Re: a joint letter on low latency and Linux

On Thu, Jun 29, 2000 at 12:44:52PM -0600, Erik Andersen wrote:
> On Thu Jun 29, 2000 at 08:41:48AM -0700, Larry McVoy wrote:
> > 
> > My advice, heeded or not, is to just say no.  We have an excellent answer
> > for realtime, it's called RT Linux.  Go use it - it's better than any
> > other answer.
> 
> Except for RTAI, I suspect, since
> 
>     a) The kernel patch is much smaller and elegant, therefore less intrusive
>     and easier to maintain.

This is a nice advertising slogan but not correct. Note that RTLinux
now works on Linux all Linux versions up to 2.4.test and PowerPC,Alpha,
as well as x86 (Mips soon) while the "easier to maintain" RTAI patch
which relies on screwing around with very x86 specific hardware
features is not there yet despite a rather large coporate investment.

The RTLinux patch was always small. The complexity is in what it does.
Despite my complaints, Ingo and Linus, with a small contribution from 
yours truly, have really simplified the 2.3 irq interface and made it
easier to track in RTLinux.

>     b) RTAI now has a feature called LXRT hard real-time (in it's early days)
>     that allows the use of hard real-time from user-space programs (thus has
>     the potential to use the process MMU protection).  Note when a thread is in
>     hard real-time mode, it still may not access Linux systems calls.

I think this is a very clever idea and would be semi-happy to add it to the 
RTLinux distribution if it were done in a way that was safe  and portable.
So far, the explanations I've seen, however, are kind of scary. 
I really don't like the use of "int xx" to provide a second copy of the
system calls, the stated need to run hard RT locks around the Linux
scheduler, and some other features (including some fuzziness about what
happens when a user forgets to mlockall or uses a shared library).
There are a couple of hard technical
problems to solve there, but when they are solved, there is no barrier
to having this system in the RTLinux distribution as well.

> 
> http://www.aero.polimi.it/projects/rtai/
> 
>  -Erik
> 
> --
> Erik B. Andersen   Web:    http://www.xmission.com/~andersen/ 
>                    email:  [e-mail]
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-- 
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