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 There's a bunch of information on that on my site (albeit outdated).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. Lee
