Dynamic scheduling

[Joel Sherrill]

Manuel Coutinho wrote:
> RTEMS has fixed-priority scheduling. RTEMS does not support dynamic
> scheduling.
>
> Actually, the rate monotonic that is referred only makes tasks to be
> periodic. It does not implement the Rate Monotonic Scheduler. The
> application must correctly assign the tasks priorities in order for the Rate
> Monotonic Scheduler to work.
>   
http://en.wikipedia.org/wiki/Rate-monotonic_scheduling is a pretty
good article.  I have never seen any claim that the RMS priority
assignment rule must be done by the operating system.  In the original
Lui/Layland paper which defined RMS and two analysis algorithms,
the only requirement laid on the OS was that it support priority
based scheduling with preemption.  As defined there, RMS and RMA
are fundamentally offline algorithms to place guarantees on
task set schedulability if you follow the priority assignment rule
and perform analysis.

With that said, I know there are software schedulers out there which
use the period information to schedule tasks.  This is not a requirement.
> Note that is good that RTEMS does this because other scheduler variants can
> be easily implemented (e.g Deadline Monotonic). Also, there are system
> models, for example, non-preemptive or in which the task releases are well
> defined, in which the Rate Monotonic Scheduler is NOT the optimal scheduler.
>
> Hope this helps
> Regards
> Manuel Coutinho
>
>   
>> -----Original Message-----
>> From: rtems-users-bounces at rtems.org [mailto:rtems-users-bounces at rtems.org]
>> On Behalf Of Nasim Zehra
>> Sent: Friday, July 17, 2009 7:58 AM
>> To: Thomas Doerfler
>> Cc: rtems-users at rtems.org
>> Subject: RE: Dynamic scheduling
>>
>>  like EDF, Minimum Laxity first.Actually i m having observation that rtems
>> has only rate monotonic .static scheduling scheme, manager.May be it does
>> not support dynamic one?I guess timer or clock directives are helpful in
>> this regard, but i am having problem using timer directive..that what
>> should be written in place of *user_data when timer is fired.Moreover
>> whether should we create timer directive in main "init" function or should
>> we create and delete then like rate monotonic periods inside the task
>> ,mentioned in the example of rate monotonic manager.
>>
>> nasim
>> ________________________________________
>> From: Thomas Doerfler [Thomas.Doerfler at embedded-brains.de]
>> Sent: Friday, July 17, 2009 12:49 PM
>> To: Nasim Zehra
>> Cc: rtems-users at rtems.org
>> Subject: Re: Dynamic scheduling
>>
>> Hi,
>>
>> what exactly do you call "dynamic scheduling"? The term seems a bit
>> "open" to me, but maybe I am simpy ignorant :-)
>>
>> wkr,
>> Thomas.
>>
>> Nasim Zehra wrote:
>>     
>>> Does rtems support dynamic scheduling??if yes what managers are useful
>>>       
>> for this purpose?
>>     
>>> nasim
>>> _______________________________________________
>>> rtems-users mailing list
>>> rtems-users at rtems.org
>>> http://www.rtems.org/mailman/listinfo/rtems-users
>>>       
>> --
>>
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>
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-- 
Joel Sherrill, Ph.D.             Director of Research & Development
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