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

# 24.19. Clustered Scheduler Configuration

This section describes configuration options related to clustered scheduling. A clustered scheduler configuration is optional. It is an advanced configuration area and only necessary in specific circumstances.

Clustered scheduling helps to control the worst-case latencies in a multiprocessor system (SMP). The goal is to reduce the amount of shared state in the system and thus prevention of lock contention. Modern multiprocessor systems tend to have several layers of data and instruction caches. With clustered scheduling it is possible to honour the cache topology of a system and thus avoid expensive cache synchronization traffic.

We have clustered scheduling in case the set of processors of a system is partitioned into non-empty pairwise-disjoint subsets. These subsets are called clusters. Clusters with a cardinality of one are partitions. Each cluster is owned by exactly one scheduler.

In order to use clustered scheduling the application designer has to answer two questions.

1. How is the set of processors partitioned into clusters?

2. Which scheduler algorithm is used for which cluster?

The schedulers are statically configured.

## 24.19.1. Configuration Step 1 - Scheduler Algorithms

Firstly, the application must select which scheduling algorithms are available with the following defines

- CONFIGURE_SCHEDULER_EDF_SMP,

- CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP,

- CONFIGURE_SCHEDULER_PRIORITY_SMP, and

- CONFIGURE_SCHEDULER_SIMPLE_SMP.

This is necessary to calculate the per-thread overhead introduced by the scheduler algorithms. After these definitions the configuration file must `#include <rtems/scheduler.h>` to have access to scheduler-specific configuration macros.

It is possible to make more than one scheduler algorithm available to the application. For example a Simple Priority SMP Scheduler could be used in a partition for low latency tasks in addition to an EDF SMP Scheduler for a general-purpose cluster. Since the per-thread overhead depends on the scheduler algorithm only the scheduler algorithms used by the application should be configured.

## 24.19.2. Configuration Step 2 - Schedulers

Each scheduler needs some data structures. Use the following macros to create the scheduler data structures for a particular scheduler identified in the configuration by `name`.

- `RTEMS_SCHEDULER_EDF_SMP(name)`,

- `RTEMS_SCHEDULER_PRIORITY_AFFINITY_SMP(name, prio_count)`,

- `RTEMS_SCHEDULER_PRIORITY_SMP(name, prio_count)`, and

- `RTEMS_SCHEDULER_SIMPLE_SMP(name)`.

The `name` parameter is used as part of a designator for scheduler-specific data structures, so the usual C/C++ designator rules apply. This `name` is not the scheduler object name. Additional parameters are scheduler-specific.

## 24.19.3. Configuration Step 3 - Scheduler Table

The schedulers are registered in the system via the scheduler table. To populate the scheduler table define `CONFIGURE_SCHEDULER_TABLE_ENTRIES` to a list of the following scheduler table entry initializers

- `RTEMS_SCHEDULER_TABLE_EDF_SMP(name, obj_name)`,

- `RTEMS_SCHEDULER_TABLE_PRIORITY_AFFINITY_SMP(name, obj_name)`,

- `RTEMS_SCHEDULER_TABLE_PRIORITY_SMP(name, obj_name)`, and

- `RTEMS_SCHEDULER_TABLE_SIMPLE_SMP(name, obj_name)`.

The `name` parameter must correspond to the parameter defining the scheduler data structures of configuration step 2. The `obj_name` determines the scheduler object name and can be used in rtems_scheduler_ident() to get the scheduler object identifier. The scheduler index is defined by the index of the scheduler table. It is a configuration error to add a scheduler multiple times to the scheduler table.

## 24.19.4. Configuration Step 4 - Processor to Scheduler Assignment

The last step is to define which processor uses which scheduler. For this purpose a scheduler assignment table must be defined. The entry count of this table must be equal to the configured maximum processors (CONFIGURE_MAXIMUM_PROCESSORS). A processor assignment to a scheduler can be optional or mandatory. The boot processor must have a scheduler assigned. In case the system needs more mandatory processors than available then a fatal run-time error will occur. To specify the scheduler assignments define `CONFIGURE_SCHEDULER_ASSIGNMENTS` to a list of

- `RTEMS_SCHEDULER_ASSIGN(scheduler_index, attr)` and

- `RTEMS_SCHEDULER_ASSIGN_NO_SCHEDULER`

macros. The `scheduler_index` parameter must be a valid index into the scheduler table defined by configuration step 3. The `attr` parameter defines the scheduler assignment attributes. By default, a scheduler assignment to a processor is optional. For the scheduler assignment attribute use one of the mutually exclusive variants

- `RTEMS_SCHEDULER_ASSIGN_DEFAULT`,

- `RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY`, and

- `RTEMS_SCHEDULER_ASSIGN_PROCESSOR_OPTIONAL`.

It is possible to add/remove processors to/from schedulers at run-time, see rtems_scheduler_add_processor() and rtems_scheduler_remove_processor().

## 24.19.5. Configuration Example

The following example shows a scheduler configuration for a hypothetical product using two chip variants. One variant has four processors which is used for the normal product line and another provides eight processors for the high-performance product line. The first processor performs hard-real time control of actuators and sensors. The second processor is not used by RTEMS at all and runs a Linux instance to provide a graphical user interface. The additional processors are used for a worker thread pool to perform data processing operations.

The processors managed by RTEMS use two Deterministic Priority SMP schedulers capable of dealing with 256 priority levels. The scheduler with index zero has the name `"IO "`. The scheduler with index one has the name `"WORK"`. The scheduler assignments of the first, third and fourth processor are mandatory, so the system must have at least four processors, otherwise a fatal run-time error will occur during system startup. The processor assignments for the fifth up to the eighth processor are optional so that the same application can be used for the normal and high-performance product lines. The second processor has no scheduler assigned and runs Linux. A hypervisor will ensure that the two systems cannot interfere in an undesirable way.

    #define CONFIGURE_MAXIMUM_PROCESSORS 8
    #define CONFIGURE_MAXIMUM_PRIORITY 255

    /* Configuration Step 1 - Scheduler Algorithms */
    #define CONFIGURE_SCHEDULER_PRIORITY_SMP
    #include <rtems/scheduler.h>

    /* Configuration Step 2 - Schedulers */
    RTEMS_SCHEDULER_PRIORITY_SMP(io, CONFIGURE_MAXIMUM_PRIORITY + 1);
    RTEMS_SCHEDULER_PRIORITY_SMP(work, CONFIGURE_MAXIMUM_PRIORITY + 1);

    /* Configuration Step 3 - Scheduler Table */
    #define CONFIGURE_SCHEDULER_TABLE_ENTRIES \
      RTEMS_SCHEDULER_TABLE_PRIORITY_SMP( \
        io, \
         rtems_build_name('I', 'O', ' ', ' ') \
      ), \
      RTEMS_SCHEDULER_TABLE_PRIORITY_SMP( \
        work, \
        rtems_build_name('W', 'O', 'R', 'K') \
      )

    /* Configuration Step 4 - Processor to Scheduler Assignment */
    #define CONFIGURE_SCHEDULER_ASSIGNMENTS \
      RTEMS_SCHEDULER_ASSIGN(0, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY), \
      RTEMS_SCHEDULER_ASSIGN_NO_SCHEDULER, \
      RTEMS_SCHEDULER_ASSIGN(1, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY), \
      RTEMS_SCHEDULER_ASSIGN(1, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY), \
      RTEMS_SCHEDULER_ASSIGN(1, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_OPTIONAL), \
      RTEMS_SCHEDULER_ASSIGN(1, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_OPTIONAL), \
      RTEMS_SCHEDULER_ASSIGN(1, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_OPTIONAL), \
      RTEMS_SCHEDULER_ASSIGN(1, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_OPTIONAL)

## 24.19.6. Configuration Errors

In case one of the scheduler indices in `CONFIGURE_SCHEDULER_ASSIGNMENTS` is invalid a link-time error will occur with an undefined reference to `RTEMS_SCHEDULER_INVALID_INDEX`.

Some fatal errors may occur in case of scheduler configuration inconsistencies or a lack of processors on the system. The fatal source is `RTEMS_FATAL_SOURCE_SMP`.

- `SMP_FATAL_BOOT_PROCESSOR_NOT_ASSIGNED_TO_SCHEDULER` - the boot processor must have a scheduler assigned.

- `SMP_FATAL_MANDATORY_PROCESSOR_NOT_PRESENT` - there exists a mandatory processor beyond the range of physically or virtually available processors. The processor demand must be reduced for this system.

- `SMP_FATAL_START_OF_MANDATORY_PROCESSOR_FAILED` - the start of a mandatory processor failed during system initialization. The system may not have this processor at all or it could be a problem with a boot loader for example. Check the `CONFIGURE_SCHEDULER_ASSIGNMENTS` definition.

- `SMP_FATAL_MULTITASKING_START_ON_UNASSIGNED_PROCESSOR` - it is not allowed to start multitasking on a processor with no scheduler assigned.