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SubjectRe: real-time threaded IO with low latency (audio)
On Fri, 23 Jul 1999 wrote:
> On Thu, Jul 22, 1999 at 05:59:43PM -0400, Raul Miller wrote:
> > <QUOTE>
> > 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.
> This is not quite correct. RT tasks under rtlinux can access all Linux services
> indirectly by passing their requests to Linux processes. For example it is quite
> easy to have a linux process log data collected by a rt task and, in fact, this
> is probably the most common use of RTLinux right now.

Its pretty close to correct. If the RT tasks need the O/S services to
perform they're rt function, as oppossed to your logging scenario where
the logging passive and doesn't feed data back into the rt system, then
they inherit the determinism of the *least* deterministic part of the
system. If you're running an 200Hz controller that expects setpoints
via a udp connection at 40 Hz, and is loggin large amounts of data, you're
"real time" controller is going to be affected by the non real-time
nature of the user-space activities. This is not good. Note that the
disk logging and the gui displaying progress charts, and the tcp are all
logically disconnected systems, the only necessary shared resources are
CPU time, and irqs, so they shouldn't have to affect each other, but they
are too closely coupled under Linux right now.


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