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Take a look at

http://kerneltrap.org/node/11752
http://rt.wiki.kernel.org/

Hard real time cannot be achieved with the vanilla kernel but this patchset can enable that. Vendors are already shipping products based on this.

http://www.redhat.com/mrg/

Yes all those links show that linux can work with *soft* real-time capability. *Hard* real-time is different as previously mentioned.

This is also why for instance linux is usually multiplexed with another RTOS for such delicate operations (e.g. modem communication on mobile phones).

Some of the documents I refer specifically use the word hard real time. As an example

https://wiki.ubuntu.com/RealTime/Feisty

Red Hat MRG product has specific guarantees below certain milli seconds for specific transactions. Refer to their whitepapers for more details.

I guess your definition of hard real time is different then which is ok as long as you clearly specify what your definition is or provide some reference as to to what you believe is a commonly acceptable definition.

Within specific industries, they start out with a definition or avoid the term and just specific the guarantees just to avoid this problem. That for example guarantees the max time slice among other things.

Edited 2008-04-19 00:29 UTC

Regardless of the document's use (or misuse) of the term "hard realtime", it's a well understood fact that hard realtime means guaranteed deadlines; unless the Linux kernel provides microsecond accuracy, it's "soft realtime". This isn't a fuzzy definition -- or a minor distinction -- hard realtime guarantees are required particularly for systems where subtle variations in performance or latency can have tragic consequences (eg. flight control systems, etc).

The vanilla kernel doesn't but we are talking about -rt patchset specifically and you can read about their goals in the kernel wiki and read about the exact guarantees within specific products offered by vendors using this patchset.

I was under the impression that hard real-time meant just guaranteed deadlines with no drop in service quality, nothing to with whether it guaranteed one or several microseconds.

You are correct. Hard real time does not mean fast. Merely a guarantee to do a particular transaction within a particular time slice. In fact, enabling hard real time can make your general performance go down in preference to less spikes.

You contradict yourself, here. If hard real time means guaranteed deadlines (and it does) what do "microseconds" have to do with it? The guarantee could be that it will definitely do something within one millisecond, or one year, but if it can be even one Planck Time over that, it fails the test. The definition of hard real time is, in fact, so pedantic, that I think I can say with reasonable certainty, that nobody, except maybe God, legitimately needs it.

In short, I think there's a Universal market of maybe 1 real time computer. And you can quote me on that. ;-)

Edited 2008-04-19 19:52 UTC

Try to remember that the next time you're riding on a plane and you're approaching for landing. Or, when you need to stop suddenly in your car. Or, when a nuke plant operator needs to regulate cooling. Because those are scenarios where it's needed.

Edited 2008-04-19 23:12 UTC

Where what is needed? Please be *very* specific in your answer.

BTW, what are the actual real time requirements of the nuclear power plant cooling system? Why should it require microsecond precision?

Edited 2008-04-19 23:38 UTC

I would bet any amount of money the Ubuntu page is wrong.

"Hard real time" isn't a vague descriptive phrase. It's an actual concept used to describe ultra-strict time constraints where something not happening exactly when it needs to has severe consequences like an explosion or people dying. It's used in stuff like nuclear power plants, avionics, etc.

Soft real time systems are those that have strict timing requirements, but missing a deadline isn't too big a deal. Playing an MP3, for example, requires refilling the audio buffer before the old one is consumed, but it's not the end of the world if it doesn't happen.

It takes a whole lot of work to make a large hard real time system, and it just doesn't make sense to put in the effort for a desktop system.

yess, well... Linux isnt just for desktops, even though this is one of its best uses

But Ubuntu is aimed squarely at the desktop.

If anybody is going to make a hard real time version of Linux, it's probably not going to be them.

'hard' real time is not so well defined as you might imagine.

A truly deterministic implementation cannot have such niceties as processor cache, interrupts or file i/o as you cannot accurately predict their latencies.

A practical 'hard' real time system still has to allow for missed deadlines, or have some kind of priority assignment for real time threads. This opens up the possibility of priority inversion. (see Mars Rover Pathfinder.)

So there are always compromises and each implementation is specific. The Linux RT patches provide one implementation, with all the problems associated with running on commodity hardware, networking, file io etc. Whether it fits your needs is another matter.

These sorts of threads always get caught up in the "what is hard realtime" question, when the more interesting question is "When is hard realtime (vs soft realtime) really needed?". When you get right down to it, hard realtime, far from desirable, is really a last resort, and should be avoided if possible, if performance is of any concern at all.

I would recommend giving 64Studio a try. Probably tuned better than Mandrake/iva would be even with this real-time patch.
Sometimes the problem with latency is overly conservative IDE settings ( you may have to get familiar with hdparm to change this) or excessive CD-ROM drive polling.

There are some good docs floating around on this but might be quicker to take 64Studio for a spin.

http://www.64studio.com/

2 HOURS to recompile the kernel - wow, there's now way it should take that long with recent hardware. I do remember when you could spend that much time at it but that was LONG ago !!!!

If it took 2 hours I seriously doubt your PC is fast and good.