的软件抽象，称为RTVR（实时虚拟资源）。这种抽象允许应用程序程序员设计嵌入式实时系统，因为每个应用程序都可以独占地访问一组专用的物理资源，这些资源以大致恒定的速率提供服务。RTVR概念的关键是服务提供率的“延迟界”参数。延迟限制规范使程序员能够指定他/她的应用程序的抖动允许。因此，我们可以简化单个应用程序的定时正确性验证，就好像应用程序任务之间没有定时干扰一样。RTVT概念还简化了保证软硬混合实时应用程序的定时/QOS要求的问题。为了实现这种抽象，我们必须在操作系统设计和实时调度理论方面取得理论和工程上的重大进展。本项目特别感兴趣的是多个实时虚拟资源的分区组成和分层分解问题。

Aloysius K. MokCCR-0207853 "REAL-TIME VIRTUAL RESOURCES"As embedded systems become more complex, a typical embedded systemwill probably involve a mix of soft and hard real-time applicationsthat share the same embedded run-time platform. In general, anapplication may consist of one or more tasks. For example, on ahand-held sensor-computer, a user may want to fix his own locationwith respect to a target landmark and also be warned of the powerconsumption status of the sensor-computer. While the power consumptionwarning task may have very loose timing constraints, the locationfixing task has fairly stringent timing and other QOS (quality ofservice) requirements. Ideally, an application developer should beable to write his application program as if it were running on adedicated computer and not have to worry about interference fromother applications. This illusion can be maintained if globalknowledge of the QOS requirements of all the applications isaccessible to the operating system scheduler, so that a globalschedulability analysis can be performed to ensure that every taskmeets its QOS requirements. However, this is not possible in theenvironment of "open systems" where the operating system may notknow about the timing requirements of all the application tasks,as is the case with most commercial embedded and real-time kernels.A simplistic solution is to assign a higher priority to timing-critical applications and let the non-real-time applications runonly when the timing-critical ones are finished. This approach isviable only for the simplest embedded systems where there is onlyone real-time task and there is no interaction among the real-timeand the non-real-time tasks. If all the applications are to variousdegrees timing-critical (hard or soft), then the distinction betweenreal-time and non-real-time applications becomes blurry. In thesensor-computer example, if the power consumption warning task isassigned a low priority because of its laxer timing constraint, thenit is possible that power may run out while the high-priority locationfixing task is monopolizing the CPU. This may in turn cause a sensorreset that may affect the validity of the location fixing result.The goal of this project is to enable the design of robust embeddedreal-time systems that must function in an open systems environment.The innovation we are investigating is an elegant software abstractioncalled RTVR (real-time virtual resource). This abstraction allowsapplication programmers to design embedded real-time systems as ifeach application had exclusive access to a set of dedicated physicalresources which provide service at a roughly constant rate. The keyto the RTVR concept is that of a "delay bound" parameter on the rateof service provision. The delay bound specification enables aprogrammer to specify the jitter allowance of his/her application.As a result, we can simplify the verification of the timing correctnessof individual applications as if there is no timing interferenceamong application tasks. The RTVT concept also simplifies the issueof guaranteeing the timing/QOS requirements of a mixture of hardand soft real-time applications. To realize this abstraction, wemust make major advances in both theory and engineering in operatingsystem design and real-time scheduling theory. Of particular interestto this project are the issues of partition composition and thehierarchical decomposition of multiple real-time virtual resources.