# 26.18. 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 SMP Scheduler is selected as the default scheduler algorithm.

For the schedulers provided by RTEMS (see Scheduling Concepts), 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.

## 26.18.1. CONFIGURE_CBS_MAXIMUM_SERVERS¶

CONSTANT:

CONFIGURE_CBS_MAXIMUM_SERVERS

OPTION TYPE:

This configuration option is an integer define.

DEFAULT VALUE:

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.

CONSTRAINTS:

The following constraints apply to this configuration option:

• The value of the configuration option shall be greater than or equal to zero.

• The value of the configuration option shall be less than or equal to SIZE_MAX.

• The value of the configuration option 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.

## 26.18.2. CONFIGURE_MAXIMUM_PRIORITY¶

CONSTANT:

CONFIGURE_MAXIMUM_PRIORITY

OPTION TYPE:

This configuration option is an integer define.

DEFAULT VALUE:

The default value is 255.

DESCRIPTION:

For the following schedulers

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.

CONSTRAINTS:

The value of the configuration option shall be equal to 3, 7, 31, 63, 127, or 255.

## 26.18.3. 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).

DESCRIPTION:

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

NOTES:

Where the system was built with SMP support enabled, this configuration option is evaluated, otherwise it is ignored.

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

CONSTRAINTS:

The following constraints apply to this configuration option:

• The value of the configuration option shall be a list of the following macros:

• RTEMS_SCHEDULER_ASSIGN( scheduler_index, attributes )

• RTEMS_SCHEDULER_ASSIGN_NO_SCHEDULER

The scheduler_index macro parameter shall be a valid index of the scheduler table defined by the CONFIGURE_SCHEDULER_TABLE_ENTRIES configuration option.

The attributes macro parameter shall be set to exactly one of the following constants:

• RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY

• RTEMS_SCHEDULER_ASSIGN_PROCESSOR_OPTIONAL

• The value of the configuration option shall be a list of exactly CONFIGURE_MAXIMUM_PROCESSORS elements.

## 26.18.4. 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 the Constant Bandwidth Server Scheduling (CBS) 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.

## 26.18.5. 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 the Earliest Deadline First 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.

## 26.18.6. 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 the Earliest Deadline First 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.

## 26.18.7. CONFIGURE_SCHEDULER_NAME¶

CONSTANT:

CONFIGURE_SCHEDULER_NAME

OPTION TYPE:

This configuration option is an integer define.

DEFAULT VALUE:

The default value is

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 rtems_scheduler_ident().

Use rtems_build_name() to define the scheduler name.

CONSTRAINTS:

The value of the configuration option shall be convertible to an integer of type rtems_name.

## 26.18.8. 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 the 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.

## 26.18.9. 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 the Arbitrary Processor Affinity 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.

## 26.18.10. 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 the 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.

## 26.18.11. 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 the 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.

## 26.18.12. 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 the Simple 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.

## 26.18.13. 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 the 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.

## 26.18.14. CONFIGURE_SCHEDULER_TABLE_ENTRIES¶

CONSTANT:

CONFIGURE_SCHEDULER_TABLE_ENTRIES

OPTION TYPE:

This configuration option is an initializer define.

DEFAULT VALUE:

The default value of this configuration option is the definition of exactly one table entry for the configured scheduler.

DESCRIPTION:

The value of this configuration option is used to initialize the table of configured schedulers.

NOTES:

Schedulers registered in the scheduler table by this configuration option are available to the application. The scheduler table entry index defines the index of the scheduler.

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

CONSTRAINTS:

The following constraints apply to this configuration option:

• The value of the configuration option shall be a list of the following macros:

• RTEMS_SCHEDULER_TABLE_CBS( name, obj_name )

• RTEMS_SCHEDULER_TABLE_EDF( name, obj_name )

• RTEMS_SCHEDULER_TABLE_EDF_SMP( name, obj_name )

• RTEMS_SCHEDULER_TABLE_PRIORITY_AFFINITY_SMP( name, obj_name )

• RTEMS_SCHEDULER_TABLE_PRIORITY( name, obj_name )

• RTEMS_SCHEDULER_TABLE_PRIORITY_SMP( name, obj_name )

• RTEMS_SCHEDULER_TABLE_SIMPLE( name, obj_name )

• RTEMS_SCHEDULER_TABLE_SIMPLE_SMP( name, obj_name )

• RTEMS_SCHEDULER_TABLE_STRONG_APA( name, obj_name )

The name macro parameter shall be the name associated with the scheduler data structures, see Clustered Scheduler Configuration.

The obj_name macro parameter shall be the scheduler object name. It is recommended to define the scheduler object name through rtems_build_name().

• Where the system was build with SMP support enabled, the table shall have one or more entries, otherwise it shall have exactly one entry.

## 26.18.15. 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.