Re: [linux-audio-dev] TimeSys Linux/RT(TM), a Real-time Version of Linux .. opinions ?

New Message Reply About this list Date view Thread view Subject view Author view Other groups

Subject: Re: [linux-audio-dev] TimeSys Linux/RT(TM), a Real-time Version of Linux .. opinions ?
From: David Olofson (david_AT_gardena.net)
Date: ti helmi  29 2000 - 19:23:08 EST


On Wed, 01 Mar 2000, Paul Barton-Davis wrote:
> >seems interesting ...
> >
> >http://www.newsalert.com/bin/story?StoryId=ColTsubWbtKvuvtaZmq&FQ=Linux&Nav=na
> >-search-&StoryTitle=Linux
> >
> >any opinions ?
>
> I see no mention of any actual times. I don't know anything about
> CMU's Resource Kernel, but I have a hard time seeing how a regular
> Linux kernel can ever be hard-RT when it is non-preemptive. Given that
> any chunk of kernel code can call cli() and leave it that way, how can
> you possibly have a real time kernel without changing the semantics of
> calls like cli() and sti() ?

You could probably implement an RT scheduler under RTLinux or RTAI,
and then hack a special "shortcut" that can schedule to user space.
However, that means that you have to force the application back to
normal Linux (possibly SCHED_FIFO) scheduling if it tries to call the
kernel, or Very Bad Things will happen. This is where it gets
complicated and uggly, but it should be possible. Then it's time to
handle the exceptions... How to find out who caused the exception if
the RT tasks can preempt the kernel?

And, this still gains *nothing* more than protected memory for the
RT tasks... As soon as you start using the kernel calls, you lose the
hard RT performance.

You *could* switch the kernel calls, so that RT tasks actually use a
different set of kernel calls, but now we're talking about a *real*
mess...

A cleaner solution would be to link the RT code with different
libraries, so that the normal kernel call entries aren't used, but
this still requires dedicated RT drivers, unless you do direct
hardware access, like a user space driver. (X, for exampel.)

Well, maybe there's some shortcut that the RTL and RTAI teams ond all
the other guys have missed... Would be cool if it could really be
done without wrecking the kernel. :-)

//David

.- M u C o S --------------------------------. .- David Olofson ------.
| A Free/Open Multimedia | | Audio Hacker |
| Plugin and Integration Standard | | Linux Advocate |
`------------> http://www.linuxdj.com/mucos -' | Open Source Advocate |
.- A u d i a l i t y ------------------------. | Singer |
| Rock Solid Low Latency Signal Processing | | Songwriter |
`---> http://www.angelfire.com/or/audiality -' `-> david_AT_linuxdj.com -'


New Message Reply About this list Date view Thread view Subject view Author view Other groups

This archive was generated by hypermail 2b28 : pe maalis 10 2000 - 07:23:28 EST