Re: Advice, tasking and hardware

[rieachus@comcast.net]

On Wednesday, June 8, 2016 at 3:30:07 AM UTC-4, Dmitry A. Kazakov wrote:
  
> That is the visible effect, the point is if restarting a task heals a 
> symptom or the illness.
> 
> P.S. It would be nice if Ada had, as well, a non-cooperative model of 
> private, protected and task. That is, private parts inacessible without 
> memory mapping only to occur through publicly visible calls. Such tasks 
> and objects could be resurrected without partition restart. They would 
> also be great for distributed systems done high-level instead of 
> low-level Annex E. Unfortunately there is no demand...

Um.  Hard real-time systems are hard, and it is harder still to recover from (transient) hardware errors.  However, there are tools, and SPARK helps a lot.  In the systems I worked on, each priority level corresponded to a thread, and was fired off every 20 ms.  There was a time requirement on restarting any task from scratch that basically resulted in a (small) collection of state variables, and the large swaths of state data like radar tracks would be discarded by hardware transient recovery. 

Oh, all physical memory allocations had to be done before the system was started.  You could, and often did have a per task memory pool, and restart just changed the allocated amount to zero.  Of course, any package which contained state had to be task specific.  The tasks that updated the HUD was relatively low priority, and some cockpit displays lower still. So telling the pilot to try cycling the power to the computer system wasn't a solution.  Hmm. I remember some older systems A6A DIANE? Where the watchdog timer was connected to a cockpit light.  That system though, did nav, radar and weapons, but was not involved in flight control.