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c-user: Clarify CONFIGURE_MAXIMUM_PRIORITY

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Update #3875.
This commit is contained in:
Sebastian Huber 2020-02-19 13:53:53 +01:00
parent 2c37918140
commit 874a5ef117
1 changed files with 32 additions and 11 deletions
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c-user/configuring_a_system.rst
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@ -618,12 +618,22 @@ DEFAULT VALUE:
to 255.
DESCRIPTION:
This configuration parameter specified the maximum numeric priority of any
task in the system and one less that the number of priority levels in the
system.
For the schedulers
Reducing the number of priorities in the system reduces the amount of memory
allocated from the RTEMS Workspace.
* :ref:`SchedulerPriority`, which is the default in uniprocessor
configurations and can be configured through the
:ref:`CONFIGURE_SCHEDULER_PRIORITY` configuration option,
* :ref:`SchedulerSMPPriority` which can be configured through the
:ref:`CONFIGURE_SCHEDULER_PRIORITY_SMP` configuration option, and
* :ref:`SchedulerSMPPriorityAffinity` which can be configured through the
:ref:`CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP` configuration option
this configuration option specifies the maximum numeric priority of any task
for these schedulers and one less that the number of priority levels for
these schedulers. For all other schedulers provided by RTEMS, this
configuration option has no effect.
NOTES:
The numerically greatest priority is the logically lowest priority in the
@ -632,11 +642,13 @@ NOTES:
Priority zero (0) is reserved for internal use by RTEMS and is not available
to applications.
With some schedulers, reducing the number of priorities can reduce the
amount of memory used by the scheduler. For example, the Deterministic
Priority Scheduler (DPS) used by default uses three pointers of storage per
priority level. Reducing the number of priorities from 256 levels to
sixteen (16) can reduce memory usage by about three (3) kilobytes.
Reducing the number of priorities through this configuration option reduces
the amount of memory allocated by the schedulers listed above. These
schedulers use a chain control structure per priority and this structure
consists of three pointers. On a 32-bit architecture, the allocated memory
is 12 bytes * (``CONFIGURE_MAXIMUM_PRIORITY`` + 1), e.g. 3072 bytes for 256
priority levels (default), 48 bytes for 4 priority levels
(``CONFIGURE_MAXIMUM_PRIORITY == 3``).
.. index:: CONFIGURE_MAXIMUM_PROCESSORS
@ -1471,7 +1483,7 @@ DATA TYPE:
RTEMS Task Priority (``rtems_task_priority``).
RANGE:
One (1) to CONFIGURE_MAXIMUM_PRIORITY.
One (1) to the maximum user priority value of the scheduler.
DEFAULT VALUE:
The default value is 1, which is the highest priority in the Classic API.
@ -3541,6 +3553,9 @@ NOTES:
:ref:`CONFIGURE_MAXIMUM_PROCESSORS <CONFIGURE_MAXIMUM_PROCESSORS>` is
exactly one.
The memory allocated for this scheduler depends on the
:ref:`CONFIGURE_MAXIMUM_PRIORITY` configuration option.
.. index:: CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP
.. _CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP:
@ -3576,6 +3591,9 @@ NOTES:
<ConfigurationSchedulersClustered>` is present, then it is used as the
scheduler for up to 32 processors.
The memory allocated for this scheduler depends on the
:ref:`CONFIGURE_MAXIMUM_PRIORITY` configuration option.
.. index:: CONFIGURE_SCHEDULER_PRIORITY_SMP
.. _CONFIGURE_SCHEDULER_PRIORITY_SMP:
@ -3610,6 +3628,9 @@ NOTES:
<ConfigurationSchedulersClustered>` is present, then it is used as the
scheduler for up to 32 processors.
The memory allocated for this scheduler depends on the
:ref:`CONFIGURE_MAXIMUM_PRIORITY` configuration option.
.. index:: CONFIGURE_SCHEDULER_SIMPLE
.. _CONFIGURE_SCHEDULER_SIMPLE: