Questions about the implementation of RMS on rtems

Gedare Bloom gedare at
Wed Mar 25 14:11:23 UTC 2015

You might like to try reading a text book on real-time systems, such
as Jane Liu's book. If you have accurate WCET measurements then RMS
works by assigning periods to tasks, computing U < 0.69 (or whatever),
and assigning priorites inversely to periods. Then you have to ensure
that the periods are enforced. RTEMS has mechanisms for creating the
periodic timers and to return a status code in case of an overrun. It
is up to the application to detect an overrun and account for it in
this case.


On Wed, Mar 25, 2015 at 8:00 AM, yuanbin zhou <hduffddybz at> wrote:
> Sorry for late reply because of paper reading, taking courses and
> unimportant things.
>> >2. How to use the processor utilization rule and first deadline rule to
>> >guarantee as more as possible tasks to run since the execution time of
>> >each tasks cannot accurately know before the task run?
>> This is in the users manual and in the original 1973 JACM paper
>> referenced.
>> It also includes the assumptions about the underlying OS and environment.
> Recently I read the paper written by C.L.LIU(Scheduling Algorithms for
> Multiprogramming in a Hard-Real-Time Environment), this paper talks about
> the algorithm of RM Scheduling and the proof of it.But now what I am more
> concerned about is how to  apply this rule to the analysis of real-time
> system.Maybe the analysis of the schedulability of this this system should
> be done by off-line tools before system start up.But the analysis of
> worse-case execution time may not correct which may result in execution
> overrun,so some papers put forward some resource reservation mechanisms to
> solve this problem.So confused about the paper reading, so I want to know
> how to handle this situation in "rtems"since that I cannot figure out any
> information from the user manual.
>> >
>> >3. Obey the rate monotonic scheduling algorithm, we can easily set the
>> >priority of every periodic tasks. But in many applications, it consists
>> >both periodic and aperiodic tasks(such as irregular interrupt which is
>> >also essential task), so how to handle the priority setting for this
>> >situation or just set the irregular task's priority by manual?
>> You need to do some academic paper research. There are a number of papers
>> describing how to address various task patterns
> I read some papers about the handle of periodic tasks and aperiodic
> tasks.There existed two kinds of method to solve this problem.
> First:the system can schedule the aperiodic tasks by periodic server which
> allocates a certain amount of budget for aperiodic execution in every period
> time.
>  second:there is no benefit for the periodic tasks to complete their tasks
> much before their deadlines. So when the aperiodic task coming, it steals
> the available slack from the periodic tasks and execute it as soon as
> possible.
> So can give more detail about the implementation of this in rtems which I
> cannot find anything from the user manual and it is really complicated to
> find the corresponding source file for all the source files located in only
> one directory!
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