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SubjectAn important fact about real time computing
The first sentence my professor maid on the real time computing
course was :

"A common misconception : Real-time computing is fast computing."

I missed the real-time and low latency debate, but reading the
kernel traffic pages [*] I saw that this misconception is very
alive.

For example Linus himself said :

> I personally would rather see that nobody ever needed RTlinux
> at all. I think hard realtime is a waste of time, myself, and
> only to be used for the case where the CPU speed is not
> overwhelmingly fast enough (and these days, for most problems
> the CPU _is_ so overwhelmingly "fast enough" that hard realtime
> should be a non-issue).

And on other place :

> If you're doing just audio that needs approximately 1%
> of the CPU resources, and you have to use hard-realtime,
> the system needs work.

And also:

> I think RTLinux is perfect for those things that truly need
> latency guarantees: no OS at _all_ in the way. But using it
> for "normal" stuff like just streaming audio and video is
> overkill. They don't have microsecond latency requirements.

Processor speed (and the size/length of the deadline) is
irrelevant. Even with a 10 GHz Pentium-X
your audio will pop if the IDE driver disables interrupts
long enough. Or if you are sorting a long list with an O(n^2)
algorithm ( I don't know an actual example from kernel, but
maybe some memory management code might do this ).

( and video _does_ have latency requirements, not microsecond
but around 20 millisecond )

Again , real time is making the dead-line, not
making the deadline fast.

David Balazic

* - http://kt.linuxcare.com/kernel-traffic/kt20000717_76.epl#6
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