Re: Hard Real-Time?

[Johnny Billquist <bqt%update.uu.se@localhost>]

On 2020-04-15 22:14, Michael Cheponis wrote:
> Is there already a way to do "hard real time" on NetBSD?
>
> To me, "hard real time" means from an external pin going 'high' to the 
> 1st instruction of my driver executing is on the order of (up to) 10 usec.
>
> Many eons ago, I did this on BSD4.3 VAX 785 and achieved <  about 100 
> usec jitter.  It was a Royal Pain to do, since so many places in the 
> kernel turned off interrupts as it massaged various data structures.  
> The pain was to find every one of those and break them down to 
> absolutely minimize the time interrupts were turned off.
>
> But it worked:  Here it is controlling various hopping machines: 
> https://www.youtube.com/watch?v=mG_ZKXo6Rlg&feature=youtu.be&t=34 ; I'm 
> sitting controlling the thing; the umbilical goes back to a UNIBUS 
> interface in the VAX.  Timesharing only slowed down; it did not stop 
> during operation.
>
> I'm especially interested in getting the RPi to operate 'hard real time' 
> on NetBSD.  I mainly want to use the RPi to control a robot -- low  
> level as well has higher-level planning/control.

I wish that people would stop coming up with their own definitions for 
well established terms.

Hard real time is a well established term, and it don't mean what you 
think. It's not about any absolute number at all, but about the 
predictability, or in another way, the consequence of missing a deadline 
(see 
https://en.wikipedia.org/wiki/Real-time_computing#Criteria_for_real-time_computing). 
Essentially, a hard real time system guarantees that it never takes 
longer than some specified time for something to happen. And as far as I 
know, NetBSD cannot give such guarantees. Various combinations of events 
and situations can lock out interrupts for some time, and there is no 
guaranteed upper bound to that time.

That said, I am sure that if you just want it to usually work, then I'm 
pretty sure NetBSD can do it well enough.

  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
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