@c @c COPYRIGHT (c) 1988-1998. @c On-Line Applications Research Corporation (OAR). @c All rights reserved. @c @c $Id$ @c @c @c Open Issues @c - nicen up the tables @c - use math mode to print formulas @c @ifinfo @node Rate Monotonic Manager, Rate Monotonic Manager Introduction, Scheduling Concepts Task State Transitions, Top @end ifinfo @chapter Rate Monotonic Manager @ifinfo @menu * Rate Monotonic Manager Introduction:: * Rate Monotonic Manager Background:: * Rate Monotonic Manager Operations:: * Rate Monotonic Manager Directives:: @end menu @end ifinfo @ifinfo @node Rate Monotonic Manager Introduction, Rate Monotonic Manager Background, Rate Monotonic Manager, Rate Monotonic Manager @end ifinfo @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{rate_monotonic_create} - Create a rate monotonic period @item @code{rate_monotonic_ident} - Get ID of a period @item @code{rate_monotonic_cancel} - Cancel a period @item @code{rate_monotonic_delete} - Delete a rate monotonic period @item @code{rate_monotonic_period} - Conclude current/Start next period @item @code{rate_monotonic_get_status} - Obtain status information on period @end itemize @ifinfo @node Rate Monotonic Manager Background, Rate Monotonic Manager Required Support, Rate Monotonic Manager Introduction, Rate Monotonic Manager @end ifinfo @section Background @ifinfo @menu * Rate Monotonic Manager Required Support:: * Rate Monotonic Manager Definitions:: * Rate Monotonic Scheduling Algorithm:: * Schedulability Analysis:: * Assumptions:: * Processor Utilization Rule:: * Processor Utilization Rule Example:: * First Deadline Rule:: * First Deadline Rule Example:: * Relaxation of Assumptions:: * Further Reading:: @end menu @end ifinfo 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 insure 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. @ifinfo @node Rate Monotonic Manager Required Support, Rate Monotonic Manager Definitions, Rate Monotonic Manager Background, Rate Monotonic Manager Background @end ifinfo @subsection Rate Monotonic Manager Required Support A clock tick is required to support the functionality provided by this manager. @ifinfo @node Rate Monotonic Manager Definitions, Rate Monotonic Scheduling Algorithm, Rate Monotonic Manager Required Support, Rate Monotonic Manager Background @end ifinfo @subsection Rate Monotonic Manager Definitions 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. 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. 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 insures a minimum time period between successive activations, but the missile must be launched by a hard deadline. @ifinfo @node Rate Monotonic Scheduling Algorithm, Schedulability Analysis, Rate Monotonic Manager Definitions, Rate Monotonic Manager Background @end ifinfo @subsection Rate Monotonic Scheduling Algorithm 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, than 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 \centerline{\vbox{\offinterlineskip\halign{ \vrule\strut#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 1.25in{\enskip\hfil#\hfil}& \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
Task Period (in milliseconds) Priority
1 100 Low
2 50 Medium
3 50 Medium
4 25 High
@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. @ifinfo @node Schedulability Analysis, Assumptions, Rate Monotonic Scheduling Algorithm, Rate Monotonic Manager Background @end ifinfo @subsection Schedulability Analysis RMS allows application designers to insure 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 @ifinfo @node Assumptions, Processor Utilization Rule, Schedulability Analysis, Rate Monotonic Manager Background @end ifinfo @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. @ifinfo @node Processor Utilization Rule, Processor Utilization Rule Example, Assumptions, Rate Monotonic Manager Background @end ifinfo @subsection 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 insure 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. @ifinfo @node Processor Utilization Rule Example, First Deadline Rule, Processor Utilization Rule, Rate Monotonic Manager Background @end ifinfo @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 @tex \centerline{\vbox{\offinterlineskip\halign{ \vrule\strut#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 1.00in{\enskip\hfil#\hfil}& \vrule#& \hbox to 1.00in{\enskip\hfil#\hfil}& \vrule#\cr\noalign{\hrule} &\bf Task&& \bf RMS && \bf Period && \bf Execution &&\bf Processor&\cr & && \bf Priority && &&\bf Time &&\bf Utilization &\cr\noalign{\hrule} & 1 && High && 100 && 15 && 0.15 &\cr\noalign{\hrule} & 2 && Medium && 200 && 50 && 0.25 &\cr\noalign{\hrule} & 3 && Low && 300 && 100 && 0.33 &\cr\noalign{\hrule} }}\hfil} @end tex @end ifset @ifset use-html @html
Task 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
@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. @ifinfo @node First Deadline Rule, First Deadline Rule Example, Processor Utilization Rule Example, Rate Monotonic Manager Background @end ifinfo @subsection 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 insure. 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 insures that all tasks begin to compete for execution time at the same instant -- when the user initialization task deletes itself. @ifinfo @node First Deadline Rule Example, Relaxation of Assumptions, First Deadline Rule, Rate Monotonic Manager Background @end ifinfo @subsection First Deadline Rule Example The First Deadline Rule can insure 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 @sp 1 @tex \centerline{\vbox{\offinterlineskip\halign{ \vrule\strut#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 1.00in{\enskip\hfil#\hfil}& \vrule#& \hbox to 1.00in{\enskip\hfil#\hfil}& \vrule#\cr\noalign{\hrule} &\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} }}\hfil} @end tex @end ifset @ifset use-html @html
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
@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 @end ifset @ifset use-tex @sp 1 @tex \centerline{\vbox{\offinterlineskip\halign{ \vrule\strut#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 0.75in{\enskip\hfil#\hfil}& \vrule#& \hbox to 2.00in{\enskip\hfil#\hfil}& \vrule#& \hbox to 1.00in{\enskip\hfil#\hfil}& \vrule#\cr\noalign{\hrule} &\bf Deadline&& \bf Task &&\bf Task&&\bf Task&&\bf Total &&\bf All Deadlines &\cr &\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} @end tex @end ifset @ifset use-html @html
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 = 175 YES
@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. @ifinfo @node Relaxation of Assumptions, Further Reading, First Deadline Rule Example, Rate Monotonic Manager Background @end ifinfo @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. @ifinfo @node Further Reading, Rate Monotonic Manager Operations, Relaxation of Assumptions, Rate Monotonic Manager Background @end ifinfo @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 @ifinfo @node Rate Monotonic Manager Operations, Creating a Rate Monotonic Period, Further Reading, Rate Monotonic Manager @end ifinfo @section Operations @ifinfo @menu * Creating a Rate Monotonic Period:: * Manipulating a Period:: * Obtaining a Period's Status:: * Canceling a Period:: * Deleting a Rate Monotonic Period:: * Examples:: * Simple Periodic Task:: * Task with Multiple Periods:: @end menu @end ifinfo @ifinfo @node Creating a Rate Monotonic Period, Manipulating a Period, Rate Monotonic Manager Operations, Rate Monotonic Manager Operations @end ifinfo @subsection Creating a Rate Monotonic Period The 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. @ifinfo @node Manipulating a Period, Obtaining a Period's Status, Creating a Rate Monotonic Period, Rate Monotonic Manager Operations @end ifinfo @subsection Manipulating a Period The rate_monotonic_period directive is used to establish and maintain periodic execution utilizing a previously created rate monotonic period. Once initiated by the 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 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 @ifinfo @node Obtaining a Period's Status, Canceling a Period, Manipulating a Period, Rate Monotonic Manager Operations @end ifinfo @subsection Obtaining a Period's Status If the rate_monotonic_period directive is invoked with a period of @code{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 rate_monotonic_period directive: @itemize @bullet @item @code{SUCCESSFUL} - period is running @item @code{TIMEOUT} - period has expired @item @code{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. @ifinfo @node Canceling a Period, Deleting a Rate Monotonic Period, Obtaining a Period's Status, Rate Monotonic Manager Operations @end ifinfo @subsection Canceling a Period The 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 rate_monotonic_period directive. @ifinfo @node Deleting a Rate Monotonic Period, Examples, Canceling a Period, Rate Monotonic Manager Operations @end ifinfo @subsection Deleting a Rate Monotonic Period The 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. @ifinfo @node Examples, Simple Periodic Task, Deleting a Rate Monotonic Period, Rate Monotonic Manager Operations @end ifinfo @subsection Examples The following sections illustrate common uses of rate monotonic periods to construct periodic tasks. @ifinfo @node Simple Periodic Task, Task with Multiple Periods, Examples, Rate Monotonic Manager Operations @end ifinfo @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_name name; rtems_id period; rtems_status_code status; name = build_name( 'P', 'E', 'R', 'D' ); (void) rate_monotonic_create( name, &period ); while ( 1 ) @{ if ( rate_monotonic_period( period, 100 ) == TIMEOUT ) break; /* Perform some periodic actions */ @} /* missed period so delete period and SELF */ (void) rate_monotonic_delete( period ); (void) task_delete( SELF ); @} @end example The above task creates a rate monotonic period as part of its initialization. The first time the loop is executed, the rate_monotonic_period directive will initiate the period for 100 ticks and return immediately. Subsequent invocations of the 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 rate_monotonic_period directive will return the TIMEOUT status. If the above task misses its deadline, it will delete the rate monotonic period and itself. @ifinfo @node Task with Multiple Periods, Rate Monotonic Manager Directives, Simple Periodic Task, Rate Monotonic Manager Operations @end ifinfo @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 task Periodic_task() @{ rtems_name name_1, name_2; rtems_id period_1, period_2; rtems_status_code status; name_1 = build_name( 'P', 'E', 'R', '1' ); name_2 = build_name( 'P', 'E', 'R', '2' ); (void ) rate_monotonic_create( name_1, &period_1 ); (void ) rate_monotonic_create( name_2, &period_2 ); while ( 1 ) @{ if ( rate_monotonic_period( period_1, 100 ) == TIMEOUT ) break; if ( rate_monotonic_period( period_2, 40 ) == TIMEOUT ) break; /* * Perform first set of actions between clock * ticks 0 and 39 of every 100 ticks. */ if ( 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 ( rate_monotonic_period( period_2, STATUS ) == TIMEOUT ) break; (void) rate_monotonic_cancel( period_2 ); @} /* missed period so delete period and SELF */ (void ) rate_monotonic_delete( period_1 ); (void ) 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 rate_monotonic_period directive will initiate the period_1 period for 100 ticks and return immediately. Subsequent invocations of the 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 rate_monotonic_cancel( period_2 ) call is performed to insure 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 rate_monotonic_period( period_1, 40 ) call is executed, except for the initial one, a directive status of TIMEOUT is returned. It is important to note that every time this call is made, the period_1 period will be initiated immediately and the task will not block. If, for any reason, the task misses any deadline, the rate_monotonic_period directive will return the TIMEOUT directive status. If the above task misses its deadline, it will delete the rate monotonic periods and itself. @ifinfo @node Rate Monotonic Manager Directives, RATE_MONOTONIC_CREATE - Create a rate monotonic period, Task with Multiple Periods, Rate Monotonic Manager @end ifinfo @section Directives @ifinfo @menu * RATE_MONOTONIC_CREATE - Create a rate monotonic period:: * RATE_MONOTONIC_IDENT - Get ID of a period:: * RATE_MONOTONIC_CANCEL - Cancel a period:: * RATE_MONOTONIC_DELETE - Delete a rate monotonic period:: * RATE_MONOTONIC_PERIOD - Conclude current/Start next period:: * RATE_MONOTONIC_GET_STATUS - Obtain status information on period:: @end menu @end ifinfo 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. @page @ifinfo @node RATE_MONOTONIC_CREATE - Create a rate monotonic period, RATE_MONOTONIC_IDENT - Get ID of a period, Rate Monotonic Manager Directives, Rate Monotonic Manager Directives @end ifinfo @subsection RATE_MONOTONIC_CREATE - Create a rate monotonic period @subheading CALLING SEQUENCE: @ifset is-C @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{SUCCESSFUL} - rate monotonic period created successfully@* @code{INVALID_NAME} - invalid task name@* @code{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. @page @ifinfo @node RATE_MONOTONIC_IDENT - Get ID of a period, RATE_MONOTONIC_CANCEL - Cancel a period, RATE_MONOTONIC_CREATE - Create a rate monotonic period, Rate Monotonic Manager Directives @end ifinfo @subsection RATE_MONOTONIC_IDENT - Get ID of a period @subheading CALLING SEQUENCE: @ifset is-C @example rtems_status_code rtems_rate_monotonic_ident( rtems_name name, rtems_id *id ); @end example @end ifset @ifset is-Ada @example procedure Rate_Monotonic_Ident ( Name : in RTEMS.Name; ID : out RTEMS.ID; Result : out RTEMS.Status_Codes ); @end example @end ifset @subheading DIRECTIVE STATUS CODES: @code{SUCCESSFUL} - period identified successfully@* @code{INVALID_NAME} - period name not found @subheading 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. @subheading NOTES: This directive will not cause the running task to be preempted. @page @ifinfo @node RATE_MONOTONIC_CANCEL - Cancel a period, RATE_MONOTONIC_DELETE - Delete a rate monotonic period, RATE_MONOTONIC_IDENT - Get ID of a period, Rate Monotonic Manager Directives @end ifinfo @subsection RATE_MONOTONIC_CANCEL - Cancel a period @subheading CALLING SEQUENCE: @ifset is-C @example rtems_status_code rtems_rate_monotonic_cancel( rtems_id id ); @end example @end ifset @ifset is-Ada @example procedure Rate_Monotonic_Cancel ( ID : in RTEMS.ID; Result : out RTEMS.Status_Codes ); @end example @end ifset @subheading DIRECTIVE STATUS CODES: @code{SUCCESSFUL} - period canceled successfully@* @code{INVALID_ID} - invalid rate monotonic period id@* @code{NOT_OWNER_OF_RESOURCE} - rate monotonic period not created by calling task @subheading DESCRIPTION: This directive cancels the rate monotonic period id. This period will be reinitiated by the next invocation of rate_monotonic_period with id. @subheading 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. @page @ifinfo @node RATE_MONOTONIC_DELETE - Delete a rate monotonic period, RATE_MONOTONIC_PERIOD - Conclude current/Start next period, RATE_MONOTONIC_CANCEL - Cancel a period, Rate Monotonic Manager Directives @end ifinfo @subsection RATE_MONOTONIC_DELETE - Delete a rate monotonic period @subheading CALLING SEQUENCE: @ifset is-C @example rtems_status_code rtems_rate_monotonic_delete( rtems_id id ); @end example @end ifset @ifset is-Ada @example procedure Rate_Monotonic_Delete ( ID : in RTEMS.ID; Result : out RTEMS.Status_Codes ); @end example @end ifset @subheading DIRECTIVE STATUS CODES: @code{SUCCESSFUL} - period deleted successfully@* @code{INVALID_ID} - invalid rate monotonic period id @subheading 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. @subheading 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. @page @ifinfo @node RATE_MONOTONIC_PERIOD - Conclude current/Start next period, RATE_MONOTONIC_GET_STATUS - Obtain status information on period, RATE_MONOTONIC_DELETE - Delete a rate monotonic period, Rate Monotonic Manager Directives @end ifinfo @subsection RATE_MONOTONIC_PERIOD - Conclude current/Start next period @subheading CALLING SEQUENCE: @ifset is-C @example rtems_status_code rtems_rate_monotonic_period( rtems_id id, rtems_interval length ); @end example @end ifset @ifset is-Ada @example procedure Rate_Monotonic_Period ( ID : in RTEMS.ID; Length : in RTEMS.Interval; Result : out RTEMS.Status_Codes ); @end example @end ifset @subheading DIRECTIVE STATUS CODES: @code{SUCCESSFUL} - period initiated successfully@* @code{INVALID_ID} - invalid rate monotonic period id@* @code{NOT_OWNER_OF_RESOURCE} - period not created by calling task@* @code{NOT_DEFINED} - period has never been initiated (only possible when period is set to PERIOD_STATUS)@* @code{TIMEOUT} - period has expired @subheading 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 @code{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. @subheading NOTES: This directive will not cause the running task to be preempted. --------------------- @page @ifinfo @node RATE_MONOTONIC_GET_STATUS - Obtain status information on period, Board Support Packages, RATE_MONOTONIC_PERIOD - Conclude current/Start next period, Rate Monotonic Manager Directives @end ifinfo @subsection RATE_MONOTONIC_GET_STATUS - Obtain status information on period @subheading CALLING SEQUENCE: @ifset is-C @example rtems_status_code rtems_rate_monotonic_get_status( rtems_id id, rtems_rate_monotonic_period_status *status ); @end example @end ifset @ifset is-Ada @example procedure Rate_Monotonic_Get_Status ( ID : in RTEMS.ID; Status : out RTEMS.Rate_Monotonic_Period_Status; Result : out RTEMS.Status_Codes ); @end example @end ifset @subheading DIRECTIVE STATUS CODES: @code{SUCCESSFUL} - period initiated successfully@* @code{INVALID_ID} - invalid rate monotonic period id@* @code{INVALID_ADDRESS} - invalid address of status@* @subheading DESCRIPTION: This directive returns status information associated with the rate monotonic period id in the following data @value{STRUCTURE}: @ifset is-C @example typedef struct @{ rtems_rate_monotonic_period_states state; unsigned32 ticks_since_last_period; unsigned32 ticks_executed_since_last_period; @} rtems_rate_monotonic_period_status; @end example @end ifset @ifset is-Ada @example type Rate_Monotonic_Period_Status is begin State : RTEMS.Rate_Monotonic_Period_States; Ticks_Since_Last_Period : RTEMS.Unsigned32; Ticks_Executed_Since_Last_Period : RTEMS.Unsigned32; end record; @end example @end ifset If the period's state is @code{RATE_MONOTONIC_INACTIVE}, both ticks_since_last_period and ticks_executed_since_last_period will be set to 0. Otherwise, ticks_since_last_period will contain the number of clock ticks which have occurred since the last invocation of the rtems_rate_monotonic_period directive. Also in this case, the ticks_executed_since_last_period will indicate how much processor time the owning task has consumed since the invocation of the rtems_rate_monotonic_period directive. @subheading NOTES: This directive will not cause the running task to be preempted.