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[DPRG] microsecond resolution realtime with Linux 2.6

Subject: [DPRG] microsecond resolution realtime with Linux 2.6
From: Ed Okerson ed at okerson.com
Date: Sat Apr 22 21:11:46 CDT 2006


What hardware platform are you running Linux on?

Ed Okerson

> The last few weeks, I've been trying to get real time performance from a
> uClibC/buildroot Linux 2.6 system. For my application, that means low
> microsecond timing resolution. The motor control system relies on bit
> banged TTL serial so timing constraints are somewhat rigid. (Yes, this is
> a bad design - I freely admit that.)
> Anyway, I think it is finally working and stable. Here's a diagram of the
> system on the robot.
> http://golem5.org/robot1/images/design20060422.jpg
> When I first came up with this, I thought it was new and different. But
> actually it is very old school. The Linux HOWTO for Yodaiken's RTLinux (of
> FSMLabs) devotes a section to this approach.
>     user space process  ----realtime FIFO---->  special hard RT process
> Robert Love, who wrote the pre-emptive Linux kernel patch (now mainstream
> in 2.6 kernels), says that soft RT is often good enough. The experience
> I've had with my robot supports this. However, the downside is increased
> time spent tuning the system and testing it. You have to really be aware
> of all of the event processing loops and I/O going on, as well as arrival
> and service rates for each loop. As a system grows, it is increasingly
> difficult to know that it will work in all situations.
> 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.
> When I started my robot, I considered using the free for non-commercial
> QNX/Neutrino distribution, RTLinux, or RTAI (another hard RT Linux
> system). In the end, I went with vanilla Linux 2.6. I believe that for
> most amateur robots, Linux 2.6 is good enough to satisfy real time timing
> constraints. It is not perfect - there are still failures. But if life and
> limb are not at risk, then it is ok.
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