Components in Real-Time Systems

Transcription

1 Contents Components in Real-Time Systems Authors: Damir Isovic and Christer Nostrom Presented by: Chris Pope Real-Time Embedded Systems Motivation for CBD in Real time Systems Challenges of using CBSE Real-time CBD Examples of Existing Components and Frameworks Conclusion Real-Time Embedded Systems Examples of Real time systems To be considered correct, programs must meet timing deadlines, as well as compute correct results Often operate with limited computational resources Operate on a wide range of hardware architectures, each with unique characteristics Must continuously operate for long periods of time Usually control or monitor physical processes Potentially disastrous consequences of failure Vehicle control systems ABS System Engine Control Unit Inertial guidance computers Aircraft Spacecraft Missiles SCADA (Supervisory Control and Data Acquisition) Power grid monitoring Industrial Control

2 Motivation for CBD CBD in Real-Time Systems Consider a brief history of real-time embedded systems 1960's - Apollo Flight Computer Required custom-engineered hardware Specific task Expert verification 1990's/2000's Capable hardware is readily available Products reach a mass market Feature-driven competition Development cycle with time/budget constraints How can we develop real time systems under these conditions, and still be sure they live up to their name (meet timing constraints, don't crash, provide correct results)? Answer? CBSE Methods (Of Course!) Reusable components could Reduce development time Increase maintainability Allow systematic verification of Program Correctness Schedulability Plus, we can make use of CBSE tools and research. Easy, right? Not Really Existing CBD technologies (J2EE, COM, CORBA) are unacceptable for real-time CBD High memory and processing requirements Unpredictable execution Increased flexibility leads to lower predictability In order to select components, we need: Required and provided interfaces for the component Detailed resource requirement information Timing characteristics of the component's operations Interprocess dependencies Reliability characteristics

3 Real-Time CBD After integrating components, we need to: Verify the components still produce correct results Verify the components' timing characteristics still hold after integration Verify the components' stated characteristics are valid for our target architecture To meet the challenges posed by CBD for Real-Time Isovic and Nostrom propose extensions to the Component Meta-model, and a development process Types Of Components Infrastructure Operating System kernel Database system Domain-specific components Domain specific components consists of one or more processes that run on infrastructure components Real-Time CBD Component Meta-model Extension Along with required/provided interfaces, components must also specify the following: Worst case execution time Memory requirements Mutual exclusion and precedence relationships (Dependencies on other components) Environment assumptions Prerequisites: A library of well documented real-time components A real-time operating system that our components can run on Component Library May be developed in-house Developed by a third party COTS Difficulty with COTS in Real-time CBD Unless the component has been built for our exact hardware platform, its execution properties may not hold Source code is not available for performing static timing analysis

4 Top Level Design - Decompose the system into manageable components Determine the interfaces required/provided by each component Identify safety issues associated with each component Detailed Design - Selecting components to be used from the candidate set Based solely on functionality, select the required components from our library Identify components that we need, but do not exist Architecture Analysis - Check that the system under development satisfies extrafuctional requirements such as maintainability, reusability, modifiability and testability Notice We make sure the system has sound non-functional requirements before we even consider whether it is operationally feasible Scheduling Analysis - Make a schedulability analysis of the system based on temporal requirements of each component Ensure specified worst case execution times (WCET) are valid on our platform Given WCET and scheduling properties of components, can we find a feasible execution schedule? Given all interprocess dependencies, can we guarantee deadlocks do not occur? Implement Missing Components Caveat must ensure worst case execution time for new components does not break the schedulability analysis performed earlier Build and Test - Verify the functional and temporal properties of the system obtained If system does not behave as expected, determine which components are at fault Examples Real-time Components Infrastructure Components VEST OS is produced by composing components offline, using a configuration tool VEST development environment verifies temporal execution properties Rubus Windows NT (subset of functionality), or Window CE

5 Conclusion Real-time embedded systems provide unique challenges CBD meta-model extensions and development process described are good steps to making the process easier Conclusion Criticism COTS components for real-time systems are not feasible A lot of effort is required to ensure the component behaves as advertised What happens if the component simply doesn t meet our timing requirements? Need components for our particular architecture Domain-Specific components are unlikely to emerge Need abstract view of hardware for portability, but this increases timing uncertainties Real-time systems interact with specific, likely proprietary, hardware Lack of tools to perform timing analysis may dissuade developers from following a structured development process