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From: https://rt.wiki.kernel.org/articles/f/r/e/Frequently_Asked_Questions_7407.html

Real-time only has impact on the kernel; Userspace does not notice the difference except for better real time behavior.

Does it mean that if we write the applications in user space, they won't get the hard real time effect?

I am not able to understand the English behind it.

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closed as off topic by Mitch, kiamlaluno, aedia λ, Marthaª, waiwai933 Apr 25 '12 at 1:01

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I think this is more a technical question about the timers in the rt kernel than the use of english. The faq isn't clear and reads as if it was written by a non-native speaker, or someone in a hurry –  mgb Apr 24 '12 at 5:03
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I'm not down voting for now, but suggest migrating to stackoverflow or another SE site. –  Kris Apr 24 '12 at 6:48
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@Kris, I agree with Anisha here. It seems to me to be an English question, albeit one that would be best answered by someone with technical knowledge of the domain. I've just tried to read the wiki page that this came from, and it's quite confusingly written. I'd like to see a good answer on this site, because I think people who struggle with understanding stuff like this at some point in the future are likely to come here. If you are able to provide an answer, please do so here. –  user16269 Apr 24 '12 at 9:15
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Totally off topic. Too domain specific. –  Mitch Apr 24 '12 at 12:56
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@Anisha I upvoted your Question because -- even if it straddles SE site boundaries -- it might as well get answered here. However, I am puzzled that you are unable to evaluate the two Answers given so far. Do they not give you enough extra information that, when you factor in your knowledge of the subject matter, the sentence becomes clearer to you? –  Eugene Seidel Apr 24 '12 at 13:04

3 Answers 3

up vote 1 down vote accepted

They mean that the patch does not add any new userspace APIs, or change the behavior of any existing APIs (in terms of what value are returned etc.). You just write your app using the normal POSIX APIs, and get better real-time behavior automatically.

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Here's what they said: spinics.net/lists/linux-rt-users/msg07910.html –  TheIndependentAquarius May 15 '12 at 12:10

Just incase a third stab at it helps (and I'm sort of crossing into SO territority, but...) :

Applying the patch/options/etc (to enable real-time support) will only have a direct impact on the kernel.

Software written in userspace will see flow-on impacts, but they will not be things like changes in the API, different return values, etc etc etc (basically - what @LaC said). They will instead be things like 'better' real-time performance because the userspace software is interfacing with something that has been set to operate in a more real-time fashion.

As a slight digression, I'd caveat the 'better' performance bit - you're not going to get true real-time operation from anything unless the whole system is designed for real-time operation (as mentioned in the SO response).. in fact, now I think about it, you could reword it to talk about the intent of the statement as follows:

If you enable these patches/options, it will make the kernel operate in a real-time mode.

This does not make your userspace programming experience any different.

You program the same way. You compile the same way. You can do everything the same as you did before. The only thing you might see, is better real-time behaviour.

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Also, can someone please tell me if I've just gone round in circles? I don't think I have, but I was finding it kinda hard to rephrase/explain without diving into a more technical zone, at which point I'd say to kill the answer here, and answer on SO... –  tanantish Apr 24 '12 at 14:51

Quoting the full paragraph:

No special API's available in the real-time patch. Real-time makes use of the standard Posix API's.

Also: If the Hires-timer is enabled in the kernel, every timer in userspace becomes a hires timer. Real-time only has impact on the kernel; Userspace does not notice the difference except for better real time behavior.

I think there are two things real-time stands for. There is a real-time patch talked about in the first paragraph. If the Hires timer is enabled, then this timer takes precedence over the real-time patch, as far as timers are concerned. The impact of the real-time patch is limited to the kernel in that case.

The last line, however, does not talk about the real-time patch anymore; instead it refers to real time as we know in normal English: the perceived effect in time. It means the usage of Hires timer leads to no significant effect other than better real-time behaviour perceived by the user.

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