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Diffstat (limited to 'Documentation/scheduler')
| -rw-r--r-- | Documentation/scheduler/sched-deadline.txt | 38 |
1 files changed, 20 insertions, 18 deletions
diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt index 18adc92a6b3b..a029891a8228 100644 --- a/Documentation/scheduler/sched-deadline.txt +++ b/Documentation/scheduler/sched-deadline.txt @@ -45,17 +45,17 @@ CONTENTS every time the task wakes up, the scheduler computes a "scheduling deadline" consistent with the guarantee (using the CBS[2,3] algorithm). Tasks are then scheduled using EDF[1] on these scheduling deadlines (the task with the - smallest scheduling deadline is selected for execution). Notice that this + earliest scheduling deadline is selected for execution). Notice that this guaranteed is respected if a proper "admission control" strategy (see Section "4. Bandwidth management") is used. Summing up, the CBS[2,3] algorithms assigns scheduling deadlines to tasks so that each task runs for at most its runtime every period, avoiding any interference between different tasks (bandwidth isolation), while the EDF[1] - algorithm selects the task with the smallest scheduling deadline as the one - to be executed first. Thanks to this feature, also tasks that do not - strictly comply with the "traditional" real-time task model (see Section 3) - can effectively use the new policy. + algorithm selects the task with the earliest scheduling deadline as the one + to be executed next. Thanks to this feature, tasks that do not strictly comply + with the "traditional" real-time task model (see Section 3) can effectively + use the new policy. In more details, the CBS algorithm assigns scheduling deadlines to tasks in the following way: @@ -64,45 +64,45 @@ CONTENTS "deadline", and "period" parameters; - The state of the task is described by a "scheduling deadline", and - a "current runtime". These two parameters are initially set to 0; + a "remaining runtime". These two parameters are initially set to 0; - When a SCHED_DEADLINE task wakes up (becomes ready for execution), the scheduler checks if - current runtime runtime - ---------------------------------- > ---------------- - scheduling deadline - current time period + remaining runtime runtime + ---------------------------------- > --------- + scheduling deadline - current time period then, if the scheduling deadline is smaller than the current time, or this condition is verified, the scheduling deadline and the - current budget are re-initialised as + remaining runtime are re-initialised as scheduling deadline = current time + deadline - current runtime = runtime + remaining runtime = runtime - otherwise, the scheduling deadline and the current runtime are + otherwise, the scheduling deadline and the remaining runtime are left unchanged; - When a SCHED_DEADLINE task executes for an amount of time t, its - current runtime is decreased as + remaining runtime is decreased as - current runtime = current runtime - t + remaining runtime = remaining runtime - t (technically, the runtime is decreased at every tick, or when the task is descheduled / preempted); - - When the current runtime becomes less or equal than 0, the task is + - When the remaining runtime becomes less or equal than 0, the task is said to be "throttled" (also known as "depleted" in real-time literature) and cannot be scheduled until its scheduling deadline. The "replenishment time" for this task (see next item) is set to be equal to the current value of the scheduling deadline; - When the current time is equal to the replenishment time of a - throttled task, the scheduling deadline and the current runtime are + throttled task, the scheduling deadline and the remaining runtime are updated as scheduling deadline = scheduling deadline + period - current runtime = current runtime + runtime + remaining runtime = remaining runtime + runtime 3. Scheduling Real-Time Tasks @@ -147,6 +147,8 @@ CONTENTS and the absolute deadlines (d_j) coincide, so a proper admission control allows to respect the jobs' absolute deadlines for this task (this is what is called "hard schedulability property" and is an extension of Lemma 1 of [2]). + Notice that if runtime > deadline the admission control will surely reject + this task, as it is not possible to respect its temporal constraints. References: 1 - C. L. Liu and J. W. Layland. Scheduling algorithms for multiprogram- @@ -156,7 +158,7 @@ CONTENTS Real-Time Systems. Proceedings of the 19th IEEE Real-time Systems Symposium, 1998. http://retis.sssup.it/~giorgio/paps/1998/rtss98-cbs.pdf 3 - L. Abeni. Server Mechanisms for Multimedia Applications. ReTiS Lab - Technical Report. http://xoomer.virgilio.it/lucabe72/pubs/tr-98-01.ps + Technical Report. http://disi.unitn.it/~abeni/tr-98-01.pdf 4. Bandwidth management ======================= |