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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 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. @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 insures 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 \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
1 100 Low
2 50 Medium
3 50 Medium
4 25 High
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 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_name name; rtems_id period; rtems_status_code status; name = rtems_build_name( 'P', 'E', 'R', 'D' ); status = 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 ( rate_monotonic_period( period, 100 ) == RTEMS_TIMEOUT ) break; /* Perform some periodic actions */ @} /* missed period so delete period and SELF */ status = rate_monotonic_delete( period ); if ( status != RTEMS_STATUS_SUCCESSFUL ) @{ printf( "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 task Periodic_task() @{ 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 ) 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 @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 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 @code{@value{DIRPREFIX}rate_monotonic_period( period_1, 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_1 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 @findex rtems_rate_monotonic_ident @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{@value{RPREFIX}SUCCESSFUL} - period identified successfully@* @code{@value{RPREFIX}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. @c @c @c @page @subsection RATE_MONOTONIC_CANCEL - Cancel a period @cindex cancel a period @subheading CALLING SEQUENCE: @ifset is-C @findex rtems_rate_monotonic_cancel @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{@value{RPREFIX}SUCCESSFUL} - period canceled successfully@* @code{@value{RPREFIX}INVALID_ID} - invalid rate monotonic period id@* @code{@value{RPREFIX}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 @code{@value{DIRPREFIX}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. @c @c @c @page @subsection RATE_MONOTONIC_DELETE - Delete a rate monotonic period @cindex delete a period @subheading CALLING SEQUENCE: @ifset is-C @findex rtems_rate_monotonic_delete @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{@value{RPREFIX}SUCCESSFUL} - period deleted successfully@* @code{@value{RPREFIX}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. @c @c @c @page @subsection RATE_MONOTONIC_PERIOD - Conclude current/Start next period @cindex conclude current period @cindex start current period @cindex period initiation @subheading CALLING SEQUENCE: @ifset is-C @findex rtems_rate_monotonic_period @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{@value{RPREFIX}SUCCESSFUL} - period initiated successfully@* @code{@value{RPREFIX}INVALID_ID} - invalid rate monotonic period id@* @code{@value{RPREFIX}NOT_OWNER_OF_RESOURCE} - period not created by calling task@* @code{@value{RPREFIX}NOT_DEFINED} - period has never been initiated (only possible when period is set to PERIOD_STATUS)@* @code{@value{RPREFIX}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{@value{RPREFIX}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. @c @c @c @page @subsection RATE_MONOTONIC_GET_STATUS - Obtain status information on period @cindex get status of period @cindex obtain status of period @subheading CALLING SEQUENCE: @ifset is-C @findex rtems_rate_monotonic_get_status @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{@value{RPREFIX}SUCCESSFUL} - period initiated successfully@* @code{@value{RPREFIX}INVALID_ID} - invalid rate monotonic period id@* @code{@value{RPREFIX}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 @findex rtems_rate_monotonic_period_status @example typedef struct @{ rtems_rate_monotonic_period_states state; rtems_unsigned32 ticks_since_last_period; rtems_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 @c RATE_MONOTONIC_INACTIVE does not have RTEMS_ in front of it. 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 @code{@value{DIRPREFIX}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 @code{@value{DIRPREFIX}rate_monotonic_period} directive. @subheading NOTES: This directive will not cause the running task to be preempted.