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Can any of this discussion be used as material to enhance<br>
the section in the manual? This stuff is very hard to explain.<br>
<br>
Tom .. any suggestions? Just emailing us a new version of<br>
the section would help. It is just a few paragraphs.<br>
<br>
On 09/05/2012 04:37 AM, Tom Smith wrote:
<blockquote
cite="mid:CAPzmA6cGCmCm6U2GqFhm5eECD3ePsUzt9Odazb=3tDLzEbFLjA@mail.gmail.com"
type="cite">
<div><font color="#888888">hi Gedare</font><br>
</div>
<div>Thank you very much for the quick reply.</div>
<div> </div>
<div>The relationship between "processor utilization rule" and
"First Deadline Rule" you stated is clear and wonderfull.</div>
<div> </div>
<div>I think I got your key point. But I still need to read more
to fully understand RMS. </div>
<div> </div>
<div>Anyhow, thank you very much</div>
<div> </div>
<div>tom</div>
<div> </div>
<div>2012/9/3 Gedare Bloom <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:gedare@rtems.org"
target="_blank">gedare@rtems.org</a>></span><br>
</div>
<div class="gmail_quote">
<blockquote style="BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px
0.8ex;PADDING-LEFT:1ex" class="gmail_quote">
<div class="im">On Mon, Sep 3, 2012 at 8:27 AM, Tom Smith <<a
moz-do-not-send="true" href="mailto:venture.g@gmail.com">venture.g@gmail.com</a>>
wrote:<br>
> Hi everyone,<br>
><br>
> I am studying Chapter 19 "Rate Monotonic Manager" of
c_user.pdf, and I get<br>
> some questions<br>
><br>
> In c_user.pdf, it says<br>
><br>
> "For a given set of independent periodic tasks, if each
task meets its first<br>
> deadline when all<br>
> tasks are started at the same time, then the deadlines
will always be met<br>
> for any combination<br>
> of start times."<br>
><br>
> my question is that:<br>
> 1. If a set of independent periodic tasks do not meet
the "Processor<br>
> Utilization Rule", but they satisfy the "First
Deadline Rule", can they be<br>
> scheduled using RMS ?<br>
><br>
</div>
Yes; you may like to consult a handbook on real-time systems
for<br>
detailed explanations, but I'll make an effort here. The
processor<br>
utilization rule is a sufficient but not necessary test for<br>
schedulability; some systems are schedulable that do not
satisfy the<br>
maximum processor utilization bounds. The "First Deadline
Rule" is a<br>
way to simplify the analysis of when tasks may start by
stating the<br>
worst-case scheduling window happens when all tasks start at
the same<br>
time; releasing all tasks at once ensures that every task has
a<br>
critical instant at the same time; this rule coincides with
the<br>
critical instance theorem. When you don't have simultaneous
release of<br>
tasks you cannot be certain when the critical instance of a
given task<br>
will occur, in which case you may need to compute the entire<br>
hyperperiod; for non-harmonic task sets the hyperperiod could
be<br>
prohibitively large. (Harmonic periodic tasks are typically
quite easy<br>
to test for schedulability.)<br>
<div class="im"><br>
> 2. Is it necessary to start all tasks at the same time
in a real<br>
> application, or is it just a trick during analysis
phase?<br>
><br>
</div>
I suppose that depends how closely you want your analysis to
match<br>
your application. If you analyze with simultaneous release but
do not<br>
ensure that during execution then what have you analyzed? You
cannot<br>
be certain your application will meet its deadlines anymore.
In this<br>
case you should use a schedulability test that does not
require<br>
simultaneous release.<br>
<span class="HOEnZb"><font color="#888888"><br>
-Gedare<br>
</font></span>
<div class="im HOEnZb"><br>
> any review on this point from anyone is welcomed. So
feel free to comment.<br>
> Best regards,<br>
><br>
> Tom Smith<br>
><br>
</div>
<div class="HOEnZb">
<div class="h5">>
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</div>
</div>
</blockquote>
</div>
<br>
</blockquote>
<br>
<br>
<pre class="moz-signature" cols="72">--
Joel Sherrill, Ph.D. Director of Research& Development
<a class="moz-txt-link-abbreviated" href="mailto:joel.sherrill@OARcorp.com">joel.sherrill@OARcorp.com</a> On-Line Applications Research
Ask me about RTEMS: a free RTOS Huntsville AL 35805
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