35. Constant Bandwidth Server Scheduler API — RTEMS Classic API Guide 5.4-rc2 (21st August 2026) documentation

# 35. Constant Bandwidth Server Scheduler API

## 35.1. Introduction

Unlike simple schedulers, the Constant Bandwidth Server (CBS) requires a special API for tasks to indicate their scheduling parameters. The directives provided by the CBS API are:

- rtems_cbs_initialize - Initialize the CBS library

- rtems_cbs_cleanup - Cleanup the CBS library

- rtems_cbs_create_server - Create a new bandwidth server

- rtems_cbs_attach_thread - Attach a thread to server

- rtems_cbs_detach_thread - Detach a thread from server

- rtems_cbs_destroy_server - Destroy a bandwidth server

- rtems_cbs_get_server_id - Get an ID of a server

- rtems_cbs_get_parameters - Get scheduling parameters of a server

- rtems_cbs_set_parameters - Set scheduling parameters of a server

- rtems_cbs_get_execution_time - Get elapsed execution time

- rtems_cbs_get_remaining_budget - Get remainig execution time

- rtems_cbs_get_approved_budget - Get scheduler approved execution time

## 35.2. Background

### 35.2.1. Constant Bandwidth Server Definitions

The Constant Bandwidth Server API enables tasks to communicate with the scheduler and indicate its scheduling parameters. The scheduler has to be set up first (by defining `CONFIGURE_SCHEDULER_CBS` macro).

The difference to a plain EDF is the presence of servers. It is a budget aware extention of the EDF scheduler, therefore, tasks attached to servers behave in a similar way as with EDF unless they exceed their budget.

The intention of servers is reservation of a certain computation time (budget) of the processor for all subsequent periods. The structure `rtems_cbs_parameters` determines the behavior of a server. It contains `deadline` which is equal to period, and `budget` which is the time the server is allowed to spend on CPU per each period. The ratio between those two parameters yields the maximum percentage of the CPU the server can use (bandwidth). Moreover, thanks to this limitation the overall utilization of CPU is under control, and the sum of bandwidths of all servers in the system yields the overall reserved portion of processor. The rest is still available for ordinary tasks that are not attached to any server.

In order to make the server effective to the executing tasks, tasks have to be attached to the servers. The `rtems_cbs_server_id` is a type denoting an id of a server and `rtems_id` a type for id of tasks. .. index:: CBS periodic tasks

### 35.2.2. Handling Periodic Tasks

Each task’s execution begins with a default background priority (see the chapter Scheduling Concepts to understand the concept of priorities in EDF). Once you decide the tasks should start periodic execution, you have two possibilities. Either you use only the Rate Monotonic manager which takes care of periodic behavior, or you declare deadline and budget using the CBS API in which case these properties are constant for all subsequent periods, unless you change them using the CBS API again. Task now only has to indicate and end of each period using `rtems_rate_monotonic_period`. .. index:: CBS overrun handler

### 35.2.3. Registering a Callback Function

In case tasks attached to servers are not aware of their execution time and happen to exceed it, the scheduler does not guarantee execution any more and pulls the priority of the task to background, which would possibly lead to immediate preemption (if there is at least one ready task with a higher pirority). However, the task is not blocked but a callback function is invoked. The callback function (`rtems_cbs_budget_overrun`) might be optionally registered upon a server creation (`rtems_cbs_create_server`).

This enables the user to define what should happen in case of budget overrun. There is obviously no space for huge operations because the priority is down and not real time any more, however, you still can at least in release resources for other tasks, restart the task or log an error information. Since the routine is called directly from kernel, use `printk()` instead of `printf()`.

The calling convention of the callback function is:

    void overrun_handler(
        rtems_cbs_server_id server_id
    );

### 35.2.4. Limitations

When using this scheduler you have to keep in mind several things:

- it_limitations

- In the current implementation it is possible to attach only a single task to each server.

- If you have a task attached to a server and you voluntatily block it in the beginning of its execution, its priority will be probably pulled to background upon unblock, thus not guaranteed deadline any more. This is because you are effectively raising computation time of the task. When unbocking, you should be always sure that the ratio between remaining computation time and remaining deadline is not higher that the utilization you have agreed with the scheduler.

## 35.3. Operations

### 35.3.1. Setting up a server

The directive `rtems_cbs_create_server` is used to create a new server that is characterized by `rtems_cbs_parameters`. You also might want to register the `rtems_cbs_budget_overrun` callback routine. After this step tasks can be attached to the server. The directive `rtems_cbs_set_parameters` can change the scheduling parameters to avoid destroying and creating a new server again.

### 35.3.2. Attaching Task to a Server

If a task is attached to a server using `rtems_cbs_attach_thread`, the task’s computation time per period is limited by the server and the deadline (period) of task is equal to deadline of the server which means if you conclude a period using `rate_monotonic_period`, the length of next period is always determined by the server’s property.

The task has a guaranteed bandwidth given by the server but should not exceed it, otherwise the priority is pulled to background until the start of next period and the `rtems_cbs_budget_overrun` callback function is invoked.

When attaching a task to server, the preemptability flag of the task is raised, otherwise it would not be possible to control the execution of the task.

### 35.3.3. Detaching Task from a Server

The directive `rtems_cbs_detach_thread` is just an inverse operation to the previous one, the task continues its execution with the initial priority.

Preemptability of the task is restored to the initial value.

### 35.3.4. Examples

The following example presents a simple common use of the API.

You can see the initialization and cleanup call here, if there are multiple tasks in the system, it is obvious that the initialization should be called before creating the task.

Notice also that in this case we decided to register an overrun handler, instead of which there could be `NULL`. This handler just prints a message to terminal, what else may be done here depends on a specific application.

During the periodic execution, remaining budget should be watched to avoid overrun.

    void overrun_handler (
        rtems_cbs_server_id server_id
    )
    {
        printk( "Budget overrun, fixing the task\n" );
        return;
    }

    rtems_task Tasks_Periodic(
        rtems_task_argument argument
    )
    {
        rtems_id             rmid;
        rtems_cbs_server_id  server_id;
        rtems_cbs_parameters params;

        params.deadline = 10;
        params.budget = 4;

        rtems_cbs_initialize();
        rtems_cbs_create_server( &params, &overrun_handler, &server_id );
        rtems_cbs_attach_thread( server_id, RTEMS_SELF );
        rtems_rate_monotonic_create( argument, &rmid );

        while ( 1 ) {
            if (rtems_rate_monotonic_period(rmid, params.deadline) == RTEMS_TIMEOUT)
                break;
            /* Perform some periodic action */
        }

        rtems_rate_monotonic_delete( rmid );
        rtems_cbs_cleanup();
        exit( 1 );
    }

## 35.4. Directives

This section details the Constant Bandwidth Server’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

### 35.4.1. CBS_INITIALIZE - Initialize the CBS library

CALLING SEQUENCE:

      int rtems_cbs_initialize( void );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successful initialization
  `RTEMS_CBS_ERROR_NO_MEMORY` | not enough memory for data

DESCRIPTION:

  This routine initializes the library in terms of allocating necessary memory for the servers. In case not enough memory is available in the system, `RTEMS_CBS_ERROR_NO_MEMORY` is returned, otherwise `RTEMS_CBS_OK`.

NOTES:

  Additional memory per each server is allocated upon invocation of `rtems_cbs_create_server`.

  Tasks in the system are not influenced, they still keep executing with their initial parameters.

### 35.4.2. CBS_CLEANUP - Cleanup the CBS library

CALLING SEQUENCE:

      int rtems_cbs_cleanup( void );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | always successful

DESCRIPTION:

  This routine detaches all tasks from their servers, destroys all servers and returns memory back to the system.

NOTES:

  All tasks continue executing with their initial priorities.

### 35.4.3. CBS_CREATE_SERVER - Create a new bandwidth server

CALLING SEQUENCE:

      int rtems_cbs_create_server (
          rtems_cbs_parameters     *params,
          rtems_cbs_budget_overrun  budget_overrun_callback,
          rtems_cbs_server_id      *server_id
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successfully created
  `RTEMS_CBS_ERROR_NO_MEMORY` | not enough memory for data
  `RTEMS_CBS_ERROR_FULL` | maximum servers exceeded
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument

DESCRIPTION:

  This routine prepares an instance of a constant bandwidth server. The input parameter `rtems_cbs_parameters` specifies scheduling parameters of the server (period and budget). If these are not valid, `RTEMS_CBS_ERROR_INVALID_PARAMETER` is returned. The `budget_overrun_callback` is an optional callback function, which is invoked in case the server’s budget within one period is exceeded. Output parameter `server_id` becomes an id of the newly created server. If there is not enough memory, the `RTEMS_CBS_ERROR_NO_MEMORY` is returned. If the maximum server count in the system is exceeded, `RTEMS_CBS_ERROR_FULL` is returned.

NOTES:

  No task execution is being influenced so far.

### 35.4.4. CBS_ATTACH_THREAD - Attach a thread to server

CALLING SEQUENCE:

      int rtems_cbs_attach_thread (
          rtems_cbs_server_id server_id,
          rtems_id            task_id
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successfully attached
  `RTEMS_CBS_ERROR_FULL` | server maximum tasks exceeded
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  Attaches a task (`task_id`) to a server (`server_id`). The server has to be previously created. Now, the task starts to be scheduled according to the server parameters and not using initial priority. This implementation allows only one task per server, if the user tries to bind another task to the same server, `RTEMS_CBS_ERROR_FULL` is returned.

NOTES:

  Tasks attached to servers become preemptible.

### 35.4.5. CBS_DETACH_THREAD - Detach a thread from server

CALLING SEQUENCE:

      int rtems_cbs_detach_thread (
          rtems_cbs_server_id server_id,
          rtems_id            task_id
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successfully detached
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  This directive detaches a thread from server. The task continues its execution with initial priority.

NOTES:

  The server can be reused for any other task.

### 35.4.6. CBS_DESTROY_SERVER - Destroy a bandwidth server

CALLING SEQUENCE:

      int rtems_cbs_destroy_server (
          rtems_cbs_server_id server_id
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successfully destroyed
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  This directive destroys a server. If any task was attached to the server, the task is detached and continues its execution according to EDF rules with initial properties.

NOTES:

  This again enables one more task to be created.

### 35.4.7. CBS_GET_SERVER_ID - Get an ID of a server

CALLING SEQUENCE:

      int rtems_cbs_get_server_id (
          rtems_id             task_id,
          rtems_cbs_server_id *server_id
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successful
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  This directive returns an id of server belonging to a given task.

### 35.4.8. CBS_GET_PARAMETERS - Get scheduling parameters of a server

CALLING SEQUENCE:

      rtems_cbs_get_parameters (
          rtems_cbs_server_id   server_id,
          rtems_cbs_parameters *params
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successful
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  This directive returns a structure with current scheduling parameters of a given server (period and execution time).

NOTES:

  It makes no difference if any task is assigned or not.

### 35.4.9. CBS_SET_PARAMETERS - Set scheduling parameters

CALLING SEQUENCE:

      int rtems_cbs_set_parameters (
          rtems_cbs_server_id   server_id,
          rtems_cbs_parameters *params
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successful
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  This directive sets new scheduling parameters to the server. This operation can be performed regardless of whether a task is assigned or not. If a task is assigned, the parameters become effective imediately, therefore it is recommended to apply the change between two subsequent periods.

NOTES:

  There is an upper limit on both period and budget equal to (2^31)-1 ticks.

### 35.4.10. CBS_GET_EXECUTION_TIME - Get elapsed execution time

CALLING SEQUENCE:

      int rtems_cbs_get_execution_time (
          rtems_cbs_server_id    server_id,
          time_t                *exec_time,
          time_t                *abs_time
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successful
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  This routine returns consumed execution time (`exec_time`) of a server during the current period.

NOTES:

  Absolute time (`abs_time`) not supported now.

### 35.4.11. CBS_GET_REMAINING_BUDGET - Get remaining execution time

CALLING SEQUENCE:

      int rtems_cbs_get_remaining_budget (
          rtems_cbs_server_id  server_id,
          time_t              *remaining_budget
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successful
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  This directive returns remaining execution time of a given server for current period.

NOTES:

  If the execution time approaches zero, the assigned task should finish computations of the current period.

### 35.4.12. CBS_GET_APPROVED_BUDGET - Get scheduler approved execution time

CALLING SEQUENCE:

      int rtems_cbs_get_approved_budget (
          rtems_cbs_server_id  server_id,
          time_t              *appr_budget
      );

DIRECTIVE STATUS CODES:

  `RTEMS_CBS_OK` | successful
  `RTEMS_CBS_ERROR_INVALID_PARAMETER` | invalid input argument
  `RTEMS_CBS_ERROR_NOSERVER` | server is not valid

DESCRIPTION:

  This directive returns server’s approved budget for subsequent periods.