rtems-docs/c_user/scheduling_concepts.rst

Scheduling Concepts
###################

.. index:: scheduling
.. index:: task scheduling

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

The concept of scheduling in real-time systems dictates the ability to
provide immediate response to specific external events, particularly
the necessity of scheduling tasks to run within a specified time limit
after the occurrence of an event.  For example, software embedded in
life-support systems used to monitor hospital patients must take instant
action if a change in the patient’s status is detected.

The component of RTEMS responsible for providing this capability is
appropriately called the scheduler.  The scheduler’s sole purpose is
to allocate the all important resource of processor time to the various
tasks competing for attention.

Scheduling Algorithms
=====================

.. index:: scheduling algorithms

RTEMS provides a plugin framework which allows it to support
multiple scheduling algorithms. RTEMS now includes multiple
scheduling algorithms in the SuperCore and the user can select which
of these they wish to use in their application.  In addition,
the user can implement their own scheduling algorithm and
configure RTEMS to use it.

Supporting multiple scheduling algorithms gives the end user the
option to select the algorithm which is most appropriate to their use
case. Most real-time operating systems schedule tasks using a priority
based algorithm, possibly with preemption control.  The classic
RTEMS scheduling algorithm which was the only algorithm available
in RTEMS 4.10 and earlier, is a priority based scheduling algorithm.
This scheduling algoritm is suitable for single core (e.g. non-SMP)
systems and is now known as the *Deterministic Priority Scheduler*.
Unless the user configures another scheduling algorithm, RTEMS will use
this on single core systems.

Priority Scheduling
-------------------
.. index:: priority scheduling

When using priority based scheduling, RTEMS allocates the processor using
a priority-based, preemptive algorithm augmented to provide round-robin
characteristics within individual priority groups.  The goal of this
algorithm is to guarantee that the task which is executing on the
processor at any point in time is the one with the highest priority
among all tasks in the ready state.

When a task is added to the ready chain, it is placed behind all other
tasks of the same priority.  This rule provides a round-robin within
priority group scheduling characteristic.  This means that in a group of
equal priority tasks, tasks will execute in the order they become ready
or FIFO order.  Even though there are ways to manipulate and adjust task
priorities, the most important rule to remember is:

- *Priority based scheduling algorithms will always select the
  highest priority task that is ready to run when allocating the processor
  to a task.*

Priority scheduling is the most commonly used scheduling algorithm.
It should be used by applications in which multiple tasks contend for
CPU time or other resources and there is a need to ensure certain tasks
are given priority over other tasks.

There are a few common methods of accomplishing the mechanics of this
algorithm.  These ways involve a list or chain of tasks in the ready state.

- The least efficient method is to randomly place tasks in the ready
  chain forcing the scheduler to scan the entire chain to determine which
  task receives the processor.

- A more efficient method is to schedule the task by placing it
  in the proper place on the ready chain based on the designated scheduling
  criteria at the time it enters the ready state.  Thus, when the processor
  is free, the first task on the ready chain is allocated the processor.

- Another mechanism is to maintain a list of FIFOs per priority.
  When a task is readied, it is placed on the rear of the FIFO for its
  priority.  This method is often used with a bitmap to assist in locating
  which FIFOs have ready tasks on them.

RTEMS currently includes multiple priority based scheduling algorithms
as well as other algorithms which incorporate deadline.  Each algorithm
is discussed in the following sections.

Deterministic Priority Scheduler
--------------------------------

This is the scheduler implementation which has always been in RTEMS.
After the 4.10 release series, it was factored into pluggable scheduler
selection.  It schedules tasks using a priority based algorithm which
takes into account preemption.  It is implemented using an array of FIFOs
with a FIFO per priority.  It maintains a bitmap which is used to track
which priorities have ready tasks.

This algorithm is deterministic (e.g. predictable and fixed) in execution
time.  This comes at the cost of using slightly over three (3) kilobytes
of RAM on a system configured to support 256 priority levels.

This scheduler is only aware of a single core.

Simple Priority Scheduler
-------------------------

This scheduler implementation has the same behaviour as the Deterministic
Priority Scheduler but uses only one linked list to manage all ready
tasks.  When a task is readied, a linear search of that linked list is
performed to determine where to insert the newly readied task.

This algorithm uses much less RAM than the Deterministic Priority
Scheduler but is *O(n)* where *n* is the number of ready tasks.
In a small system with a small number of tasks, this will not be a
performance issue.  Reducing RAM consumption is often critical in small
systems which are incapable of supporting a large number of tasks.

This scheduler is only aware of a single core.

Simple SMP Priority Scheduler
-----------------------------

This scheduler is based upon the Simple Priority Scheduler and is designed
to have the same behaviour on a single core system.  But this scheduler
is capable of scheduling threads across multiple cores in an SMP system.
When given a choice of replacing one of two threads at equal priority
on different cores, this algorithm favors replacing threads which are
preemptible and have executed the longest.

This algorithm is non-deterministic. When scheduling, it must consider
which tasks are to be executed on each core while avoiding superfluous
task migrations.

Earliest Deadline First Scheduler
---------------------------------
.. index:: earliest deadline first scheduling

This is an alternative scheduler in RTEMS for single core applications.
The primary EDF advantage is high total CPU utilization (theoretically
up to 100%). It assumes that tasks have priorities equal to deadlines.

This EDF is initially preemptive, however, individual tasks may be declared
not-preemptive. Deadlines are declared using only Rate Monotonic manager which
goal is to handle periodic behavior. Period is always equal to deadline. All
ready tasks reside in a single ready queue implemented using a red-black tree.

This implementation of EDF schedules two different types of task
priority types while each task may switch between the two types within
its execution. If a task does have a deadline declared using the Rate
Monotonic manager, the task is deadline-driven and its priority is equal
to deadline.  On the contrary if a task does not have any deadline or
the deadline is cancelled using the Rate Monotonic manager, the task is
considered a background task with priority equal to that assigned
upon initialization in the same manner as for priority scheduler. Each
background task is of a lower importance than each deadline-driven one
and is scheduled when no deadline-driven task and no higher priority
background task is ready to run.

Every deadline-driven scheduling algorithm requires means for tasks
to claim a deadline.  The Rate Monotonic Manager is responsible for
handling periodic execution. In RTEMS periods are equal to deadlines,
thus if a task announces a period, it has to be finished until the
end of this period. The call of ``rtems_rate_monotonic_period``
passes the scheduler the length of oncoming deadline. Moreover, the``rtems_rate_monotonic_cancel`` and ``rtems_rate_monotonic_delete``
calls clear the deadlines assigned to the task.

Constant Bandwidth Server Scheduling (CBS)
------------------------------------------
.. index:: constant bandwidth server scheduling

This is an alternative scheduler in RTEMS for single core applications.
The CBS is a budget aware extension of EDF scheduler. The main goal of this
scheduler is to ensure temporal isolation of tasks meaning that a task’s
execution in terms of meeting deadlines must not be influenced by other
tasks as if they were run on multiple independent processors.

Each task can be assigned a server (current implementation supports only
one task per server). The server is characterized by period (deadline)
and computation time (budget). The ratio budget/period yields bandwidth,
which is the fraction of CPU to be reserved by the scheduler for each
subsequent period.

The CBS is equipped with a set of rules applied to tasks attached to servers
ensuring that deadline miss because of another task cannot occur.
In case a task breaks one of the rules, its priority is pulled to background
until the end of its period and then restored again. The rules are:

- Task cannot exceed its registered budget,

- Task cannot be
  unblocked when a ratio between remaining budget and remaining deadline
  is higher than declared bandwidth.

The CBS provides an extensive API. Unlike EDF, the``rtems_rate_monotonic_period`` does not declare a deadline because
it is carried out using CBS API. This call only announces next period.

Scheduling Modification Mechanisms
==================================

.. index:: scheduling mechanisms

RTEMS provides four mechanisms which allow the user to alter the task
scheduling decisions:

- user-selectable task priority level

- task preemption control

- task timeslicing control

- manual round-robin selection

Each of these methods provides a powerful capability to customize sets
of tasks to satisfy the unique and particular requirements encountered
in custom real-time applications.  Although each mechanism operates
independently, there is a precedence relationship which governs the
effects of scheduling modifications.  The evaluation order for scheduling
characteristics is always priority, preemption mode, and timeslicing.
When reading the descriptions of timeslicing and manual round-robin
it is important to keep in mind that preemption (if enabled) of a task
by higher priority tasks will occur as required, overriding the other
factors presented in the description.

Task Priority and Scheduling
----------------------------
.. index:: task priority

The most significant task scheduling modification mechanism is the ability
for the user to assign a priority level to each individual task when it
is created and to alter a task’s priority at run-time.  RTEMS supports
up to 255 priority levels.  Level 255 is the lowest priority and level
1 is the highest.

Preemption
----------.. index:: preemption

Another way the user can alter the basic scheduling algorithm is by
manipulating the preemption mode flag (``RTEMS_PREEMPT_MASK``)
of individual tasks.  If preemption is disabled for a task
(``RTEMS_NO_PREEMPT``), then the task will not relinquish
control of the processor until it terminates, blocks, or re-enables
preemption.  Even tasks which become ready to run and possess higher
priority levels will not be allowed to execute.  Note that the preemption
setting has no effect on the manner in which a task is scheduled.
It only applies once a task has control of the processor.

Timeslicing
-----------.. index:: timeslicing
.. index:: round robin scheduling

Timeslicing or round-robin scheduling is an additional method which
can be used to alter the basic scheduling algorithm.  Like preemption,
timeslicing is specified on a task by task basis using the timeslicing
mode flag (``RTEMS_TIMESLICE_MASK``).  If timeslicing is
enabled for a task (``RTEMS_TIMESLICE``), then RTEMS will
limit the amount of time the task can execute before the processor is
allocated to another task.  Each tick of the real-time clock reduces
the currently running task’s timeslice.  When the execution time equals
the timeslice, RTEMS will dispatch another task of the same priority
to execute.  If there are no other tasks of the same priority ready to
execute, then the current task is allocated an additional timeslice and
continues to run.  Remember that a higher priority task will preempt
the task (unless preemption is disabled) as soon as it is ready to run,
even if the task has not used up its entire timeslice.

Manual Round-Robin
------------------.. index:: manual round robin

The final mechanism for altering the RTEMS scheduling algorithm is
called manual round-robin.  Manual round-robin is invoked by using the``rtems_task_wake_after`` directive with a time interval
of ``RTEMS_YIELD_PROCESSOR``.  This allows a task to give
up the processor and be immediately returned to the ready chain at the
end of its priority group.  If no other tasks of the same priority are
ready to run, then the task does not lose control of the processor.

Dispatching Tasks
=================.. index:: dispatching

The dispatcher is the RTEMS component responsible for
allocating the processor to a ready task.  In order to allocate
the processor to one task, it must be deallocated or retrieved
from the task currently using it.  This involves a concept
called a context switch.  To perform a context switch, the
dispatcher saves the context of the current task and restores
the context of the task which has been allocated to the
processor.  Saving and restoring a task’s context is the
storing/loading of all the essential information about a task to
enable it to continue execution without any effects of the
interruption.  For example, the contents of a task’s register
set must be the same when it is given the processor as they were
when it was taken away.  All of the information that must be
saved or restored for a context switch is located either in the
TCB or on the task’s stacks.

Tasks that utilize a numeric coprocessor and are created with the``RTEMS_FLOATING_POINT`` attribute require additional
operations during a context switch.  These additional operations
are necessary to save and restore the floating point context of``RTEMS_FLOATING_POINT`` tasks.  To avoid unnecessary save
and restore operations, the state of the numeric coprocessor is only
saved when a ``RTEMS_FLOATING_POINT`` task is dispatched
and that task was not the last task to utilize the coprocessor.

Task State Transitions
======================.. index:: task state transitions

Tasks in an RTEMS system must always be in one of the
five allowable task states.  These states are: executing, ready,
blocked, dormant, and non-existent.

A task occupies the non-existent state before
a ``rtems_task_create`` has been issued on its behalf.
A task enters the non-existent state from any other state in the system
when it is deleted with the ``rtems_task_delete`` directive.
While a task occupies this state it does not have a TCB or a task ID
assigned to it; therefore, no other tasks in the system may reference
this task.

When a task is created via the ``rtems_task_create``
directive it enters the dormant state.  This state is not entered through
any other means.  Although the task exists in the system, it cannot
actively compete for system resources.  It will remain in the dormant
state until it is started via the ``rtems_task_start``
directive, at which time it enters the ready state.  The task is now
permitted to be scheduled for the processor and to compete for other
system resources.

.. code:: c

    +-------------------------------------------------------------+
    |                         Non-existent                        |
    |  +-------------------------------------------------------+  |
    |  |                                                       |  |
    |  |                                                       |  |
    |  |      Creating        +---------+     Deleting         |  |
    |  | -------------------> | Dormant | -------------------> |  |
    |  |                      +---------+                      |  |
    |  |                           |                           |  |
    |  |                  Starting |                           |  |
    |  |                           |                           |  |
    |  |                           V          Deleting         |  |
    |  |             +-------> +-------+ ------------------->  |  |
    |  |  Yielding  /   +----- | Ready | ------+               |  |
    |  |           /   /       +-------+ <--+   \\              |  |
    |  |          /   /                      \\   \\ Blocking    |  |
    |  |         /   / Dispatching   Readying \\   \\            |  |
    |  |        /   V                          \\   V           |  |
    |  |      +-----------+    Blocking     +---------+        |  |
    |  |      | Executing | --------------> | Blocked |        |  |
    |  |      +-----------+                 +---------+        |  |
    |  |                                                       |  |
    |  |                                                       |  |
    |  +-------------------------------------------------------+  |
    |                         Non-existent                        |
    +-------------------------------------------------------------+

A task occupies the blocked state whenever it is unable to be scheduled
to run.  A running task may block itself or be blocked by other tasks in
the system.  The running task blocks itself through voluntary operations
that cause the task to wait.  The only way a task can block a task other
than itself is with the ``rtems_task_suspend`` directive.
A task enters the blocked state due to any of the following conditions:

- A task issues a ``rtems_task_suspend`` directive
  which blocks either itself or another task in the system.

- The running task issues a ``rtems_barrier_wait``
  directive.

- The running task issues a ``rtems_message_queue_receive``
  directive with the wait option and the message queue is empty.

- The running task issues an ``rtems_event_receive``
  directive with the wait option and the currently pending events do not
  satisfy the request.

- The running task issues a ``rtems_semaphore_obtain``
  directive with the wait option and the requested semaphore is unavailable.

- The running task issues a ``rtems_task_wake_after``
  directive which blocks the task for the given time interval.  If the time
  interval specified is zero, the task yields the processor and remains
  in the ready state.

- The running task issues a ``rtems_task_wake_when``
  directive which blocks the task until the requested date and time arrives.

- The running task issues a ``rtems_rate_monotonic_period``
  directive and must wait for the specified rate monotonic period
  to conclude.

- The running task issues a ``rtems_region_get_segment``
  directive with the wait option and there is not an available segment large
  enough to satisfy the task’s request.

A blocked task may also be suspended.  Therefore, both the suspension
and the blocking condition must be removed before the task becomes ready
to run again.

A task occupies the ready state when it is able to be scheduled to run,
but currently does not have control of the processor.  Tasks of the same
or higher priority will yield the processor by either becoming blocked,
completing their timeslice, or being deleted.  All tasks with the same
priority will execute in FIFO order.  A task enters the ready state due
to any of the following conditions:

- A running task issues a ``rtems_task_resume``
  directive for a task that is suspended and the task is not blocked
  waiting on any resource.

- A running task issues a ``rtems_message_queue_send``,``rtems_message_queue_broadcast``, or a``rtems_message_queue_urgent`` directive
  which posts a message to the queue on which the blocked task is
  waiting.

- A running task issues an ``rtems_event_send``
  directive which sends an event condition to a task which is blocked
  waiting on that event condition.

- A running task issues a ``rtems_semaphore_release``
  directive which releases the semaphore on which the blocked task is
  waiting.

- A timeout interval expires for a task which was blocked
  by a call to the ``rtems_task_wake_after`` directive.

- A timeout period expires for a task which blocked by a
  call to the ``rtems_task_wake_when`` directive.

- A running task issues a ``rtems_region_return_segment``
  directive which releases a segment to the region on which the blocked task
  is waiting and a resulting segment is large enough to satisfy
  the task’s request.

- A rate monotonic period expires for a task which blocked
  by a call to the ``rtems_rate_monotonic_period`` directive.

- A timeout interval expires for a task which was blocked
  waiting on a message, event, semaphore, or segment with a
  timeout specified.

- A running task issues a directive which deletes a
  message queue, a semaphore, or a region on which the blocked
  task is waiting.

- A running task issues a ``rtems_task_restart``
  directive for the blocked task.

- The running task, with its preemption mode enabled, may
  be made ready by issuing any of the directives that may unblock
  a task with a higher priority.  This directive may be issued
  from the running task itself or from an ISR.
  A ready task occupies the executing state when it has
  control of the CPU.  A task enters the executing state due to
  any of the following conditions:

- The task is the highest priority ready task in the
  system.

- The running task blocks and the task is next in the
  scheduling queue.  The task may be of equal priority as in
  round-robin scheduling or the task may possess the highest
  priority of the remaining ready tasks.

- The running task may reenable its preemption mode and a
  task exists in the ready queue that has a higher priority than
  the running task.

- The running task lowers its own priority and another
  task is of higher priority as a result.

- The running task raises the priority of a task above its
  own and the running task is in preemption mode.

.. COMMENT: COPYRIGHT (c) 1988-2008.

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

.. COMMENT: All rights reserved.