5.4. SMP Schedulers

All SMP schedulers included in RTEMS are priority based. The processors managed by a scheduler instance are allocated to the highest priority tasks allowed to run.

5.4.1. Earliest Deadline First SMP Scheduler

This is a job-level fixed-priority scheduler using the Earliest Deadline First (EDF) method. By convention, the maximum priority level is \(min(INT\_MAX, 2^{62} - 1)\) for background tasks. Tasks without an active deadline are background tasks. In case deadlines are not used, then the EDF scheduler behaves exactly like a fixed-priority scheduler. The tasks with an active deadline have a higher priority than the background tasks. This scheduler supports task processor affinities of one-to-one and one-to-all, e.g., a task can execute on exactly one processor or all processors managed by the scheduler instance. The processor affinity set of a task must contain all online processors to select the one-to-all affinity. This is to avoid pathological cases if processors are added/removed to/from the scheduler instance at run-time. In case the processor affinity set contains not all online processors, then a one-to-one affinity will be used selecting the processor with the largest index within the set of processors currently owned by the scheduler instance. This scheduler algorithm supports thread pinning. The ready queues use a red-black tree with the task priority as the key.

This scheduler algorithm is the default scheduler in SMP configurations if more than one processor is configured (CONFIGURE_MAXIMUM_PROCESSORS).

5.4.2. Deterministic Priority SMP Scheduler

A fixed-priority scheduler which uses a table of chains with one chain per priority level for the ready tasks. The maximum priority level is configurable. By default, the maximum priority level is 255 (256 priority levels), see CONFIGURE_MAXIMUM_PRIORITY.

5.4.3. Simple Priority SMP Scheduler

A fixed-priority scheduler which uses a sorted chain for the ready tasks. By convention, the maximum priority level is 255. The implementation limit is actually \(2^{63} - 1\).

5.4.4. Arbitrary Processor Affinity Priority SMP Scheduler

A fixed-priority scheduler which uses a table of chains with one chain per priority level for the ready tasks. The maximum priority level is configurable. By default, the maximum priority level is 255 (256 priority levels), see CONFIGURE_MAXIMUM_PRIORITY. This scheduler supports arbitrary task processor affinities. The worst-case run-time complexity of some scheduler operations exceeds \(O(n)\) while \(n\) is the count of ready tasks.