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[DPRG] microsecond resolution realtime with Linux 2.6

Subject: [DPRG] microsecond resolution realtime with Linux 2.6
From: Dave Hylands dhylands at gmail.com
Date: Sun Apr 23 17:01:46 CDT 2006

Hi Chris,

I intended my original reply to go to the list, but I forgot to do a
Reply-All, so I'm reposting to the list.

> But the good news is that it does work. Vanilla Linux is good enough to
> give microsecond resolution soft real time performance entirely in user
> space. You don't need a special hard RT platform or to write any device
> drivers. You can stay out of the kernel and still get the performance you
> need. It may take some work, though.

I respectfully disagree (on some points). I agree that soft RT is
typically good enough. I also think that Vanilla Linux is good enough
for most robots and many applications where people think that they
might need RT. Especially if you use the real time priorities.

The point I disagree about is the microsecond timing resolution. There
are many drivers within linux that will disable interrupts for more
than a microscond (and sometimes for more than a few milliseconds).
This will cause latency in any type of scheduling.

Obviously, this will be alleviated with faster processors. Most of my
experience is working with relatively slower processors (measured in
the couple of hundred MHz). Obviously a GHz processor will have less
latency.

But given, that I've seen cases where interrupts are disabled for a
small number of milliseconds on a 200 Mhz processor, they would be
disabled for hundreds of microseconds on a GHz machine.

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/


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