21 Feb
2006
21 Feb
'06
10:14 p.m.
On Tue, 2006-02-21 at 20:00 -0600, Jan Depner wrote:
On Tue, 2006-02-21 at 20:49 -0500, Lee Revell wrote:
Well, of course, Monta Vista is selling their own RT kernel!
What I mean is that the difference between a hard RT system and a soft
RT system is not between a guaranteed 15us response and a guaranteed
15ms response, it's between the ability to guarantee ANY response time
and only being able to meet the deadline 99.9999999% of the time.
Lee
On Tue, 2006-02-21 at 19:30 -0600, Jan Depner
wrote:
I beg to differ. Hard real-time guarantees the response time. Most
good hard real-time systems actually do respond in the 10-15 microsecond
range (though that is not a requirement of hard real-time). Theory has
no place in hard real-time. Check with Monta Vista and see if they
think the kernel with RT patches is hard real-time.
On Tue, 2006-02-21 at 15:28 -0500, Lee Revell
wrote:
Hard RT is not about what the response time is, it's about whether you
can guarantee to make some arbitrary deadline, which the -rt patch can
theoretically do (I say theoretically because you still would have to
audit a limited set of code paths for RT safeness).
On Tue, 2006-02-21 at 13:19 -0600, Jan Depner
wrote:
> There's a bunch of information on that on my site (albeit outdated).
> Tuning the disk drives is a must and it *will* help but there are
> instances where the disk drive is busy and you can't get to it no
> matter
> how well tuned it is. I prefer to minimize any chance of that. You
> have to remember that unless you're running RTLinux or VXWorks (or DOS
> or VMS) you're not running a hard real time system. Shit happens.
>
The -rt kernel with fuill preemption actually is a hard real time system
(no one claims it is in the same league of reliability as QNX or
VXWorks, yet...) - it should be able to guarantee response times.
While I agree that it's very good it's not hard real-time. It can't
do guaranteed 15 microsecond interrupt response. It is a very good soft
real-time system.