Some confusion about the "First Deadline Rule"

[Tom Smith]

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>

> On Mon, Sep 3, 2012 at 8:27 AM, Tom Smith <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
> >
>  > _______________________________________________
> > rtems-users mailing list
> > rtems-users at rtems.org
> > http://www.rtems.org/mailman/listinfo/rtems-users
> >
>
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