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Re: Task priorities in non strictly real-time systems

From Hans-Bernhard Bröker <HBBroeker@t-online.de>
Newsgroups comp.arch.embedded
Subject Re: Task priorities in non strictly real-time systems
Date 2020-01-06 18:01 +0100
Message-ID <h7h7fdFr0f8U1@mid.dfncis.de> (permalink)
References <qung9m$m79$1@dont-email.me> <qunihq$450$1@dont-email.me> <quradg$h90$1@dont-email.me> <qutd6p$om0$3@dont-email.me> <quttqq$vhf$1@dont-email.me>

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Am 06.01.2020 um 01:08 schrieb Don Y:

> Preemptive frameworks can be implemented in a variety of ways.
> It need NOT mean that the processor can be pulled out from under
> your feet at any "random" time.

I think I have to disagree with that.

Let's start with linguistics.  To "preempt" means: to get in ahead of
someone (or something) else.

In the RT world, that something is a running task, and preemption
happens when some other task gets to use the CPU before that task is
done with it.  I.e. a preemptive framework, pretty much by definition of
the words, does pull the CPU out from under you, and that _will_ happen
at, essentially, random times.

If the running task can routinely (as opposed to exceptionally) avoid
that happening to itself, that means it's essentially the other type of
framework: a cooperative one.

In other words: for the distinction of preemptive vs. cooperative
multitasking to make any sense at all, it has to be about who routinely
gets to decide whether task switching is allowed at any given time.  If
the decision normally is with individual tasks, that's cooperative.  If
it's centralized in the OS, that's preemptive.

Now we in c.a.embedded don't usually get to make that distinction quite
so cleanly of course, because for us, ISRs are a fact of life, and those
are always preemptive.  So purely cooperative multitasking is out.  We
only get to decide between largely cooperative systems with some
preemptive task disturbing the "clean" picture, and almose purely
preemptive system setups, with all the extra cost in terms of data
exchange synchronization between tasks that entails.


> So, barring "events" signalled by an ISR, you can conceivably
> execute code inside a single task for DAYS and never lose control of
> the processor.

And still, when eventually you do lose it, the time when that happens
is, for all practical intents and purposes, random.  Because otherwise
you wouldn't "lose" control, you'ld be "giving it up".   In other words,
in such a case the used RTOS framework may be preemptive, but you're not
using it that way.

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Task priorities in non strictly real-time systems pozz <pozzugno@gmail.com> - 2020-01-03 14:41 +0100
  Re: Task priorities in non strictly real-time systems David Brown <david.brown@hesbynett.no> - 2020-01-03 15:19 +0100
    Re: Task priorities in non strictly real-time systems pozz <pozzugno@gmail.com> - 2020-01-05 01:25 +0100
      Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-05 13:32 -0600
        Re: Task priorities in non strictly real-time systems Richard Damon <Richard@Damon-Family.org> - 2020-01-05 16:49 -0500
          Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-05 21:05 -0600
        Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-05 17:08 -0700
          Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-05 21:26 -0600
            Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-05 22:10 -0700
              Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-06 07:37 -0600
                Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-06 17:21 -0700
                Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-07 19:23 -0600
              Re: Task priorities in non strictly real-time systems Richard Damon <Richard@Damon-Family.org> - 2020-01-06 20:36 -0500
                Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-07 01:38 -0600
                Re: Task priorities in non strictly real-time systems Richard Damon <Richard@Damon-Family.org> - 2020-01-07 06:53 -0500
            Re: Task priorities in non strictly real-time systems George Neuner <gneuner2@comcast.net> - 2020-01-06 07:13 -0500
            Re: Task priorities in non strictly real-time systems David Brown <david.brown@hesbynett.no> - 2020-01-07 10:11 +0100
              Re: Task priorities in non strictly real-time systems Richard Damon <Richard@Damon-Family.org> - 2020-01-07 07:05 -0500
              Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-07 19:34 -0600
          Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-06 10:44 +0200
            Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-06 02:51 -0700
              Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-06 15:39 +0200
                Re: Task priorities in non strictly real-time systems George Neuner <gneuner2@comcast.net> - 2020-01-06 13:29 -0500
                Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-06 14:38 -0700
                Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-07 07:05 +0200
                Re: Task priorities in non strictly real-time systems George Neuner <gneuner2@comcast.net> - 2020-01-07 13:09 -0500
                Re: Task priorities in non strictly real-time systems Dimiter_Popoff <dp@tgi-sci.com> - 2020-01-07 20:53 +0200
                Re: Task priorities in non strictly real-time systems Clifford Heath <no.spam@please.net> - 2020-01-08 08:43 +1100
                Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-08 00:33 +0200
                Re: Task priorities in non strictly real-time systems Dimiter_Popoff <dp@tgi-sci.com> - 2020-01-08 15:26 +0200
                Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-08 00:08 +0200
                Re: Task priorities in non strictly real-time systems David Brown <david.brown@hesbynett.no> - 2020-01-07 10:37 +0100
                Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-06 16:10 -0700
                Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-07 07:37 +0200
                Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-07 01:49 -0600
                Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-07 06:43 -0700
                Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-07 10:21 +0200
                Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-07 06:42 -0700
                Re: Task priorities in non strictly real-time systems Richard Damon <Richard@Damon-Family.org> - 2020-01-07 22:23 -0500
                Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-09 22:07 +0200
                Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-07 10:54 +0200
                Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-07 06:43 -0700
          Re: Task priorities in non strictly real-time systems Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-06 18:01 +0100
            Re: Task priorities in non strictly real-time systems Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-06 19:47 +0200
    Re: Task priorities in non strictly real-time systems pozz <pozzugno@gmail.com> - 2020-01-05 01:58 +0100
      Re: Task priorities in non strictly real-time systems Clifford Heath <no.spam@please.net> - 2020-01-05 14:26 +1100
        Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-05 11:21 +0200
          Re: Task priorities in non strictly real-time systems David Brown <david.brown@hesbynett.no> - 2020-01-05 14:07 +0100
            Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-05 17:46 +0200
              Re: Task priorities in non strictly real-time systems David Brown <david.brown@hesbynett.no> - 2020-01-05 19:08 +0100
        Re: Task priorities in non strictly real-time systems David Brown <david.brown@hesbynett.no> - 2020-01-05 13:56 +0100
        Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-05 13:39 -0600
          Re: Task priorities in non strictly real-time systems Clifford Heath <no.spam@please.net> - 2020-01-06 07:44 +1100
            Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-05 21:00 -0600
              Re: Task priorities in non strictly real-time systems Clifford Heath <no.spam@please.net> - 2020-01-06 14:50 +1100
                Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-06 07:12 -0600
      Re: Task priorities in non strictly real-time systems Paul Rubin <no.email@nospam.invalid> - 2020-01-04 22:34 -0800
        Re: Task priorities in non strictly real-time systems David Brown <david.brown@hesbynett.no> - 2020-01-05 14:12 +0100
          Re: Task priorities in non strictly real-time systems Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-05 18:00 +0200
        Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-05 13:44 -0600
      Re: Task priorities in non strictly real-time systems George Neuner <gneuner2@comcast.net> - 2020-01-06 14:52 -0500
      Re: Task priorities in non strictly real-time systems George Neuner <gneuner2@comcast.net> - 2020-01-06 15:05 -0500
        Re: Task priorities in non strictly real-time systems gtwrek@sonic.net (gtwrek) - 2020-01-06 22:41 +0000
        Re: Task priorities in non strictly real-time systems Don Y <blockedofcourse@foo.invalid> - 2020-01-06 19:50 -0700
          Re: Task priorities in non strictly real-time systems George Neuner <gneuner2@comcast.net> - 2020-01-07 13:26 -0500
  Re: Task priorities in non strictly real-time systems Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-03 16:28 +0200
    Re: Task priorities in non strictly real-time systems pozz <pozzugno@gmail.com> - 2020-01-05 01:37 +0100
      Re: Task priorities in non strictly real-time systems Les Cargill <lcargill99@comcast.com> - 2020-01-05 13:46 -0600
        Re: Task priorities in non strictly real-time systems Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-05 22:44 +0200
  Re: Task priorities in non strictly real-time systems Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-03 11:49 -0800
  Re: Task priorities in non strictly real-time systems upsidedown@downunder.com - 2020-01-03 22:51 +0200
    Re: Task priorities in non strictly real-time systems pozz <pozzugno@gmail.com> - 2020-01-05 01:48 +0100
  Re: Task priorities in non strictly real-time systems Joe Chisolm <jchisolm6@earthlink.net> - 2020-01-03 15:34 -0600
  Re: Task priorities in non strictly real-time systems Richard Damon <Richard@Damon-Family.org> - 2020-01-03 21:46 -0500

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