Re: Real Time response/latency question

[Dan Malek <dan@netx4.com>]

Graham Stoney wrote:
> 
> Hi folks,
> 
> I'm looking at software architecture questions, and wondering how much
> application domain work I can move out of driver land and into user space,

I believe this is a good programming practice, and in many
cases I don't even write a driver when the application is
allowed to run with root privilege.

> .... Can anyone give me a rough idea of the maximum
> latency in user space I could expect on an 860T based system with multiple
> threads, where only one thread has SCHED_RR realtime scheduling priority?


You can't answer this question due to the design and implementation
of the kernel.  Most "real-time" systems can't either once you
throw protocol stacks, file systems, disk drives, and other
external devices or distributed services into the feature mix.

I have successfully written applications that required
sub-millisecond latency.  You just have to be careful about
other applications in the system, and remember that you can't
preempt the kernel.

If you have hard real-time requirements, check into the RT-Linux
features.  These are starting to mature, and can make scheduling
guarantees within the confined real-time domain.


	-- Dan

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