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Hard and soft real time

Hard and soft real time

Posted Nov 4, 2010 19:20 UTC (Thu) by dlang (subscriber, #313)
In reply to: Hard and soft real time by csimmonds
Parent article: KS2010: Deadline scheduling

but even with hard-real-time work, most of the situations allow you to miss a deadline once in a while. If your production line has to be stopped daily or hourly, you can't accept it, but if it stops once a year because the deadline was missed, that's probably acceptable.

you could also have a hard-real-time task that has a 5 second deadline to get the task done. non-real-time linux can accomplish that today (except in the face of failing hardware)

even things with fairly short deadlines can be made statistically reliable in many cases. It's only when you start getting into extremely short deadlines or systems that also handle non-critical processes that compete for resources that you start having real problems.

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"hard" real-time

Posted Nov 4, 2010 20:06 UTC (Thu) by dmarti (subscriber, #11625) [Link]

"Hard" real-time is where if you miss a deadline, the robot chops off the user's head, or the user's plane crashes.

"hard" real-time

Posted Nov 4, 2010 20:37 UTC (Thu) by dlang (subscriber, #313) [Link]

even in the case of aircraft, missing one deadline does not make the plane crash. having the system lock up will make the plane crash, making the system miss deadlines too badly, or too frequently may make the plane crash.

there are _very_ few situations where a single missed deadline proves fatal to the system (or the user :-)

in engineering, the assumption is that you may have unexpected loads, or you may have sub-par materials, so every design includes a safety margin, which means that they make it statistically unlikely that too many things will go wrong and the item will fail.

Even on the Space Shuttle, something as critical as the heat resistant tiles are not individually critical, it's expected that some number of them will be damaged or fall off on any flight. When too many of them get damages, you have the Columbia disintegrating, but that doesn't translate into zero tolerance of tile failure

"hard" real-time

Posted Nov 4, 2010 22:26 UTC (Thu) by csimmonds (subscriber, #3130) [Link]

We seem to be getting off the topic here. The point is that Linux is used in real-time applications - I have worked on several myself. Real time is about determinism and deadlines, and typically there are a range of acceptable behaviours. Dead line scheduling is a valuable tool in designing systems that have to meet deadlines (in essence, it makes the analysis easier). So, it would make sense to have such a scheduler as an option in the Linux. People seem to be arguing against improving the kernel, and I don't understand why.


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