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Rate Monotonic Manager

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.. index:: rate mononitonic tasks
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.. 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

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.. 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

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.. index:: Rate Monotonic Scheduling Algorithm, definition
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.. 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 (in milliseconds) Priority
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

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.. 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

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.. 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 Priority Period Execution Time Processor 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

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.. 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:

Task RMS Priority Period Execution Time Processor 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.

Deadline Time Task 1 Task 2 Task 3 Total Execution Time All Deadlines 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

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.. index:: create a period

CALLING SEQUENCE:

procedure Rate_Monotonic_Create (
Name   : in     RTEMS.Name;
ID     :    out RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

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

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.. index:: get ID of a period
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.. index:: obtain ID of a period

CALLING SEQUENCE:

procedure Rate_Monotonic_Ident (
Name   : in     RTEMS.Name;
ID     :    out RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

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

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.. index:: cancel a period

CALLING SEQUENCE:

procedure Rate_Monotonic_Cancel (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

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

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.. index:: delete a period

CALLING SEQUENCE:

procedure Rate_Monotonic_Delete (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

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

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.. index:: conclude current period
?
.. index:: start current period
?
.. index:: period initiation

CALLING SEQUENCE:

procedure Rate_Monotonic_Period (
ID      : in     RTEMS.ID;
Length  : in     RTEMS.Interval;
Result  :    out RTEMS.Status_Codes
);

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

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.. index:: get status of period
?
.. index:: obtain status of period

CALLING SEQUENCE:

procedure Rate_Monotonic_Get_Status (
ID      : in     RTEMS.ID;
Status  :    out RTEMS.Rate_Monotonic_Period_Status;
Result  :    out RTEMS.Status_Codes
);

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 record:

type Rate_Monotonic_Period_Status is
begin
Owner                      : RTEMS.ID;
State                      : RTEMS.Rate_Monotonic_Period_States;
Since_Last_Period          : RTEMS.Unsigned32;
Executed_Since_Last_Period : RTEMS.Unsigned32;
end record;

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

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.. index:: get statistics of period
?
.. index:: obtain statistics of period

CALLING SEQUENCE:

NOT SUPPORTED FROM Ada BINDING

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 record:

NOT SUPPORTED FROM Ada BINDING

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

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.. index:: reset statistics of period

CALLING SEQUENCE:

procedure Rate_Monotonic_Reset_Statistics (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

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

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.. index:: reset statistics of all periods

CALLING SEQUENCE:

procedure Rate_Monotonic_Reset_All_Statistics;

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

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.. index:: print period statistics report
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.. index:: period statistics report

CALLING SEQUENCE:

procedure Rate_Monotonic_Report_Statistics;

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.

NOTES:

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

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