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@c

@c  COPYRIGHT (c) 1988-2002.

@c  On-Line Applications Research Corporation (OAR).

@c  All rights reserved.

@c

@c  rtmon.t,v 1.23 2002/03/27 17:35:15 joel Exp

@c

@c

@c  Open Issues

@c    - nicen up the tables

@c    - use math mode to print formulas

@c

@chapter Rate Monotonic Manager

@cindex rate mononitonic tasks

@cindex periodic tasks

@section Introduction

The rate monotonic manager provides facilities to

implement tasks which execute in a periodic fashion.  The

directives provided by the rate monotonic manager are:

@itemize @bullet

@item @code{@value{DIRPREFIX}rate_monotonic_create} - Create a rate monotonic period

@item @code{@value{DIRPREFIX}rate_monotonic_ident} - Get ID of a period

@item @code{@value{DIRPREFIX}rate_monotonic_cancel} - Cancel a period

@item @code{@value{DIRPREFIX}rate_monotonic_delete} - Delete a rate monotonic period

@item @code{@value{DIRPREFIX}rate_monotonic_period} - Conclude current/Start next period

@item @code{@value{DIRPREFIX}rate_monotonic_get_status} - Obtain status information on period

@end itemize

@section 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.

@subsection Rate Monotonic Manager Required Support

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

@subsection Rate Monotonic Manager Definitions

@cindex 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.

@cindex 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.

@cindex 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.

@subsection Rate Monotonic Scheduling Algorithm

@cindex Rate Monotonic Scheduling Algorithm, definition

@cindex 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:

@itemize @code{ }

@b{RMS is an optimal static priority algorithm for

scheduling independent, preemptible, periodic tasks

on a single processor.}

@end itemize

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:

@ifset use-ascii

@example

@group

+--------------------+---------------------+---------------------+

|        Task        |       Period        |      Priority       |

|                    |  (in milliseconds)  |                     |

+--------------------+---------------------+---------------------+

|         1          |         100         |         Low         |

+--------------------+---------------------+---------------------+

|         2          |          50         |       Medium        |

+--------------------+---------------------+---------------------+

|         3          |          50         |       Medium        |

+--------------------+---------------------+---------------------+

|         4          |          25         |        High         |

+--------------------+---------------------+---------------------+

@end group

@end example

@end ifset

@ifset use-tex

@sp 1

@tex

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\vrule#&

\hbox to 1.25in{\enskip\hfil#\hfil}&

\vrule#\cr\noalign{\hrule}

&\bf Task&& \bf Period && \bf Priority &\cr

& && \bf (in milliseconds) && &\cr\noalign{\hrule}

& 1 && 100 && Low &\cr\noalign{\hrule}

& 2 && 50 && Medium &\cr\noalign{\hrule}

& 3 && 50 && Medium &\cr\noalign{\hrule}

& 4 && 25 && High &\cr\noalign{\hrule}

}}\hfil}

@end tex

@end ifset

@ifset use-html

@html

  

      
      

         
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@end html

@end ifset

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.

@subsection Schedulability Analysis

@cindex 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.

@lowersections

@subsection Assumptions

The schedulability analysis rules for RMS were

developed based on the following assumptions:

@itemize @bullet

@item The requests for all tasks for which hard deadlines

exist are periodic, with a constant interval between requests.

@item Each task must complete before the next request for it

occurs.

@item The tasks are independent in that a task does not depend

on the initiation or completion of requests for other tasks.

@item The execution time for each task without preemption or

interruption is constant and does not vary.

@item Any non-periodic tasks in the system are special.  These

tasks displace periodic tasks while executing and do not have

hard, critical deadlines.

@end itemize

Once the basic schedulability analysis is understood,

some of the above assumptions can be relaxed and the

side-effects accounted for.

@subsection Processor Utilization Rule

@cindex 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:

@example

@group

Utilization = 0

for index = 1 to maximum_tasks

  Utilization = Utilization + (Time(index)/Period(index))

@end group

@end example

To ensure schedulability even under transient

overload, the processor utilization must adhere to the following

rule:

@example

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

@end example

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.

@subsection 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:

@ifset use-ascii

@example

@group

    +------------+----------+--------+-----------+-------------+

    |    Task    |   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     |

    +------------+----------+--------+-----------+-------------+

@end group

@end example

@end ifset

@ifset use-tex

@sp 1

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& 2 && Medium && 200 && 50 && 0.25 &\cr\noalign{\hrule}

& 3 && Low && 300 && 100 && 0.33 &\cr\noalign{\hrule}

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@ifset use-html

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@end html

@end ifset

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.

@subsection First Deadline Rule

@cindex 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.

@subsection 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:

@ifset use-ascii

@example

@group

    +------------+----------+--------+-----------+-------------+

    |    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     |

    +------------+----------+--------+-----------+-------------+

@end group

@end example

@end ifset

@ifset use-tex

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&\bf Task&& \bf RMS && \bf Period && \bf Execution &&\bf Processor&\cr

& && \bf Priority && &&\bf Time &&\bf Utilization &\cr\noalign{\hrule}

& 1 && High && 100 && 25 && 0.25 &\cr\noalign{\hrule}

& 2 && Medium && 200 && 50 && 0.25 &\cr\noalign{\hrule}

& 3 && Low && 300 && 100 && 0.33 &\cr\noalign{\hrule}

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@end ifset

@ifset use-html

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@end html

@end ifset

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.

@ifset use-ascii

@example

@group

+----------+------+------+------+----------------------+---------------+

| 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       |

+----------+------+------+------+----------------------+---------------+

@end group

@end example

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&\bf Time    && \bf 1    &&\bf 2   &&\bf 3   &&\bf Execution Time &&\bf Net?&\cr\noalign{\hrule}

& 100&& 1 && 1 && 1 && 25 + 50 + 100 = 175 && NO &\cr\noalign{\hrule}

& 200&& 2 && 1 && 1 && 50 + 50 + 100 = 200 && YES &\cr\noalign{\hrule}

}}\hfil}

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@end ifset

@ifset use-html

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@end html

@end ifset

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.

@subsection 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.

@subsection Further Reading

For more information on Rate Monotonic Scheduling and

its schedulability analysis, the reader is referred to the

following:

@itemize @code{ }

@item @cite{C. L. Liu and J. W. Layland. "Scheduling Algorithms for

Multiprogramming in a Hard Real Time Environment." @b{Journal of

the Association of Computing Machinery}. January 1973. pp. 46-61.}

@item @cite{John Lehoczky, Lui Sha, and Ye Ding. "The Rate Monotonic

Scheduling Algorithm: Exact Characterization and Average Case

Behavior."  @b{IEEE Real-Time Systems Symposium}. 1989. pp. 166-171.}

@item @cite{Lui Sha and John Goodenough. "Real-Time Scheduling

Theory and Ada."  @b{IEEE Computer}. April 1990. pp. 53-62.}

@item @cite{Alan Burns. "Scheduling hard real-time systems: a

review."  @b{Software Engineering Journal}. May 1991. pp. 116-128.}

@end itemize

@raisesections

@section Operations

@subsection Creating a Rate Monotonic Period

The @code{@value{DIRPREFIX}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.

@subsection Manipulating a Period

The @code{@value{DIRPREFIX}rate_monotonic_period} directive is used to

establish and maintain periodic execution utilizing a previously

created rate monotonic period.   Once initiated by the

@code{@value{DIRPREFIX}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:

@itemize @bullet

@item 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.

@item 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.

@item If the rate monotonic period has expired before the task

invokes the @code{@value{DIRPREFIX}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.

@end itemize

@subsection Obtaining the Status of a Period

If the @code{@value{DIRPREFIX}rate_monotonic_period} directive is invoked

with a period of @code{@value{RPREFIX}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

@code{@value{DIRPREFIX}rate_monotonic_period}

directive:

@itemize @bullet

@item @code{@value{RPREFIX}SUCCESSFUL} - period is running

@item @code{@value{RPREFIX}TIMEOUT} - period has expired

@item @code{@value{RPREFIX}NOT_DEFINED} - period has never been initiated

@end itemize

Obtaining the status of a rate monotonic period does

not alter the state or length of that period.

@subsection Canceling a Period

The @code{@value{DIRPREFIX}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 @code{@value{DIRPREFIX}rate_monotonic_period} directive.

@subsection Deleting a Rate Monotonic Period

The @code{@value{DIRPREFIX}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.

@subsection Examples

The following sections illustrate common uses of rate

monotonic periods to construct periodic tasks.

@subsection Simple Periodic Task

This example consists of a single periodic task

which, after initialization, executes every 100 clock ticks.

@page

@example

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 );

@}

@end example

The above task creates a rate monotonic period as

part of its initialization.  The first time the loop is

executed, the @code{@value{DIRPREFIX}rate_monotonic_period}

directive will initiate the period for 100 ticks and return

immediately.  Subsequent invocations of the

@code{@value{DIRPREFIX}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 @code{@value{DIRPREFIX}rate_monotonic_period}

directive will return the @code{@value{RPREFIX}TIMEOUT} status.

If the above task misses its deadline, it will delete the rate

monotonic period and itself.

@subsection 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.

@page

@example

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 );

@}

@end example

The above task creates two rate monotonic periods as

part of its initialization.  The first time the loop is

executed, the @code{@value{DIRPREFIX}rate_monotonic_period}

directive will initiate the period_1 period for 100 ticks

and return immediately.  Subsequent invocations of the

@code{@value{DIRPREFIX}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

@code{@value{DIRPREFIX}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 @code{@value{DIRPREFIX}rate_monotonic_period( period_2, 40 )}

call is executed, except for the initial one, a directive status

of @code{@value{RPREFIX}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

@code{@value{DIRPREFIX}rate_monotonic_period} directive will

return the @code{@value{RPREFIX}TIMEOUT}

directive status.  If the above task misses its deadline, it

will delete the rate monotonic periods and itself.

@section 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.

@c

@c

@c

@page

@subsection RATE_MONOTONIC_CREATE - Create a rate monotonic period

@cindex create a period

@subheading CALLING SEQUENCE:

@ifset is-C

@findex rtems_rate_monotonic_create

@example

rtems_status_code rtems_rate_monotonic_create(

  rtems_name  name,

  rtems_id   *id

);

@end example

@end ifset

@ifset is-Ada

@example

procedure Rate_Monotonic_Create (

   Name   : in     RTEMS.Name;

   ID     :    out RTEMS.ID;

   Result :    out RTEMS.Status_Codes

);

@end example

@end ifset

@subheading DIRECTIVE STATUS CODES:

@code{@value{RPREFIX}SUCCESSFUL} - rate monotonic period created successfully@*

@code{@value{RPREFIX}INVALID_NAME} - invalid task name@*

@code{@value{RPREFIX}TOO_MANY} - too many periods created

@subheading 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.

@subheading NOTES:

This directive will not cause the calling task to be

preempted.

@c

@c

@c

@page

@subsection RATE_MONOTONIC_IDENT - Get ID of a period

@cindex get ID of a period

@cindex obtain ID of a period

@subheading CALLING SEQUENCE:

@ifset is-C