Realtime aspects Bernhard Weirich 0126357 Realtime Systems Realtime

Real-time aspects Bernhard Weirich 0126357

Real-time Systems • Real-time systems need to accomplish their task s before the deadline. – Hard real-time: violation of the deadline can lead to a total system failure – Soft real-time: violation of the deadline degrades the service quality • To be able to guarantee all deadlines to be met, the worstcase execution time (WCET) has to be known.

Hardware • In synchronous designs the worst case gate level delays and propagation time must be considered in any case. They are provided by the Timing analyzer. • The response time of a circuit to a certain input in terms of clock ticks is easy to calculate • But: – Information flow can be very difficult to predict. It often depends on history

Problematic Hardware • DRAM: access can be delayed by refresh cycles • Computer Architecture: – Caches – Pipelining – Branch Prediction • Network • Harddisk • …

Problems with Software • Software depends on the hardware • But knowledge of the software can be used to better predict hardware behavior • Additional problems: – Multitasking, conditional execution – Multiprocessing – Virtual Memory

WCET is more than SW or HW Development • WCET analysis requires knowledge of software and hardware at the same time • Abstraction to simple hardware interfaces is not possible

Approaches • Real-time operating systems • Real-time programming languages (e. g. Ada) • Increase predictability: – Cache-locking – Function cache • Predictable network protocols (TT-ethernet, TTP)

WCET estimation • WCET analysis during program compilation • Static WCET analysis of source code or compiled executables – DFA (data-flow analysis) cache modelling – Pipeline modelling – Branch prediction modelling – Model DRAM refreshs as periodic interrupts • Measure ET

Static WCET analyzer Software • Bound – T http: //www. tidorum. fi/bound-t/ • ai. T http: //www. absint. com/ait/ • Rapi. Time http: //www. rapitasystems. com/ • Calc WCET 167 http: //www. wcet. at/tools. html

Resources • http: //ti. tuwien. ac. at/rts/teaching/courses/wcet • http: //en. wikipedia. org/wiki/Worstcase_execution_time • Modeling the Function Cache for Worst-Case Execution (Raimund Kirner, Martin Schoeberl) • Experimental Assessment of Worst-case Program Execution Times (Pavel Atanassov)