On Tue, 2006-02-21 at 21:14 -0500, Lee Revell wrote:
> On Tue, 2006-02-21 at 20:00 -0600, Jan Depner wrote:
> > On Tue, 2006-02-21 at 20:49 -0500, Lee Revell wrote:
> > > On Tue, 2006-02-21 at 19:30 -0600, Jan Depner wrote:
> > > > On Tue, 2006-02-21 at 15:28 -0500, Lee Revell wrote:
> > > > > 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 There's a bunch of information on that on my site (albeit outdated).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.
> > > >
> > >
> > > 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).
> > >
> >
> >
> > 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.
> >
>
> 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.
>
Agreed. For hard real-time I would expect 0 xruns ever and that's
not what I get even with my new system (which is of course tweaked to
within an inch of it's life). I'm pretty happy with the kernel as it is
'cause I sure don't get very many xruns. About one per hour on average
and it's usually just when I'm starting and stopping things. I think I
can live with that ;-)
--
Jan 'Evil Twin' Depner
The Fuzzy Dice
http://myweb.cableone.net/eviltwin69/fuzzy.html
"As we enjoy great advantages from the invention of others, we should be
glad of an opportunity to serve others by any invention of ours, and
this we should do freely and generously."
Benjamin Franklin, on declining patents offered by the governor of
Pennsylvania for his "Pennsylvania Fireplace", c. 1744
