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SubjectRe: real-time threaded IO with low latency (audio)
Raul Miller wrote:
> David Olofson <> wrote:
> > I think this whole discussion is beginning to get confused by different
> > definitions of "real time". (Not the first time...) Basically, there are
> > two kinds:
> >
> > 1) Soft real time
> > * Typically ms precision timing.
> > * No guaranteed deadlines.
> >
> > 2) Hard real time
> > * Usually significantly better than ms precision
> > * Guaranteed deadlines.
> Maybe three kinds, see:

Yes, I've seen the KURT site, but not looked much closer at it.

Perhaps I should have mentioned Firm Real Time as a third kind. (It's
more or less what I sugested for multimedia later on in the original
post. However, I really don't like the normal mode/real time mode
switching idea...) But as I was more focused on the issue that makes all
the difference to some applications; guaranteed maximum latency; I
didn't consider firm real time significantly different from soft real
time. It is indeed in real life, but not to applications that really
require hard real time.


> Some types of processing (eg. multimedia) do not fit well into the
> hard or soft real-time categories. The periodic requests made by
> multimedia applications are sensitive to variations in timing. As
> such, they are not well served by the loose guarantees provided by
> soft real-time systems. Hard real-time systems are often faced with
> providing guarantees at the expense of providing services. Thus, while
> they can meet the timing requirements of a multimedia application,
> many times they cannot meet it's other service requirements. For
> example, real-time processes running under RTLinux have no access to
> any of the Linux services.
> A purely binary distinction between hard and soft real-time is clearly
> not acceptable for all applications. Many applications have
> requirements spanning a continuum between the two. ...
> </QUOTE>
> [With supposedly a demo where a modified xanim performs better on a
> loaded system.]
> --
> Raul

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