Logging/printf's in RT env

Martin Pohlack mp26 at os.inf.tu-dresden.de
Mon Aug 8 21:57:53 CEST 2005


> Yes, it goes away.
> But I thought that, if you have a real-time application running it will
> not be influenced by some "time-sharing" application as the logserver
> should be.

Just a short answer from me:  This is of course only true if you don't
use these services!  How should the system protect you if you call a
non-realtime service???

Btw. using the logserver or any other printf-style output inside a
real-time loop is a *very* bad idea.  This services ususally use very
slow output devices (serial line, text-mode display).

As a general rule:  You must decouple the output between the output
service and you rt-app.  Of course, this can not be achieved with
synchronous IPC to a non-rt service ...

One would typically use a memory buffer and either collect the data and
output it *after* the experiment or use a asynchronous memory protocol
to transfer the information to another io service (Note: do not use LOG
here as it also slows down the rt-part of the system as it causes
serious slowdown in hardware IO (Jork once meassured a lot more than one
µs per character !!!).

You might want to have a look in the "rt_mon" or the "grtmon" package,
which have been used with some success in similar situations.  Both
packages should have demo applications available and for grtmon there is
a Diploma Thesis (A Generalized Approach to Runtime Monitoring for
Real-Time Systems) available from:


Hope this helps ...

Martin Pohlack

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