|
In computer science, rate-monotonic scheduling (RMS)〔.〕 is a scheduling algorithm used in real-time operating systems (RTOS) with a static-priority scheduling class.〔, http://oreilly.com/catalog/linuxkernel/chapter/ch10.html#85347.〕 The static priorities are assigned on the basis of the cycle duration of the job: the shorter the cycle duration is, the higher is the job's priority. These operating systems are generally preemptive and have deterministic guarantees with regard to response times. Rate monotonic analysis is used in conjunction with those systems to provide scheduling guarantees for a particular application. ==Introduction== A simple version of rate-monotonic analysis assumes that threads have the following properties: *No resource sharing (processes do not share resources, ''e.g.'' a hardware resource, a queue, or any kind of semaphore blocking or non-blocking (busy-waits)) *Deterministic deadlines are exactly equal to periods *Static priorities (the task with the highest static priority that is runnable immediately preempts all other tasks) *Static priorities assigned according to the ''rate monotonic'' conventions (tasks with shorter periods/deadlines are given higher priorities) *Context switch times and other thread operations are free and have no impact on the model It is a mathematical model that contains a calculated simulation of periods in a closed system, where round-robin and time-sharing schedulers fail to meet the scheduling needs otherwise. Rate monotonic scheduling looks at a run modeling of all threads in the system and determines how much time is needed to meet the guarantees for the set of threads in question. proved that for a set of periodic tasks with unique periods, a feasible schedule that will always meet deadlines exists if the CPU utilization is below a specific bound (depending on the number of tasks). The schedulability test for RMS is: : where is the computation time, is the release period (with deadline one period later), and is the number of processes to be scheduled. For example, for two processes. When the number of processes tends towards infinity, this expression will tend towards: : Therefore, a rough estimate is that RMS can meet all of the deadlines if CPU utilization is less than 69.32%. The other 30.7% of the CPU can be dedicated to lower-priority non real-time tasks. It is known that a randomly generated periodic task system will meet all deadlines when the utilization is 85% or less,〔.〕 however this fact depends on knowing the exact task statistics (periods, deadlines) which cannot be guaranteed for all task sets. The rate-monotonic priority assignment is ''optimal'', meaning that if any static-priority scheduling algorithm can meet all the deadlines, then the rate-monotonic algorithm can too. The deadline-monotonic scheduling algorithm is also optimal with equal periods and deadlines, in fact in this case the algorithms are identical; in addition, deadline monotonic scheduling is optimal when deadlines are less than periods.〔.〕 For the task model in which deadlines can be greater than periods, Audsley's algorithm endowed with an exact schedulability test for this model finds an optimal priority assignment. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rate-monotonic scheduling」の詳細全文を読む スポンサード リンク
|