[RTEMS Project] #4885: Feature Request: synchronous job release for periodic tasks

[RTEMS trac]

#4885: Feature Request: synchronous job release for periodic tasks
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  Reporter:  Junjie Shi  |      Owner:  (none)
      Type:              |     Status:  new
  enhancement            |
  Priority:  normal      |  Milestone:  4.11.5
 Component:  admin       |    Version:  6
  Severity:  normal      |   Keywords:  synchronous job release, periodic
                         |  tasks
Blocked By:              |   Blocking:
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 Currently, executing tasks in a periodic manner is supported by RTEMS
 through the usage of the rate monotonic manager. All further usage of the
 term periodicity complies with the definition in the RTEMS documentation
 chapter 11.2.3.

 We’ve taken a look into the usage of periodic tasks with RTEMS and
 noticed, that mainly two functionalities are needed to realize periodic
 tasks for ones application:

 1. rtems_rate_monotonic_create(…)
 2. rtems_rate_monotonic_period(…)
    create(…) is used to create the control block for the period.
    period(…) actually starts and maintains the created period. This
    means the time of execution of a tasks first job and its
    following jobs is dependent on the first function call of
    period(…) and the specified period. The function call usually
    happens from within a tasks body (as it can be seen in the
    example from the RTEMS documentation chapter 11.3.7)

 This implementation approach leads to the problem, that without additional
 action it is not possible to release multiple tasks synchronously. A
 synchronous release means that the first job of each intended task becomes
 available/arrives at the same time. This problem is caused by the the
 fact, that the first job of a higher priority task T1 delays the
 initialization of the period through calling period(…) in the first job of
 a lower priority task T2.

 Nevertheless, in the research/teaching field as well as in practical
 applications, a synchronous release is often desired/used. Even though
 this problem could be avoided through a smart workaround on the level of
 application, we suggest that this feature of synchronous release should be
 implemented as part of the RTEMS rate monotonic manager. We propose, that
 the API should be extended such that a functionality becomes available,
 which allows the user to mark tasks for synchronous release while creating
 them. The first usage of period(…) to initialize the period would become
 redundant for these special cases, but would still work the same for all
 non-synchronized tasks.

 One illustrative example has been attached, which shows that jobs are not
 released synchronously even if the release opertions are issued as early
 as possible. More details can be found in the comments in the attached
 source code.

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Ticket URL: <http://devel.rtems.org/ticket/4885>
RTEMS Project <http://www.rtems.org/>
RTEMS Project