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38 basically an implementation of the Earliest Deadline First (EDF) scheduling
40 that makes it possible to isolate the behavior of tasks between each other.
48 "runtime" microseconds of execution time every "period" microseconds, and
50 from the beginning of the period. In order to implement this behavior,
73 - The state of the task is described by a "scheduling deadline", and
93 - When a SCHED_DEADLINE task executes for an amount of time t, its
105 value of the scheduling deadline;
107 - When the current time is equal to the replenishment time of a
125 * satisfied :P) to be sure of fully understanding all technical details.
128 There are no limitations on what kind of task can exploit this new
136 A typical real-time task is composed of a repetition of computation phases
139 Each job J_j (where J_j is the j^th job of the task) is characterized by an
140 arrival time r_j (the time when the job starts), an amount of computation
150 The utilization of a real-time task is defined as the ratio between its
152 the fraction of CPU time needed to execute the task.
155 to the number of CPUs), then the scheduler is unable to respect all the
157 Note that total utilization is defined as the sum of the utilizations
159 multiple real-time tasks, the parameters of the i-th task are indicated
166 As a matter of fact, in this case it is possible to provide an upper bound
168 between the finishing time of a job and its absolute deadline).
170 maximum tardiness of each task is smaller or equal than
179 If M=1 (uniprocessor system), or in case of partitioned scheduling (each
183 of all the tasks executing on a CPU if and only if the total utilization
184 of the tasks running on such a CPU is smaller or equal than 1.
185 If D_i != P_i for some task, then it is possible to define the density of
187 of all the tasks running on a CPU if the sum of the densities of the tasks
199 Of course it is possible to test the exact schedulability of tasks with
203 computing the total amount of CPU time h(t) needed by all the tasks to
204 respect all of their deadlines in a time interval of size t, and comparing
206 the amount of time needed by the tasks in a time interval of size t is
207 smaller than the size of the interval) for all the possible values of t, then
208 EDF is able to schedule the tasks respecting all of their deadlines. Since
209 performing this check for all possible values of t is impossible, it has been
210 proven[4,5,6] that it is sufficient to perform the test for values of t
211 between 0 and a maximum value L. The cited papers contain all of the
213 In any case, this kind of analysis is too complex as well as too
224 of the number of CPUs.
226 Consider a set {Task_1,...Task_{M+1}} of M+1 tasks on a system with M
230 period smaller than the one of the first task. Hence, if all the tasks
233 smaller than the absolute deadline of Task_1, which is t + P). As a
235 time t + e + P, after its absolute deadline. The total utilization of the
237 values of e this can become very close to 1. This is known as "Dhall's
250 just confirms the Dhall's effect. A more complete survey of the literature
258 tasks are not starved and that the tardiness of real-time tasks has an upper
262 the total utilization is smaller or equal than M then the response times of
277 guaranteeing that all the jobs' deadlines of a task are respected.
287 called "hard schedulability property" and is an extension of Lemma 1 of [2]).
293 ming in a hard-real-time environment. Journal of the Association for
296 Real-Time Systems. Proceedings of the 19th IEEE Real-time Systems
300 4 - J. Y. Leung and M.L. Merril. A Note on Preemptive Scheduling of
304 Hard-Real-Time Sporadic Tasks on One Processor. Proceedings of the
307 Concerning the Preemptive Scheduling of Periodic Real-Time tasks on
313 Analysis. Proceedings of the 24th IEEE Real-Time Systems Symposium, 2003.
314 9 - T. Baker. An Analysis of EDF Schedulability on a Multiprocessor.
317 10 - J. Goossens, S. Funk and S. Baruah, Priority-Driven Scheduling of
320 11 - R. Davis and A. Burns. A Survey of Hard Real-Time Scheduling for
326 13 - P. Valente and G. Lipari. An Upper Bound to the Lateness of Soft
327 Real-Time Tasks Scheduled by EDF on Multiprocessors. Proceedings of
330 Global EDF. Proceedings of the 22nd Euromicro Conference on
340 of the available fractions of CPU time to the various tasks under control.
342 no guarantee can be given on the actual scheduling of the -deadline tasks.
345 correctly schedule a set of real-time tasks is that the total utilization
347 the sum of the ratio between runtime and period for all tasks is smaller
349 of a "traditional" real-time task, and is also often referred to as
368 respecting their needs in terms of granularity. Therefore, using this simple
369 interface we can put a cap on total utilization of -deadline tasks (i.e.,
382 direct subset of dl_bw.
385 can be created while the sum of their bandwidths stays below:
390 be thus free of oversubscribing the system up to any arbitrary level.
397 Specifying a periodic/sporadic task that executes for a given amount of
398 runtime at each instance, and that is scheduled according to the urgency of
399 its own timing constraints needs, in general, a way of declaring:
416 tasks can use at most 95%, multiplied by the number of CPUs that compose the
418 This means that non -deadline tasks will receive at least 5% of the CPU time,
422 Section 5), then this simple setting of the bandwidth management is able to
439 An example of a simple configuration (pin a -deadline task to CPU0)
462 of retaining bandwidth isolation among non-interacting tasks. This is
463 being studied from both theoretical and practical points of view, and
467 address), which is the best way to allow unprivileged use of the mechanisms
472 the preliminary phases of the merge and we really seek feedback that would
479 are part of a wider Linux Scheduler validation suite. The suite is
499 of 5 seconds.
507 of the command line options. Please refer to rt-app documentation for more
510 The second testing application is a modification of schedtool, called
520 of 10ms every 100ms (note that parameters are expressed in microseconds).