24.18. General Scheduler Configuration — RTEMS Classic API Guide 5.4-rc2 (21st August 2026) documentation

# 24.18. General Scheduler Configuration

This section describes configuration options related to selecting a scheduling algorithm for an application. A scheduler configuration is optional and only necessary in very specific circumstances. A normal application configuration does not need any of the configuration options described in this section.

By default, the Deterministic Priority Scheduler algorithm is used in uniprocessor configurations. In case SMP is enabled and the configured maximum processors (CONFIGURE_MAXIMUM_PROCESSORS) is greater than one, then the Earliest Deadline First (EDF) SMP Scheduler is selected as the default scheduler algorithm.

For the schedulers built into RTEMS, the configuration is straightforward. All that is required is to define the configuration option which specifies which scheduler you want for in your application.

The pluggable scheduler interface also enables the user to provide their own scheduling algorithm. If you choose to do this, you must define multiple configuration option.

## 24.18.1. CONFIGURE_CBS_MAXIMUM_SERVERS

CONSTANT:

  `CONFIGURE_CBS_MAXIMUM_SERVERS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is CONFIGURE_MAXIMUM_TASKS.

VALUE CONSTRAINTS:

  The value of this configuration option shall satisfy all of the following constraints:

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to `SIZE_MAX`.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

DESCRIPTION:

  The value of this configuration option defines the maximum number Constant Bandwidth Servers that can be concurrently active.

NOTES:

  This configuration option is only evaluated if the configuration option CONFIGURE_SCHEDULER_CBS is defined.

## 24.18.2. CONFIGURE_MAXIMUM_PRIORITY

CONSTANT:

  `CONFIGURE_MAXIMUM_PRIORITY`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 255.

VALUE CONSTRAINTS:

  The value of this configuration option shall be an element of {3, 7, 31, 63, 127, 255}.

DESCRIPTION:

  For the following schedulers

  - Deterministic Priority Scheduler, which is the default in uniprocessor configurations and can be configured through the CONFIGURE_SCHEDULER_PRIORITY configuration option,

  - Deterministic Priority SMP Scheduler which can be configured through the CONFIGURE_SCHEDULER_PRIORITY_SMP configuration option, and

  - Arbitrary Processor Affinity Priority SMP Scheduler which can be configured through the 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 system and will thus be used by the IDLE task.

  Priority zero is reserved for internal use by RTEMS and is not available to applications.

  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`).

  The default value is 255, because RTEMS shall support 256 priority levels to be compliant with various standards. These priorities range from 0 to 255.

## 24.18.3. CONFIGURE_SCHEDULER_ASSIGNMENTS

CONSTANT:

  `CONFIGURE_SCHEDULER_ASSIGNMENTS`

OPTION TYPE:

  This configuration option is an initializer define.

DEFAULT VALUE:

  The default value of this configuration option is computed so that the default scheduler is assigned to each configured processor (up to 32).

VALUE CONSTRAINTS:

  The value of this configuration option shall satisfy all of the following constraints:

  - It shall be a list of the following macros:

    - `RTEMS_SCHEDULER_ASSIGN( processor_index, attributes )`

    - `RTEMS_SCHEDULER_ASSIGN_NO_SCHEDULER`

  - It shall be a list of exactly CONFIGURE_MAXIMUM_PROCESSORS elements.

DESCRIPTION:

  The value of this configuration option is used to initialize the initial scheduler to processor assignments.

NOTES:

  This configuration option is only evaluated in SMP configurations.

  This is an advanced configuration option, see Clustered Scheduler Configuration.

## 24.18.4. CONFIGURE_SCHEDULER_CBS

CONSTANT:

  `CONFIGURE_SCHEDULER_CBS`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Constant Bandwidth Server (CBS) Scheduler algorithm is made available to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  In case no explicit clustered scheduler configuration is present, then it is used as the scheduler for exactly one processor.

## 24.18.5. CONFIGURE_SCHEDULER_EDF

CONSTANT:

  `CONFIGURE_SCHEDULER_EDF`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Earliest Deadline First (EDF) Scheduler algorithm is made available to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  In case no explicit clustered scheduler configuration is present, then it is used as the scheduler for exactly one processor.

## 24.18.6. CONFIGURE_SCHEDULER_EDF_SMP

CONSTANT:

  `CONFIGURE_SCHEDULER_EDF_SMP`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Earliest Deadline First (EDF) SMP Scheduler algorithm is made available to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  This scheduler algorithm is only available when RTEMS is built with SMP support enabled.

  In case no explicit clustered scheduler configuration is present, then it is used as the scheduler for up to 32 processors.

  This scheduler algorithm is the default in SMP configurations if CONFIGURE_MAXIMUM_PROCESSORS is greater than one.

## 24.18.7. CONFIGURE_SCHEDULER_NAME

CONSTANT:

  `CONFIGURE_SCHEDULER_NAME`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is

  - `"MEDF"` for the EDF SMP Scheduler,

  - `"MPA "` for the Arbitrary Processor Affinity Priority SMP Scheduler,

  - `"MPD "` for the Deterministic Priority SMP Scheduler,

  - `"MPS "` for the Simple Priority SMP Scheduler,

  - `"UCBS"` for the Uniprocessor CBS Scheduler,

  - `"UEDF"` for the Uniprocessor EDF Scheduler,

  - `"UPD "` for the Uniprocessor Deterministic Priority Scheduler, and

  - `"UPS "` for the Uniprocessor Simple Priority Scheduler.

VALUE CONSTRAINTS:

  The value of this configuration option shall be a valid integer of type `rtems_name`.

DESCRIPTION:

  The value of this configuration option defines the name of the default scheduler.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  Schedulers can be identified via c:func:rtems_scheduler_ident.

  Use `rtems_build_name()` to define the scheduler name.

## 24.18.8. CONFIGURE_SCHEDULER_PRIORITY

CONSTANT:

  `CONFIGURE_SCHEDULER_PRIORITY`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Deterministic Priority Scheduler algorithm is made available to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  In case no explicit clustered scheduler configuration is present, then it is used as the scheduler for exactly one processor.

  This scheduler algorithm is the default when CONFIGURE_MAXIMUM_PROCESSORS is exactly one.

  The memory allocated for this scheduler depends on the CONFIGURE_MAXIMUM_PRIORITY configuration option.

## 24.18.9. CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP

CONSTANT:

  `CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Arbitrary Processor Affinity SMP Scheduler algorithm is made available to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  This scheduler algorithm is only available when RTEMS is built with SMP support enabled.

  In case no explicit clustered scheduler configuration is present, then it is used as the scheduler for up to 32 processors.

  The memory allocated for this scheduler depends on the CONFIGURE_MAXIMUM_PRIORITY configuration option.

## 24.18.10. CONFIGURE_SCHEDULER_PRIORITY_SMP

CONSTANT:

  `CONFIGURE_SCHEDULER_PRIORITY_SMP`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Deterministic Priority SMP Scheduler algorithm is made available to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  This scheduler algorithm is only available when RTEMS is built with SMP support enabled.

  In case no explicit clustered scheduler configuration is present, then it is used as the scheduler for up to 32 processors.

  The memory allocated for this scheduler depends on the CONFIGURE_MAXIMUM_PRIORITY configuration option.

## 24.18.11. CONFIGURE_SCHEDULER_SIMPLE

CONSTANT:

  `CONFIGURE_SCHEDULER_SIMPLE`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Simple Priority Scheduler algorithm is made available to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  In case no explicit clustered scheduler configuration is present, then it is used as the scheduler for exactly one processor.

## 24.18.12. CONFIGURE_SCHEDULER_SIMPLE_SMP

CONSTANT:

  `CONFIGURE_SCHEDULER_SIMPLE_SMP`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Simple Priority SMP Scheduler algorithm is made available to the application. application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  This scheduler algorithm is only available when RTEMS is built with SMP support enabled.

  In case no explicit clustered scheduler configuration is present, then it is used as the scheduler for up to 32 processors.

## 24.18.13. CONFIGURE_SCHEDULER_STRONG_APA

CONSTANT:

  `CONFIGURE_SCHEDULER_STRONG_APA`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then Strong APA algorithm is made available to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  This scheduler algorithm is only available when RTEMS is built with SMP support enabled.

  This scheduler algorithm is not correctly implemented. Do not use it.

## 24.18.14. CONFIGURE_SCHEDULER_USER

CONSTANT:

  `CONFIGURE_SCHEDULER_USER`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the described feature is not enabled.

DESCRIPTION:

  In case this configuration option is defined, then the user shall provide a scheduler algorithm to the application.

NOTES:

  This scheduler configuration option is an advanced configuration option. Think twice before you use it.

  RTEMS allows the application to provide its own task/thread scheduling algorithm. In order to do this, one shall define `CONFIGURE_SCHEDULER_USER` to indicate the application provides its own scheduling algorithm. If `CONFIGURE_SCHEDULER_USER` is defined then the following additional macros shall be defined:

  - `CONFIGURE_SCHEDULER` shall be defined to a static definition of the scheduler data structures of the user scheduler.

  - `CONFIGURE_SCHEDULER_TABLE_ENTRIES` shall be defined to a scheduler table entry initializer for the user scheduler.

  - `CONFIGURE_SCHEDULER_USER_PER_THREAD` shall be defined to the type of the per-thread information of the user scheduler.

  At this time, the mechanics and requirements for writing a new scheduler are evolving and not fully documented. It is recommended that you look at the existing Deterministic Priority Scheduler in `cpukit/score/src/schedulerpriority*.c` for guidance. For guidance on the configuration macros, please examine `cpukit/sapi/include/confdefs.h` for how these are defined for the Deterministic Priority Scheduler.