Re: [rtl] using RTLinux for video stream handling

[Corey Minyard <minyard@acm.org>]

Marko Rauhamaa <marko@tekelec.com> writes:

> > X is a user space process, thus cannot disable interrupts, and
> > therefore cannot block the system.

This is not true.  X is a user space processes, but it is possible for
user space processes to disable interrupts.  I have personally done
it.  You have to modify your iopl and you have to be a supervisor
process, but you can do it.  And some X servers do this, I'm pretty
sure.  I know they go directly to the hardware.

And since X directly calls the cli and sti instructions instead of
calling the Linux functions in the kernel, even RTLinux cannot stop
this.

> 
> Besides, as a user process, X has absolutely no CPU bandwidth guarantees
> no matter what it's priority is. The other processes heavily interfere
> with timing. And ordinarily you don't have control over the processes
> that the user launches on the system. If the load is high enough, I
> guess it might take literally seconds before X wakes up after it has
> spent its time slice. ("I guess" because I don't know the specifics of
> the Linux scheduler.)

I believe this is true, but it doesn't have to be, if the X server
used Posix.1c process priorities.

-- 
Corey Minyard               Internet:  minyard@acm.org
  Work: minyard@nortel.ca       UUCP:  minyard@wf-rch.cirr.com
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