Some confusion about the "First Deadline Rule"

[Joel Sherrill]

Can any of this discussion be used as material to enhance
the section in the manual? This stuff is very hard to explain.

Tom .. any suggestions? Just emailing us a new version of
the section would help. It is just a few paragraphs.

On 09/05/2012 04:37 AM, Tom Smith wrote:
> hi Gedare
> Thank you very much for the quick reply.
> The relationship between "processor utilization rule" and "First 
> Deadline Rule"  you stated is clear and wonderfull.
> I think I got your key point. But I still need to read more to fully 
> understand RMS.
> Anyhow, thank you very much
> tom
> 2012/9/3 Gedare Bloom <gedare at rtems.org <mailto:gedare at rtems.org>>
>
>     On Mon, Sep 3, 2012 at 8:27 AM, Tom Smith <venture.g at gmail.com
>     <mailto:venture.g at gmail.com>> wrote:
>     > Hi everyone,
>     >
>     > I am studying Chapter 19 "Rate Monotonic Manager"  of
>     c_user.pdf, and I get
>     > some questions
>     >
>     > In c_user.pdf, it says
>     >
>     > "For a given set of independent periodic tasks, if each task
>     meets its first
>     > deadline when all
>     > tasks are started at the same time, then the deadlines will
>     always be met
>     > for any combination
>     > of start times."
>     >
>     > my question is that:
>     > 1. If a set of  independent periodic tasks do not meet the
>     "Processor
>     > Utilization Rule",  but they satisfy the "First Deadline Rule",
>      can they be
>     > scheduled  using RMS ?
>     >
>     Yes; you may like to consult a handbook on real-time systems for
>     detailed explanations, but I'll make an effort here. The processor
>     utilization rule is a sufficient but not necessary test for
>     schedulability; some systems are schedulable that do not satisfy the
>     maximum processor utilization bounds. The "First Deadline Rule" is a
>     way to simplify the analysis of when tasks may start by stating the
>     worst-case scheduling window happens when all tasks start at the same
>     time; releasing all tasks at once ensures that every task has a
>     critical instant at the same time; this rule coincides with the
>     critical instance theorem. When you don't have simultaneous release of
>     tasks you cannot be certain when the critical instance of a given task
>     will occur, in which case you may need to compute the entire
>     hyperperiod; for non-harmonic task sets the hyperperiod could be
>     prohibitively large. (Harmonic periodic tasks are typically quite easy
>     to test for schedulability.)
>
>     > 2. Is it necessary to start all tasks at the same time in a real
>     > application, or  is it just a trick during analysis phase?
>     >
>     I suppose that depends how closely you want your analysis to match
>     your application. If you analyze with simultaneous release but do not
>     ensure that during execution then what have you analyzed? You cannot
>     be certain your application will meet its deadlines anymore. In this
>     case you should use a schedulability test that does not require
>     simultaneous release.
>
>     -Gedare
>
>     > any review on this point from anyone is welcomed. So feel  free
>     to  comment.
>     > Best regards,
>     >
>     > Tom Smith
>     >
>     > _______________________________________________
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>     > rtems-users at rtems.org <mailto:rtems-users at rtems.org>
>     > http://www.rtems.org/mailman/listinfo/rtems-users
>     >
>
>


-- 
Joel Sherrill, Ph.D.             Director of Research&   Development
joel.sherrill at OARcorp.com        On-Line Applications Research
Ask me about RTEMS: a free RTOS  Huntsville AL 35805
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