source: rtems-docs/c_user/rate_monotonic_manager.rst @ d389819

Rate Monotonic Manager
######################

.. index:: rate mononitonic tasks
.. index:: periodic tasks

Introduction
============

The rate monotonic manager provides facilities to implement tasks which execute
in a periodic fashion.  Critically, it also gathers information about the
execution of those periods and can provide important statistics to the
user which can be used to analyze and tune the application.  The directives
provided by the rate monotonic manager are:

- ``rtems_rate_monotonic_create`` - Create a rate monotonic period

- ``rtems_rate_monotonic_ident`` - Get ID of a period

- ``rtems_rate_monotonic_cancel`` - Cancel a period

- ``rtems_rate_monotonic_delete`` - Delete a rate monotonic period

- ``rtems_rate_monotonic_period`` - Conclude current/Start next period

- ``rtems_rate_monotonic_get_status`` - Obtain status from a period

- ``rtems_rate_monotonic_get_statistics`` - Obtain statistics from a period

- ``rtems_rate_monotonic_reset_statistics`` - Reset statistics for a period

- ``rtems_rate_monotonic_reset_all_statistics`` - Reset statistics for all periods

- ``rtems_rate_monotonic_report_statistics`` - Print period statistics report

Background
==========

The rate monotonic manager provides facilities to
manage the execution of periodic tasks.  This manager was
designed to support application designers who utilize the Rate
Monotonic Scheduling Algorithm (RMS) to ensure that their
periodic tasks will meet their deadlines, even under transient
overload conditions.  Although designed for hard real-time
systems, the services provided by the rate monotonic manager may
be used by any application which requires periodic tasks.

Rate Monotonic Manager Required Support
---------------------------------------

A clock tick is required to support the functionality provided by this manager.

Period Statistics
-----------------

This manager maintains a set of statistics on each period object.  These
statistics are reset implictly at period creation time and may be reset or
obtained at any time by the application.  The following is a list of the
information kept:

- ``owner``
  is the id of the thread that owns this period.

- ``count``
  is the total number of periods executed.

- ``missed_count``
  is the number of periods that were missed.

- ``min_cpu_time``
  is the minimum amount of CPU execution time consumed
  on any execution of the periodic loop.

- ``max_cpu_time``
  is the maximum amount of CPU execution time consumed
  on any execution of the periodic loop.

- ``total_cpu_time``
  is the total amount of CPU execution time consumed
  by executions of the periodic loop.

- ``min_wall_time``
  is the minimum amount of wall time that passed
  on any execution of the periodic loop.

- ``max_wall_time``
  is the maximum amount of wall time that passed
  on any execution of the periodic loop.

- ``total_wall_time``
  is the total amount of wall time that passed
  during executions of the periodic loop.

Each period is divided into two consecutive phases.  The period starts with the
active phase of the task and is followed by the inactive phase of the task.  In
the inactive phase the task is blocked and waits for the start of the next
period.  The inactive phase is skipped in case of a period miss.  The wall time
includes the time during the active phase of the task on which the task is not
executing on a processor.  The task is either blocked (for example it waits for
a resource) or a higher priority tasks executes, thus preventing it from
executing.  In case the wall time exceeds the period time, then this is a
period miss.  The gap between the wall time and the period time is the margin
between a period miss or success.

The period statistics information is inexpensive to maintain
and can provide very useful insights into the execution
characteristics of a periodic task loop.  But it is just information.
The period statistics reported must be analyzed by the user in terms
of what the applications is.  For example, in an application where
priorities are assigned by the Rate Monotonic Algorithm, it would
be very undesirable for high priority (i.e. frequency) tasks to
miss their period.  Similarly, in nearly any application, if a
task were supposed to execute its periodic loop every 10 milliseconds
and it averaged 11 milliseconds, then application requirements
are not being met.

The information reported can be used to determine the "hot spots"
in the application.  Given a period's id, the user can determine
the length of that period.  From that information and the CPU usage,
the user can calculate the percentage of CPU time consumed by that
periodic task.  For example, a task executing for 20 milliseconds
every 200 milliseconds is consuming 10 percent of the processor's
execution time.  This is usually enough to make it a good candidate
for optimization.

However, execution time alone is not enough to gauge the value of
optimizing a particular task.  It is more important to optimize
a task executing 2 millisecond every 10 milliseconds (20 percent
of the CPU) than one executing 10 milliseconds every 100 (10 percent
of the CPU).  As a general rule of thumb, the higher frequency at
which a task executes, the more important it is to optimize that
task.

Rate Monotonic Manager Definitions
----------------------------------
.. index:: periodic task, definition

A periodic task is one which must be executed at a
regular interval.  The interval between successive iterations of
the task is referred to as its period.  Periodic tasks can be
characterized by the length of their period and execution time.
The period and execution time of a task can be used to determine
the processor utilization for that task.  Processor utilization
is the percentage of processor time used and can be calculated
on a per-task or system-wide basis.  Typically, the task's
worst-case execution time will be less than its period.  For
example, a periodic task's requirements may state that it should
execute for 10 milliseconds every 100 milliseconds.  Although
the execution time may be the average, worst, or best case, the
worst-case execution time is more appropriate for use when
analyzing system behavior under transient overload conditions... index:: aperiodic task, definition

In contrast, an aperiodic task executes at irregular
intervals and has only a soft deadline.  In other words, the
deadlines for aperiodic tasks are not rigid, but adequate
response times are desirable.  For example, an aperiodic task
may process user input from a terminal... index:: sporadic task, definition

Finally, a sporadic task is an aperiodic task with a
hard deadline and minimum interarrival time.  The minimum
interarrival time is the minimum period of time which exists
between successive iterations of the task.  For example, a
sporadic task could be used to process the pressing of a fire
button on a joystick.  The mechanical action of the fire button
ensures a minimum time period between successive activations,
but the missile must be launched by a hard deadline.

Rate Monotonic Scheduling Algorithm
-----------------------------------
.. index:: Rate Monotonic Scheduling Algorithm, definition
.. index:: RMS Algorithm, definition

The Rate Monotonic Scheduling Algorithm (RMS) is
important to real-time systems designers because it allows one
to guarantee that a set of tasks is schedulable.  A set of tasks
is said to be schedulable if all of the tasks can meet their
deadlines.  RMS provides a set of rules which can be used to
perform a guaranteed schedulability analysis for a task set.
This analysis determines whether a task set is schedulable under
worst-case conditions and emphasizes the predictability of the
system's behavior.  It has been proven that:

- *RMS is an optimal static priority algorithm for
  scheduling independent, preemptible, periodic tasks
  on a single processor.*

RMS is optimal in the sense that if a set of tasks
can be scheduled by any static priority algorithm, then RMS will
be able to schedule that task set.  RMS bases it schedulability
analysis on the processor utilization level below which all
deadlines can be met.

RMS calls for the static assignment of task
priorities based upon their period.  The shorter a task's
period, the higher its priority.  For example, a task with a 1
millisecond period has higher priority than a task with a 100
millisecond period.  If two tasks have the same period, then RMS
does not distinguish between the tasks.  However, RTEMS
specifies that when given tasks of equal priority, the task
which has been ready longest will execute first.  RMS's priority
assignment scheme does not provide one with exact numeric values
for task priorities.  For example, consider the following task
set and priority assignments:

+--------------------+---------------------+---------------------+
| Task               | Period              | Priority            |
|                    | (in milliseconds)   |                     |
+====================+=====================+=====================+
|         1          |         100         |         Low         |
+--------------------+---------------------+---------------------+
|         2          |          50         |       Medium        |
+--------------------+---------------------+---------------------+
|         3          |          50         |       Medium        |
+--------------------+---------------------+---------------------+
|         4          |          25         |        High         |
+--------------------+---------------------+---------------------+

RMS only calls for task 1 to have the lowest
priority, task 4 to have the highest priority, and tasks 2 and 3
to have an equal priority between that of tasks 1 and 4.  The
actual RTEMS priorities assigned to the tasks must only adhere
to those guidelines.

Many applications have tasks with both hard and soft
deadlines.  The tasks with hard deadlines are typically referred
to as the critical task set, with the soft deadline tasks being
the non-critical task set.  The critical task set can be
scheduled using RMS, with the non-critical tasks not executing
under transient overload, by simply assigning priorities such
that the lowest priority critical task (i.e. longest period) has
a higher priority than the highest priority non-critical task.
Although RMS may be used to assign priorities to the
non-critical tasks, it is not necessary.  In this instance,
schedulability is only guaranteed for the critical task set.

Schedulability Analysis
-----------------------

.. index:: RMS schedulability analysis

RMS allows application designers to ensure that tasks
can meet all deadlines, even under transient overload, without
knowing exactly when any given task will execute by applying
proven schedulability analysis rules.

Assumptions
~~~~~~~~~~~

The schedulability analysis rules for RMS were
developed based on the following assumptions:

- The requests for all tasks for which hard deadlines
  exist are periodic, with a constant interval between requests.

- Each task must complete before the next request for it
  occurs.

- The tasks are independent in that a task does not depend
  on the initiation or completion of requests for other tasks.

- The execution time for each task without preemption or
  interruption is constant and does not vary.

- Any non-periodic tasks in the system are special.  These
  tasks displace periodic tasks while executing and do not have
  hard, critical deadlines.

Once the basic schedulability analysis is understood,
some of the above assumptions can be relaxed and the
side-effects accounted for.

Processor Utilization Rule
~~~~~~~~~~~~~~~~~~~~~~~~~~
.. index:: RMS Processor Utilization Rule

The Processor Utilization Rule requires that
processor utilization be calculated based upon the period and
execution time of each task.  The fraction of processor time
spent executing task index is Time(index) / Period(index).  The
processor utilization can be calculated as follows:
.. code:: c

    Utilization = 0
    for index = 1 to maximum_tasks
    Utilization = Utilization + (Time(index)/Period(index))

To ensure schedulability even under transient
overload, the processor utilization must adhere to the following
rule:
.. code:: c

    Utilization = maximum_tasks * (2**(1/maximum_tasks) - 1)

As the number of tasks increases, the above formula
approaches ln(2) for a worst-case utilization factor of
approximately 0.693.  Many tasks sets can be scheduled with a
greater utilization factor.  In fact, the average processor
utilization threshold for a randomly generated task set is
approximately 0.88.

Processor Utilization Rule Example
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This example illustrates the application of the
Processor Utilization Rule to an application with three critical
periodic tasks.  The following table details the RMS priority,
period, execution time, and processor utilization for each task:


+------------+----------+--------+-----------+-------------+
| Tas   k    | RMS      | Period | Execution | Processor   |
|            | Priority |        | Time      | Utilization |
+============+==========+========+===========+=============+
|     1      |   High   |  100   |    15     |    0.15     |
+------------+----------+--------+-----------+-------------+
|     2      |  Medium  |  200   |    50     |    0.25     |
+------------+----------+--------+-----------+-------------+
|     3      |   Low    |  300   |   100     |    0.33     |
+------------+----------+--------+-----------+-------------+

The total processor utilization for this task set is
0.73 which is below the upper bound of 3 * (2**(1/3) - 1), or
0.779, imposed by the Processor Utilization Rule.  Therefore,
this task set is guaranteed to be schedulable using RMS.

First Deadline Rule
~~~~~~~~~~~~~~~~~~~
.. index:: RMS First Deadline Rule

If a given set of tasks do exceed the processor
utilization upper limit imposed by the Processor Utilization
Rule, they can still be guaranteed to meet all their deadlines
by application of the First Deadline Rule.  This rule can be
stated as follows:

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.

A key point with this rule is that ALL periodic tasks
are assumed to start at the exact same instant in time.
Although this assumption may seem to be invalid,  RTEMS makes it
quite easy to ensure.  By having a non-preemptible user
initialization task, all application tasks, regardless of
priority, can be created and started before the initialization
deletes itself.  This technique ensures that all tasks begin to
compete for execution time at the same instant - when the user
initialization task deletes itself.

First Deadline Rule Example
~~~~~~~~~~~~~~~~~~~~~~~~~~~

The First Deadline Rule can ensure schedulability
even when the Processor Utilization Rule fails.  The example
below is a modification of the Processor Utilization Rule
example where task execution time has been increased from 15 to
25 units.  The following table details the RMS priority, period,
execution time, and processor utilization for each task:
.. code:: c

+------------+----------+--------+-----------+-------------+
| Task       | RMS      | Period | Execution | Processor   |
|            | Priority |        | Time      | Utilization |
+============+==========+========+===========+=============+
|     1      |   High   |  100   |    25     |    0.25     |
+------------+----------+--------+-----------+-------------+
|     2      |  Medium  |  200   |    50     |    0.25     |
+------------+----------+--------+-----------+-------------+
|     3      |   Low    |  300   |   100     |    0.33     |
+------------+----------+--------+-----------+-------------+

The total processor utilization for the modified task
set is 0.83 which is above the upper bound of 3 * (2**(1/3) - 1),
or 0.779, imposed by the Processor Utilization Rule.  Therefore,
this task set is not guaranteed to be schedulable using RMS.
However, the First Deadline Rule can guarantee the
schedulability of this task set.  This rule calls for one to
examine each occurrence of deadline until either all tasks have
met their deadline or one task failed to meet its first
deadline.  The following table details the time of each deadline
occurrence, the maximum number of times each task may have run,
the total execution time, and whether all the deadlines have
been met.
.. code:: c

+----------+------+------+------+----------------------+---------------+
| Deadline | Task | Task | Task | Total                | All Deadlines |
| Time     | 1    | 2    | 3    | Execution Time       | Met?          |
+==========+======+======+======+======================+===============+
|   100    |  1   |  1   |  1   |  25 + 50 + 100 = 175 |      NO       |
+----------+------+------+------+----------------------+---------------+
|   200    |  2   |  1   |  1   |  50 + 50 + 100 = 200 |     YES       |
+----------+------+------+------+----------------------+---------------+

The key to this analysis is to recognize when each
task will execute.  For example at time 100, task 1 must have
met its first deadline, but tasks 2 and 3 may also have begun
execution.  In this example, at time 100 tasks 1 and 2 have
completed execution and thus have met their first deadline.
Tasks 1 and 2 have used (25 + 50) = 75 time units, leaving (100
- 75) = 25 time units for task 3 to begin.  Because task 3 takes
100 ticks to execute, it will not have completed execution at
time 100.  Thus at time 100, all of the tasks except task 3 have
met their first deadline.

At time 200, task 1 must have met its second deadline
and task 2 its first deadline.  As a result, of the first 200
time units, task 1 uses (2 * 25) = 50 and task 2 uses 50,
leaving (200 - 100) time units for task 3.  Task 3 requires 100
time units to execute, thus it will have completed execution at
time 200.  Thus, all of the tasks have met their first deadlines
at time 200, and the task set is schedulable using the First
Deadline Rule.

Relaxation of Assumptions
~~~~~~~~~~~~~~~~~~~~~~~~~

The assumptions used to develop the RMS
schedulability rules are uncommon in most real-time systems.
For example, it was assumed that tasks have constant unvarying
execution time.  It is possible to relax this assumption, simply
by using the worst-case execution time of each task.

Another assumption is that the tasks are independent.
This means that the tasks do not wait for one another or
contend for resources.  This assumption can be relaxed by
accounting for the amount of time a task spends waiting to
acquire resources.  Similarly, each task's execution time must
account for any I/O performed and any RTEMS directive calls.

In addition, the assumptions did not account for the
time spent executing interrupt service routines.  This can be
accounted for by including all the processor utilization by
interrupt service routines in the utilization calculation.
Similarly, one should also account for the impact of delays in
accessing local memory caused by direct memory access and other
processors accessing local dual-ported memory.

The assumption that nonperiodic tasks are used only
for initialization or failure-recovery can be relaxed by placing
all periodic tasks in the critical task set.  This task set can
be scheduled and analyzed using RMS.  All nonperiodic tasks are
placed in the non-critical task set.  Although the critical task
set can be guaranteed to execute even under transient overload,
the non-critical task set is not guaranteed to execute.

In conclusion, the application designer must be fully
cognizant of the system and its run-time behavior when
performing schedulability analysis for a system using RMS.
Every hardware and software factor which impacts the execution
time of each task must be accounted for in the schedulability
analysis.

Further Reading
~~~~~~~~~~~~~~~

For more information on Rate Monotonic Scheduling and
its schedulability analysis, the reader is referred to the
following:

- *C. L. Liu and J. W. Layland. "Scheduling Algorithms for
  Multiprogramming in a Hard Real Time Environment." *Journal of
  the Association of Computing Machinery*. January 1973. pp. 46-61.*

- *John Lehoczky, Lui Sha, and Ye Ding. "The Rate Monotonic
  Scheduling Algorithm: Exact Characterization and Average Case
  Behavior."  *IEEE Real-Time Systems Symposium*. 1989. pp. 166-171.*

- *Lui Sha and John Goodenough. "Real-Time Scheduling
  theory and Ada."  *IEEE Computer*. April 1990. pp. 53-62.*

- *Alan Burns. "Scheduling hard real-time systems: a
  review."  *Software Engineering Journal*. May 1991. pp. 116-128.*

Operations
==========

Creating a Rate Monotonic Period
--------------------------------

The ``rtems_rate_monotonic_create`` directive creates a rate
monotonic period which is to be used by the calling task to
delineate a period.  RTEMS allocates a Period Control Block
(PCB) from the PCB free list.  This data structure is used by
RTEMS to manage the newly created rate monotonic period.  RTEMS
returns a unique period ID to the application which is used by
other rate monotonic manager directives to access this rate
monotonic period.

Manipulating a Period
---------------------

The ``rtems_rate_monotonic_period`` directive is used to
establish and maintain periodic execution utilizing a previously
created rate monotonic period.   Once initiated by the``rtems_rate_monotonic_period`` directive, the period is
said to run until it either expires or is reinitiated.  The state of the rate
monotonic period results in one of the following scenarios:

- If the rate monotonic period is running, the calling
  task will be blocked for the remainder of the outstanding period
  and, upon completion of that period, the period will be
  reinitiated with the specified period.

- If the rate monotonic period is not currently running
  and has not expired, it is initiated with a length of period
  ticks and the calling task returns immediately.

- If the rate monotonic period has expired before the task
  invokes the ``rtems_rate_monotonic_period`` directive,
  the period will be initiated with a length of period ticks and the calling task
  returns immediately with a timeout error status.

Obtaining the Status of a Period
--------------------------------

If the ``rtems_rate_monotonic_period`` directive is invoked
with a period of ``RTEMS_PERIOD_STATUS`` ticks, the current
state of the specified rate monotonic period will be returned.  The following
table details the relationship between the period's status and
the directive status code returned by the``rtems_rate_monotonic_period``
directive:

- ``RTEMS_SUCCESSFUL`` - period is running

- ``RTEMS_TIMEOUT`` - period has expired

- ``RTEMS_NOT_DEFINED`` - period has never been initiated

Obtaining the status of a rate monotonic period does
not alter the state or length of that period.

Canceling a Period
------------------

The ``rtems_rate_monotonic_cancel`` directive is used to stop
the period maintained by the specified rate monotonic period.
The period is stopped and the rate monotonic period can be
reinitiated using the ``rtems_rate_monotonic_period`` directive.

Deleting a Rate Monotonic Period
--------------------------------

The ``rtems_rate_monotonic_delete`` directive is used to delete
a rate monotonic period.  If the period is running and has not
expired, the period is automatically canceled.  The rate
monotonic period's control block is returned to the PCB free
list when it is deleted.  A rate monotonic period can be deleted
by a task other than the task which created the period.

Examples
--------

The following sections illustrate common uses of rate
monotonic periods to construct periodic tasks.

Simple Periodic Task
--------------------

This example consists of a single periodic task
which, after initialization, executes every 100 clock ticks.
.. code:: c

    rtems_task Periodic_task(rtems_task_argument arg)
    {
    rtems_name        name;
    rtems_id          period;
    rtems_status_code status;
    name = rtems_build_name( 'P', 'E', 'R', 'D' );
    status = rtems_rate_monotonic_create( name, &period );
    if ( status != RTEMS_STATUS_SUCCESSFUL ) {
    printf( "rtems_monotonic_create failed with status of %d.\\n", rc );
    exit( 1 );
    }
    while ( 1 ) {
    if ( rtems_rate_monotonic_period( period, 100 ) == RTEMS_TIMEOUT )
    break;
    /* Perform some periodic actions \*/
    }
    /* missed period so delete period and SELF \*/
    status = rtems_rate_monotonic_delete( period );
    if ( status != RTEMS_STATUS_SUCCESSFUL ) {
    printf( "rtems_rate_monotonic_delete failed with status of %d.\\n", status );
    exit( 1 );
    }
    status = rtems_task_delete( SELF );    /* should not return \*/
    printf( "rtems_task_delete returned with status of %d.\\n", status );
    exit( 1 );
    }

The above task creates a rate monotonic period as
part of its initialization.  The first time the loop is
executed, the ``rtems_rate_monotonic_period``
directive will initiate the period for 100 ticks and return
immediately.  Subsequent invocations of the``rtems_rate_monotonic_period`` directive will result
in the task blocking for the remainder of the 100 tick period.
If, for any reason, the body of the loop takes more than 100
ticks to execute, the ``rtems_rate_monotonic_period``
directive will return the ``RTEMS_TIMEOUT`` status.
If the above task misses its deadline, it will delete the rate
monotonic period and itself.

Task with Multiple Periods
--------------------------

This example consists of a single periodic task
which, after initialization, performs two sets of actions every
100 clock ticks.  The first set of actions is performed in the
first forty clock ticks of every 100 clock ticks, while the
second set of actions is performed between the fortieth and
seventieth clock ticks.  The last thirty clock ticks are not
used by this task.
.. code:: c

    rtems_task Periodic_task(rtems_task_argument arg)
    {
    rtems_name        name_1, name_2;
    rtems_id          period_1, period_2;
    rtems_status_code status;
    name_1 = rtems_build_name( 'P', 'E', 'R', '1' );
    name_2 = rtems_build_name( 'P', 'E', 'R', '2' );
    (void ) rtems_rate_monotonic_create( name_1, &period_1 );
    (void ) rtems_rate_monotonic_create( name_2, &period_2 );
    while ( 1 ) {
    if ( rtems_rate_monotonic_period( period_1, 100 ) == TIMEOUT )
    break;
    if ( rtems_rate_monotonic_period( period_2, 40 ) == TIMEOUT )
    break;
    /*
    *  Perform first set of actions between clock
    *  ticks 0 and 39 of every 100 ticks.
    \*/
    if ( rtems_rate_monotonic_period( period_2, 30 ) == TIMEOUT )
    break;
    /*
    *  Perform second set of actions between clock 40 and 69
    *  of every 100 ticks.  THEN ...
    *
    *  Check to make sure we didn't miss the period_2 period.
    \*/
    if ( rtems_rate_monotonic_period( period_2, STATUS ) == TIMEOUT )
    break;
    (void) rtems_rate_monotonic_cancel( period_2 );
    }
    /* missed period so delete period and SELF \*/
    (void ) rtems_rate_monotonic_delete( period_1 );
    (void ) rtems_rate_monotonic_delete( period_2 );
    (void ) task_delete( SELF );
    }

The above task creates two rate monotonic periods as
part of its initialization.  The first time the loop is
executed, the ``rtems_rate_monotonic_period``
directive will initiate the period_1 period for 100 ticks
and return immediately.  Subsequent invocations of the``rtems_rate_monotonic_period`` directive
for period_1 will result in the task blocking for the remainder
of the 100 tick period.  The period_2 period is used to control
the execution time of the two sets of actions within each 100
tick period established by period_1.  The``rtems_rate_monotonic_cancel( period_2 )``
call is performed to ensure that the period_2 period
does not expire while the task is blocked on the period_1
period.  If this cancel operation were not performed, every time
the ``rtems_rate_monotonic_period( period_2, 40 )``
call is executed, except for the initial one, a directive status
of ``RTEMS_TIMEOUT`` is returned.  It is important to
note that every time this call is made, the period_2 period will be
initiated immediately and the task will not block.

If, for any reason, the task misses any deadline, the``rtems_rate_monotonic_period`` directive will
return the ``RTEMS_TIMEOUT``
directive status.  If the above task misses its deadline, it
will delete the rate monotonic periods and itself.

Directives
==========

This section details the rate monotonic manager's
directives.  A subsection is dedicated to each of this manager's
directives and describes the calling sequence, related
constants, usage, and status codes.

RATE_MONOTONIC_CREATE - Create a rate monotonic period
------------------------------------------------------
.. index:: create a period

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_create

.. code:: c

    rtems_status_code rtems_rate_monotonic_create(
    rtems_name  name,
    rtems_id   \*id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - rate monotonic period created successfully
``RTEMS_INVALID_NAME`` - invalid period name
``RTEMS_TOO_MANY`` - too many periods created

**DESCRIPTION:**

This directive creates a rate monotonic period.  The
assigned rate monotonic id is returned in id.  This id is used
to access the period with other rate monotonic manager
directives.  For control and maintenance of the rate monotonic
period, RTEMS allocates a PCB from the local PCB free pool and
initializes it.

**NOTES:**

This directive will not cause the calling task to be
preempted.

RATE_MONOTONIC_IDENT - Get ID of a period
-----------------------------------------
.. index:: get ID of a period
.. index:: obtain ID of a period

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_ident

.. code:: c

    rtems_status_code rtems_rate_monotonic_ident(
    rtems_name  name,
    rtems_id   \*id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - period identified successfully
``RTEMS_INVALID_NAME`` - period name not found

**DESCRIPTION:**

This directive obtains the period id associated with
the period name to be acquired.  If the period name is not
unique, then the period id will match one of the periods with
that name.  However, this period id is not guaranteed to
correspond to the desired period.  The period id is used to
access this period in other rate monotonic manager directives.

**NOTES:**

This directive will not cause the running task to be
preempted.

RATE_MONOTONIC_CANCEL - Cancel a period
---------------------------------------
.. index:: cancel a period

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_cancel

.. code:: c

    rtems_status_code rtems_rate_monotonic_cancel(
    rtems_id id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - period canceled successfully
``RTEMS_INVALID_ID`` - invalid rate monotonic period id
``RTEMS_NOT_OWNER_OF_RESOURCE`` - rate monotonic period not created by calling task

**DESCRIPTION:**

This directive cancels the rate monotonic period id.
This period will be reinitiated by the next invocation of``rtems_rate_monotonic_period`` with id.

**NOTES:**

This directive will not cause the running task to be
preempted.

The rate monotonic period specified by id must have
been created by the calling task.

RATE_MONOTONIC_DELETE - Delete a rate monotonic period
------------------------------------------------------
.. index:: delete a period

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_delete

.. code:: c

    rtems_status_code rtems_rate_monotonic_delete(
    rtems_id id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - period deleted successfully
``RTEMS_INVALID_ID`` - invalid rate monotonic period id

**DESCRIPTION:**

This directive deletes the rate monotonic period
specified by id.  If the period is running, it is automatically
canceled.  The PCB for the deleted period is reclaimed by RTEMS.

**NOTES:**

This directive will not cause the running task to be
preempted.

A rate monotonic period can be deleted by a task
other than the task which created the period.

RATE_MONOTONIC_PERIOD - Conclude current/Start next period
----------------------------------------------------------
.. index:: conclude current period
.. index:: start current period
.. index:: period initiation

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_period

.. code:: c

    rtems_status_code rtems_rate_monotonic_period(
    rtems_id       id,
    rtems_interval length
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - period initiated successfully
``RTEMS_INVALID_ID`` - invalid rate monotonic period id
``RTEMS_NOT_OWNER_OF_RESOURCE`` - period not created by calling task
``RTEMS_NOT_DEFINED`` - period has never been initiated (only
possible when period is set to PERIOD_STATUS)
``RTEMS_TIMEOUT`` - period has expired

**DESCRIPTION:**

This directive initiates the rate monotonic period id
with a length of period ticks.  If id is running, then the
calling task will block for the remainder of the period before
reinitiating the period with the specified period.  If id was
not running (either expired or never initiated), the period is
immediately initiated and the directive returns immediately.

If invoked with a period of ``RTEMS_PERIOD_STATUS`` ticks, the
current state of id will be returned.  The directive status
indicates the current state of the period.  This does not alter
the state or period of the period.

**NOTES:**

This directive will not cause the running task to be preempted.

RATE_MONOTONIC_GET_STATUS - Obtain status from a period
-------------------------------------------------------
.. index:: get status of period
.. index:: obtain status of period

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_get_status

.. code:: c

    rtems_status_code rtems_rate_monotonic_get_status(
    rtems_id                            id,
    rtems_rate_monotonic_period_status \*status
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - period initiated successfully
``RTEMS_INVALID_ID`` - invalid rate monotonic period id
``RTEMS_INVALID_ADDRESS`` - invalid address of status

**DESCRIPTION:**

This directive returns status information associated with
the rate monotonic period id in the following data structure:.. index:: rtems_rate_monotonic_period_status

.. code:: c

    typedef struct {
    rtems_id                              owner;
    rtems_rate_monotonic_period_states    state;
    rtems_rate_monotonic_period_time_t    since_last_period;
    rtems_thread_cpu_usage_t              executed_since_last_period;
    }  rtems_rate_monotonic_period_status;

.. COMMENT: RATE_MONOTONIC_INACTIVE does not have RTEMS_ in front of it.

A configure time option can be used to select whether the time information is
given in ticks or seconds and nanoseconds.  The default is seconds and
nanoseconds.  If the period's state is ``RATE_MONOTONIC_INACTIVE``, both
time values will be set to 0.  Otherwise, both time values will contain
time information since the last invocation of the``rtems_rate_monotonic_period`` directive.  More
specifically, the ticks_since_last_period value contains the elapsed time
which has occurred since the last invocation of the``rtems_rate_monotonic_period`` directive and the
ticks_executed_since_last_period contains how much processor time the
owning task has consumed since the invocation of the``rtems_rate_monotonic_period`` directive.

**NOTES:**

This directive will not cause the running task to be preempted.

RATE_MONOTONIC_GET_STATISTICS - Obtain statistics from a period
---------------------------------------------------------------
.. index:: get statistics of period
.. index:: obtain statistics of period

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_get_statistics

.. code:: c

    rtems_status_code rtems_rate_monotonic_get_statistics(
    rtems_id                                id,
    rtems_rate_monotonic_period_statistics \*statistics
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - period initiated successfully
``RTEMS_INVALID_ID`` - invalid rate monotonic period id
``RTEMS_INVALID_ADDRESS`` - invalid address of statistics

**DESCRIPTION:**

This directive returns statistics information associated with
the rate monotonic period id in the following data structure:.. index:: rtems_rate_monotonic_period_statistics

.. code:: c

    typedef struct {
    uint32_t     count;
    uint32_t     missed_count;
    #ifdef RTEMS_ENABLE_NANOSECOND_CPU_USAGE_STATISTICS
    struct timespec min_cpu_time;
    struct timespec max_cpu_time;
    struct timespec total_cpu_time;
    #else
    uint32_t  min_cpu_time;
    uint32_t  max_cpu_time;
    uint32_t  total_cpu_time;
    #endif
    #ifdef RTEMS_ENABLE_NANOSECOND_RATE_MONOTONIC_STATISTICS
    struct timespec min_wall_time;
    struct timespec max_wall_time;
    struct timespec total_wall_time;
    #else
    uint32_t  min_wall_time;
    uint32_t  max_wall_time;
    uint32_t  total_wall_time;
    #endif
    }  rtems_rate_monotonic_period_statistics;

This directive returns the current statistics information for
the period instance assocaited with ``id``.  The information
returned is indicated by the structure above.

**NOTES:**

This directive will not cause the running task to be preempted.

RATE_MONOTONIC_RESET_STATISTICS - Reset statistics for a period
---------------------------------------------------------------
.. index:: reset statistics of period

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_reset_statistics

.. code:: c

    rtems_status_code rtems_rate_monotonic_reset_statistics(
    rtems_id  id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - period initiated successfully
``RTEMS_INVALID_ID`` - invalid rate monotonic period id

**DESCRIPTION:**

This directive resets the statistics information associated with
this rate monotonic period instance.

**NOTES:**

This directive will not cause the running task to be preempted.

RATE_MONOTONIC_RESET_ALL_STATISTICS - Reset statistics for all periods
----------------------------------------------------------------------
.. index:: reset statistics of all periods

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_reset_all_statistics

.. code:: c

    void rtems_rate_monotonic_reset_all_statistics(void);

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive resets the statistics information associated with
all rate monotonic period instances.

**NOTES:**

This directive will not cause the running task to be preempted.

RATE_MONOTONIC_REPORT_STATISTICS - Print period statistics report
-----------------------------------------------------------------
.. index:: print period statistics report
.. index:: period statistics report

**CALLING SEQUENCE:**

.. index:: rtems_rate_monotonic_report_statistics

.. code:: c

    void rtems_rate_monotonic_report_statistics(void);

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive prints a report on all active periods which have
executed at least one period. The following is an example of the
output generated by this directive... index:: rtems_rate_monotonic_period_statistics

.. code:: c

    ID      OWNER   PERIODS  MISSED    CPU TIME    WALL TIME
    MIN/MAX/AVG  MIN/MAX/AVG
    0x42010001  TA1       502     0       0/1/0.99    0/0/0.00
    0x42010002  TA2       502     0       0/1/0.99    0/0/0.00
    0x42010003  TA3       501     0       0/1/0.99    0/0/0.00
    0x42010004  TA4       501     0       0/1/0.99    0/0/0.00
    0x42010005  TA5        10     0       0/1/0.90    0/0/0.00

**NOTES:**

This directive will not cause the running task to be preempted.

.. COMMENT: COPYRIGHT (c) 1988-2007.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.