On Monday 14 July 2008 20:36:57 andi wrote: > Hi! > > It think first you have to define what real-time is. There are so many > definitions around, but the one I prefer is the one given in the book > "Real-Time Systems - Design Principles for Distributed Applications" by > Hermann Kopetz: > > "A real-time computer system is a computer system in which > the correctness of the system behavior depends not only on > the logical results of the computations, but also on the > physical time, when these results are produced. > The point in time when a result has to be produced is called a > deadline. > Deadlines are dictated by the environment of the real-time > computer system." > > Furthermore he defines: > "If the result has utility even after the deadline, we call the > deadline soft. Systems with soft deadlines are not the focus > of these lectures. > If the result has no utility after the deadline has passed, the > deadline is called firm. > If a catastrophe could result if a strict deadline is missed, the > deadline is called hard. > A real-time computer system that has to meet at least one hard > deadline is called a hard real-time system. > Hard- and soft real-time system design are fundamentally > different." You should also consider firm realtime systems, where the deadlines can be missed without bring down the entire system, but the result might be useless or degraded. > You wanted a good online explanation, and I cited you the book, but you > can find the definition on slides from Hermann Kopetz here: > http://ti.tuwien.ac.at/rts/teaching/courses/ezs/Pdf_Slides_WS07/1_Introduct >ion.pdf He also suggests some other ways to classify real-time systems. > > I know my answer does not tell you anything about real-time linux. I > don't know what you mean with WRT linux? is this some real-time linux > project? i only know openwrt, but thats nothing about real-time. I think he means "With Respect To" (or regard?) If you follow the strict definition of hard real-time systems, I don't think you could call Linux a real-time OS, as you *cannot* possibly guarantee to meet all deadlines (as you have interrupts, soft-irqs, etc etc). The kernel is *way* to complex. However, you can make a pretty good attempt at making it a hard real-time system, but I'd say it's rather "very firm" than hard. If you're looking for a hard real-time linux system, I'd say throw an email to either the AVR32-camp or the ARM-people. As a Norwegian, I'm naturally inclined to promoted AVR ;) -- mvh Henrik Austad
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