[squeak-dev] Re: Delay time question

Robert F. Scheer rfscheer at speakeasy.net
Mon Mar 3 03:01:47 UTC 2008


On Mon, 2008-03-03 at 13:51 +1300, Michael van der Gulik wrote:
> 
> 
> On Mon, Mar 3, 2008 at 2:01 PM, Robert F. Scheer
> <rfscheer at speakeasy.net> wrote:
>         Also, after installing the linux-rt ("realtime") kernel from
>         Ubuntu
>         repositories, the delay bag performance was horrible, as
>         mentioned
>         earlier, but not in the same league of hurt as the results
>         you've just
>         posted.
> 
> 
> I can't tell if there's just the one real-time Linux which has
> multiple names or if they're separate projects; I'm seeing linux-rt,
> Linux/RT, rtlinux, RT-Linux, ...
> 

The version in the Ubuntu repository is really a low-latency kernel
based on work by Thomas Gleixner and Ingo Molnar.  It is basically the
standard Ubuntu kernel with their patch tailored for some Ubuntu
differences.

It is intended to support automation, robotics, telco and multimedia
authoring for example.

Supposedly, it adds support for high resolution timers and full
preemption.  (I haven't had time to get into what that really means or
requires, sorry).

It is not a real "real-time" Linux kernel by any means.  It was an easy
experiment that I did but not a very serious one.

- Robert

> Anyway, if they're all the same thing, then you're not using any of
> the real-time features:
> 
> http://www.faqs.org/docs/Linux-HOWTO/RTLinux-HOWTO.html
> 
> In order to get real-time performance, it sounds like you need to
> write your real-time code as a kernel module, and the rest of your
> application (the parts that don't need to be real-time) would run in
> user-land and communicate using a real-time FIFO character device.
> 
> Also, by real-time, they really do mean real-time: "Worst case
> interrupt latency on a 486/33Mhz PC measures well under 30
> microseconds, close to the hardware limit." (ref:
> http://www.linuxdevices.com/articles/AT3694406595.html, assuming it's
> the same RT-Linux).
> 
> Gulik.
> 
> -- 
> http://people.squeakfoundation.org/person/mikevdg
> http://gulik.pbwiki.com/ 




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