Re: RT patch acceptance

Gene Heskett
2005-05-24T14:47:54Z

On Tuesday 24 May 2005 09:58, Esben Nielsen wrote:

As a user working on makeing a small mill into a CNC machine, using
emc, I'll comment as needed here.

>On Tue, 24 May 2005, Karim Yaghmour wrote:
>> Esben Nielsen wrote:
>> > I find that a bad approach:
>> > 1) You don't have RT in userspace.
>> > 2) You can't use Linux drivers for standeard hardware when you
>> > want it to be part of your deterministic RT application.
>>
>> Please have a look at RTAI/fusion. For the record, RTAI has been
>> providing hard-rt in standard Linux user-space for over 5 years
>> now. With RTAI/Fusion this gets even better as there isn't even a
>> special API ...
>
>The tests I have read (I can't remember the links, but it was on
> lwn.net) states that the worst case latency is even worse than for
> a standeard 2.6 kernel!

It is worse, by a quite noticeable amount. Keyboard events often take
noticeable fractions of a second to show, or to take effect if its
the apps own interface that you are entering control data to.

>If you gonna make usefull deterministic real-time in userspace you
> got to change stuff in kernel space and implement stuff like
> priority inheritance, priority ceiling or similar. It can only
> turn up to be an ugly hack which will end up being as intruesive
> into the kernel as Ingo's approach. If you don't do anything like
> that you can not use _any_ Linux kernel resources from your RT
> processes even though you have reimplemented the pthread library to
> know about the "super RT" priorities.
>
>But I give you: You will gain better interrupt latencies because
>interrupts are executed below the Linux proper. I.e. when the Linux
>kernel runs with interrupt disabled, they are really enabled in the
> RTAI subsystem.
>
>My estimate is that RTAI is good when you have a very small
> subsystem you need to run RT with very low latencies. Forinstance,
> controlling a fast device with limiting hardware resources to
> buffer events.

This is true, and in order to be able to run emc with anything like a
decent motor speed at the motors, I had to buy a new board and video
card to replace the one I was going to use, which was a 266mhz p2.
The p2 could do it, but there was no time left to run linux, so
controlling the application wasn't possible. Without changing the
RTAI cycle time, a 1400mhz athlon runs linux at fairly normal speed
while emc is running.

>For large control systems I don't think it is the proper way to do
> it. There it is much better to run the control tasks as normal
> Linux user-space processes with RT-priority. I can see Ingo's
> kernel doing that, I can't see RTAI doing it except for very
> special situations where you don't make _any_ Linux system calls at
> all! You can't even use a normal Linux network device or character
> device from your RT application!

I agree. Use RTAI if you are building a specialized box that will
never be asked to do anything else, mostly because thats all it will
be capable of doing unless it has horsepower to burn, lots of it.

Ingo's RT patches allow me to do some play time and driver development
on this box for that application with a reasonable expectation that
it will work on that box when I haul the code down there and
recompile it there.

I've also noted that Jack users need this as its not very usable
without it, or wasn't half a year ago. I'm not a die hard Jack user,
but I'd really like to import my movie camera without any dropped
frames, so I expect what fixes one will fix the other.

If, at the same time, it will give me back a keyboard when SA is
filtering the incoming mail on this machine, thats a huge plus. I'm
about to find that out as I just built the -07 version.

>> Here are a few links if you're interested:
>> http://www.rtai.org/modules.php?name=Content&pa=showpage&pid=1
>> http://marc.theaimsgroup.com/?l=linux-kernel&m=111634653913840&w=2
>>
>> Karim
>
>Esben

--
Cheers, Gene
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Copyright 2005 by Maurice Eugene Heskett, all rights reserved.