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Re: From cooperative to preemptive scheduler [Stack]

From pozz <pozzugno@gmail.com>
Newsgroups comp.arch.embedded
Subject Re: From cooperative to preemptive scheduler [Stack]
Date 2020-01-15 09:36 +0100
Organization A noiseless patient Spider
Message-ID <qvmitp$c8o$1@dont-email.me> (permalink)
References <qv0lmu$up1$1@dont-email.me> <qvkc2v$3p8$1@dont-email.me> <h86i1qFaiccU1@mid.dfncis.de>

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Il 14/01/2020 20:10, Hans-Bernhard Bröker ha scritto:
> Am 14.01.2020 um 13:27 schrieb pozz:
>> Another trouble I found with a preemptive RTOS is how to size the stack
>> of tasks.
> 
> That trouble is not at all particular to preemptive scheduling.
> 
>> In my superloop cooperative scheduler:
> 
>> the stack is assigned to all the unused memory available in the system
>> (that is all the memory minus variables and heap size, if you use it).
> 
> And how do you know that that's sufficient?

I'm not too smart to estimate the stack usage of a function (I know the 
compiler can produce some useful info about this, but you should add 
interrupt stack usage and so on), so my approach is only tests.

I fill the memory with a known value, run the application for many 
hours/days and eventually check the memory content to see the greatest 
(or lowest) address reached by the stack.

This isn't a simple job, but I do it one time, because there's a single 
global stack in superloop architecture.


>> Another issue with preemptive approach is that you should be smart
>> enough to size N stacks (where N is the number of tasks).
>> With the superloop architecture above, you should size only *one* global
>> stack, that can be calculated over one task, the one that needs more stack.
> 
> If you can't calculate the stack for those small-ish tasks, then you
> can't do it for the functions called by your super-loop, either.
> 
> If you can judge which of the super-loop's sub-tasks consumes the most
> stack, and how much that is, then you can do it for preemptive
> scheduling, too.
> 
> The difficulty of (reliably) computing stack usage is the same,
> regardless what tasking concept is used.

I don't agree.
Most of the time I can guess what is the task (or a few tasks) that 
consumes more stack. So the effort to estimate stack usage is limited.

In preemptive scheduler, you would need to multiplicate your effort to 
estimate stack usage for every single task to avoid wasting precious memory.


Anyway, suppose we are very smart to calculate stack usage of each task:
- task1 needs 10kB
- task2 needs 10kB

With preemptive scheduler you need to reserve 20kB for the stack, in 
superloop you can reserve only 10kB.

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From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-07 02:08 +0100
  Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-06 19:37 -0700
    Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-07 10:11 +0100
      Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-07 07:51 -0700
        Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-07 23:51 +0100
          Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-09 01:39 +0200
            Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-10 00:42 +0100
              Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-15 18:39 +0200
          Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-08 23:12 -0700
            Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-10 00:49 +0100
          Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-08 23:12 -0700
          Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-09 01:33 -0700
  Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-07 09:38 +0200
    Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-08 00:02 +0100
      Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-09 00:54 +0200
        Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-09 12:19 +0100
          Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-09 14:03 +0200
            Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-10 00:32 +0100
              Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-15 18:53 +0200
                Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-16 14:48 +0100
                Re: From cooperative to preemptive scheduler: a real example upsidedown@downunder.com - 2020-01-16 21:04 +0200
                Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-16 22:24 +0200
                Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-17 02:05 +0100
                Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-18 23:03 +0200
  Re: From cooperative to preemptive scheduler [Stack] pozz <pozzugno@gmail.com> - 2020-01-14 13:27 +0100
    Re: From cooperative to preemptive scheduler [Stack] Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-14 20:10 +0100
      Re: From cooperative to preemptive scheduler [Stack] pozz <pozzugno@gmail.com> - 2020-01-15 09:36 +0100
        Re: From cooperative to preemptive scheduler [Stack] Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2020-01-15 11:30 +0100
          Re: From cooperative to preemptive scheduler [Stack] pozz <pozzugno@gmail.com> - 2020-01-15 11:51 +0100
        Re: From cooperative to preemptive scheduler [Stack] Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-15 21:51 +0100
    Re: From cooperative to preemptive scheduler [Stack] Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-15 21:26 +0200
      Re: From cooperative to preemptive scheduler [Stack] Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-15 22:12 +0100
        Re: From cooperative to preemptive scheduler [Stack] Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-15 23:30 +0200

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