diff options
| -rw-r--r-- | c-user/symmetric_multiprocessing_services.rst | 50 |
1 files changed, 0 insertions, 50 deletions
diff --git a/c-user/symmetric_multiprocessing_services.rst b/c-user/symmetric_multiprocessing_services.rst index 226d98d..b05e145 100644 --- a/c-user/symmetric_multiprocessing_services.rst +++ b/c-user/symmetric_multiprocessing_services.rst @@ -187,56 +187,6 @@ Schedulers`. To set the scheduler of a task see :ref:`SCHEDULER_IDENT - Get ID of a scheduler` and :ref:`TASK_SET_SCHEDULER - Set scheduler of a task`. -Task Priority Queues --------------------- - -Due to the support for clustered scheduling the task priority queues need -special attention. It makes no sense to compare the priority values of two -different scheduler instances. Thus, it is not possible to simply use one -plain priority queue for tasks of different scheduler instances. - -One solution to this problem is to use two levels of queues. The top level -queue provides FIFO ordering and contains priority queues. Each priority queue -is associated with a scheduler instance and contains only tasks of this -scheduler instance. Tasks are enqueued in the priority queue corresponding to -their scheduler instance. In case this priority queue was empty, then it is -appended to the FIFO. To dequeue a task the highest priority task of the first -priority queue in the FIFO is selected. Then the first priority queue is -removed from the FIFO. In case the previously first priority queue is not -empty, then it is appended to the FIFO. So there is FIFO fairness with respect -to the highest priority task of each scheduler instances. See also -:cite:`Brandenburg:2013:OMIP`. - -Such a two level queue may need a considerable amount of memory if fast enqueue -and dequeue operations are desired (depends on the scheduler instance count). -To mitigate this problem an approch of the FreeBSD kernel was implemented in -RTEMS. We have the invariant that a task can be enqueued on at most one task -queue. Thus, we need only as many queues as we have tasks. Each task is -equipped with spare task queue which it can give to an object on demand. The -task queue uses a dedicated memory space independent of the other memory used -for the task itself. In case a task needs to block, then there are two options - -- the object already has task queue, then the task enqueues itself to this - already present queue and the spare task queue of the task is added to a list - of free queues for this object, or - -- otherwise, then the queue of the task is given to the object and the task - enqueues itself to this queue. - -In case the task is dequeued, then there are two options - -- the task is the last task on the queue, then it removes this queue from the - object and reclaims it for its own purpose, or - -- otherwise, then the task removes one queue from the free list of the object - and reclaims it for its own purpose. - -Since there are usually more objects than tasks, this actually reduces the -memory demands. In addition the objects contain only a pointer to the task -queue structure. This helps to hide implementation details and makes it -possible to use self-contained synchronization objects in Newlib and GCC (C++ -and OpenMP run-time support). - Scheduler Helping Protocol -------------------------- |