From: Con Kolivas Add comprehensive documentation of the RSDL cpu scheduler design. Signed-off-by: Con Kolivas Cc: Ingo Molnar Cc: Nick Piggin Cc: "Siddha, Suresh B" Signed-off-by: Andrew Morton --- Documentation/sched-design.txt | 273 ++++++++++++++++++++++++++++++- 1 files changed, 267 insertions(+), 6 deletions(-) diff -puN Documentation/sched-design.txt~sched-document-rsdl-cpu-scheduler Documentation/sched-design.txt --- a/Documentation/sched-design.txt~sched-document-rsdl-cpu-scheduler +++ a/Documentation/sched-design.txt @@ -1,11 +1,14 @@ - Goals, Design and Implementation of the - new ultra-scalable O(1) scheduler + Goals, Design and Implementation of the ultra-scalable O(1) scheduler by + Ingo Molnar and the Rotating Staircase Deadline cpu scheduler policy + designed by Con Kolivas. - This is an edited version of an email Ingo Molnar sent to - lkml on 4 Jan 2002. It describes the goals, design, and - implementation of Ingo's new ultra-scalable O(1) scheduler. - Last Updated: 18 April 2002. + This was originally an edited version of an email Ingo Molnar sent to + lkml on 4 Jan 2002. It describes the goals, design, and implementation + of Ingo's ultra-scalable O(1) scheduler. It now contains a description + of the Rotating Staircase Deadline priority scheduler that was built on + this design. + Last Updated: Sun Feb 25 2007 Goal @@ -163,3 +166,261 @@ certain code paths and data constructs. code is smaller than the old one. Ingo + + +Rotating Staircase Deadline cpu scheduler policy +================================================ + +Design summary +============== + +A novel design which incorporates a foreground-background descending priority +system (the staircase) with runqueue managed minor and major epochs (rotation +and deadline). + + +Features +======== + +A starvation free, strict fairness O(1) scalable design with interactivity +as good as the above restrictions can provide. There is no interactivity +estimator, no sleep/run measurements and only simple fixed accounting. +The design has strict enough a design and accounting that task behaviour +can be modelled and maximum scheduling latencies can be predicted by +the virtual deadline mechanism that manages runqueues. The prime concern +in this design is to maintain fairness at all costs determined by nice level, +yet to maintain as good interactivity as can be allowed within the +constraints of strict fairness. + + +Design description +================== + +RSDL works off the principle of providing each task a quota of runtime that +it is allowed to run at each priority level equal to its static priority +(ie. its nice level) and every priority below that. When each task is queued, +the cpu that it is queued onto also keeps a record of that quota. If the +task uses up its quota it is decremented one priority level. Also, if the cpu +notices a quota full has been used for that priority level, it pushes +everything remaining at that priority level to the next lowest priority +level. Once every runtime quota has been consumed of every priority level, +a task is queued on the "expired" array. When no other tasks exist with +quota, the expired array is activated and fresh quotas are handed out. This +is all done in O(1). + + +Design details +============== + +Each cpu has its own runqueue which micromanages its own epochs, and each +task keeps a record of its own entitlement of cpu time. Most of the rest +of these details apply to non-realtime tasks as rt task management is +straight forward. + +Each runqueue keeps a record of what major epoch it is up to in the +rq->prio_rotation field which is incremented on each major epoch. It also +keeps a record of quota available to each priority value valid for that +major epoch in rq->prio_quota[]. + +Each task keeps a record of what major runqueue epoch it was last running +on in p->rotation. It also keeps a record of what priority levels it has +already been allocated quota from during this epoch in a bitmap p->bitmap. + +The only tunable that determines all other details is the RR_INTERVAL. This +is set to 6ms (minimum on 1000HZ, higher at different HZ values). + +All tasks are initially given a quota based on RR_INTERVAL. This is equal to +RR_INTERVAL between nice values of 0 and 19, and progressively larger for +nice values from -1 to -20. This is assigned to p->quota and only changes +with changes in nice level. + +As a task is first queued, it checks in recalc_task_prio to see if it has +run at this runqueue's current priority rotation. If it has not, it will +have its p->prio level set to equal its p->static_prio (nice level) and will +be given a p->time_slice equal to the p->quota, and has its allocation +bitmap bit set in p->bitmap for its static priority (nice value). This +quota is then also added to the current runqueue's rq->prio_quota[p->prio]. +It is then queued on the current active priority array. + +If a task has already been running during this major epoch, if it has +p->time_slice left and the rq->prio_quota for the task's p->prio still +has quota, it will be placed back on the active array, but no more quota +will be added to either the task or the runqueue quota. + +If a task has been running during this major epoch, but does not have +p->time_slice left or the runqueue's prio_quota for this task's p->prio +does not have quota, it will find the next lowest priority in its bitmap +that it has not been allocated quota from. It then gets the a full quota +in p->time_slice and adds that to the quota value for the relevant priority +rq->prio_quota. It is then queued on the current active priority array at +the newly determined lower priority. + +If a task has been running during this major epoch, and does not have +any entitlement left in p->bitmap and no time_slice left, it will have its +bitmap cleared, and be queued at its p->static_prio again, but on the expired +priority array. No quota will be allocated until this task is scheduled. + +When a task is queued, it has its static_prio bit set in the current +runqueue's rq->static_bitmap, and the relevant bit in the rq->dyn_bitmap. +In order to minimise the number of bitmap lookups, the bitmap of queued +tasks on the expired array is at the end of the same bitmap as the active +array. The number of tasks queued at the current static_prio is kept in +rq->prio_queued[]. + +During a scheduler_tick where a task is running, the p->time_slice is +decremented, and if it reaches zero then the recalc_task_prio is readjusted +and the task rescheduled. + +During a task running tick, the runqueue prio_quota is also decremented. If +it empties then a priority rotation occurs (a major or minor epoch). If the +current runqueue's priority level is better than that of nice 19 tasks, a +minor rotation is performed, otherwise a major rotation will occur. + +A minor rotation takes the remaining tasks at this priority level queue and +merges them with a list_splice_tail with the queue from the next lowest +priority level. At this time, any tasks that have been merged will now +have invalid values in p->prio so this must be considered when dequeueing +the task, and for testing for preemption. + +A major rotation takes the remaining tasks at this priority level queue and +merges them with a list_splice_tail with the best priority task running on +the expired array, and swaps the priority arrays. The priority quotas are +reset at this time. Any tasks that have been merged will now have invalid +values in p->array and possibly p->prio so this must be considered. The +rq->prio_rotation is incremented at this time. + +When a task is dequeued, the dyn_bitmap bit is unset only after testing +that the relevant queue is actually empty since p->prio may be inaccurate +and no hard accounting of the number of tasks at that level is possible. + +When selecting a new task for scheduling, after the first dynamic bit is +found on the dyn_bitmap, it is checked to see that a task is really queued +at that priority or if it is a false positive due to the task being +dequeued at a time when its p->prio does not match which queue it is on +after some form of priority rotation. This is a rare occurrence as it tends +to only occur if a task that is already waiting on a runqueue gets dequeued. +If the bitmap value is in the expired array range, a major priority rotation +is performed. If the chosen task has not been running during this major or +minor rotation it has new quota allocated at this time, and added to the +runqueue's quota. + + +Modelling deadline behaviour +============================ + +As the accounting in this design is hard and not modified by sleep average +calculations or interactivity modifiers, it is possible to accurately +predict the maximum latency that a task may experience under different +conditions. This is a virtual deadline mechanism enforced by mandatory +runqueue epochs, and not by trying to keep complicated accounting of each +task. + +The maximum duration a task can run during one major epoch is determined +by its nice value. Nice 0 tasks can run at 19 different priority levels +for RR_INTERVAL duration during each epoch (the equivalent of nice 0 to nice +19). Nice 10 tasks can run at 9 priority levels for each epoch, and so on. + +Therefore the maximum duration a runqueue epoch can take is determined by +the number of tasks running, and their nice level. After that, the maximum +duration it can take before a task can wait before it get scheduled is +determined by the difference between its nice value and the nice value of +the highest priority task queued. + +In the following examples, these are _worst case scenarios_ and would rarely +occur, but can be modelled nonetheless to determine the maximum possible +latency. + +So for example, if two nice 0 tasks are running, and one has just expired as +another is activated for the first time receiving a full quota for this +runqueue rotation, the first task will wait: + +nr_tasks * max_duration + nice_difference * rr_interval +1 * 19 * RR_INTERVAL + 0 = 114ms + +In the presence of a nice 10 task, a nice 0 task would wait a maximum of +1 * 10 * RR_INTERVAL + 0 = 60ms + +In the presence of a nice 0 task, a nice 10 task would wait a maximum of +1 * 19 * RR_INTERVAL + 9 * RR_INTERVAL = 168ms + +Using a more complicated example, if there are 4 tasks running fully cpu +bound, one each at nice -20, nice 0, nice 10 and nice 19, we can calculate +the maximum latency possible for the nice 10 task. Note that -20 tasks are +heavily biased for so this will be a long time, but can be modelled. + +The nice -20 task has quota = RR_INTERVAL + 20*RR_INTERVAL = 21*RR_INTERVAL. +It can run at 39 priority levels so its maximum duration = +39 * 21 * RR_INTERVAL. +The nice 0 task works out to +19 * RR_INTERVAL +The nice 19 task works out to +RR_INTERVAL. + +So major epoch can take up a maximum of +39 * 21 * RR_INTERVAL + 19 * RR_INTERVAL + RR_INTERVAL = 1229 * RR_INTERVAL; + +Then before the nice 10 task will run, the nice -20 and nice 0 task will +run for 28 * 21 * RR_INTERVAL and 9 * RR_INTERVAL respectively for a total +of 597 * RR_INTERVAL. + +This means the maximum duration a nice 10 task can wait in the presence of +these other tasks is 1826*RR_INTERVAL. This is a long time of course and is +heavily penalised by the presence of nice -20 tasks which would not be part +of a normal environment. + +While this section describes the maximum latency a task can have, this size +latencies will only be seen by fully cpu bound tasks. + + +Achieving interactivity +======================= + +A requirement of this scheduler design was to achieve good interactivity +despite being a completely fair deadline based design. The disadvantage of +designs that try to achieve interactivity is that they usually do so at +the expense of maintaining fairness. As cpu speeds increase, the requirement +for some sort of metered unfairness towards interactive tasks becomes a less +desirable phenomenon, but low latency and fairness remains mandatory to +good interactive performance. + +This design relies on the fact that interactive tasks, by their nature, +sleep often. Most fair scheduling designs end up penalising such tasks +indirectly giving them less than their fair possible share because of the +sleep, and have to use a mechanism of bonusing their priority to offset +this based on the duration they sleep. This becomes increasingly inaccurate +as the number of running tasks rises and more tasks spend time waiting on +runqueues rather than sleeping, and it is impossible to tell whether the +task that's waiting on a runqueue only intends to run for a short period and +then sleep again after than runqueue wait. Furthermore, all such designs rely +on a period of time to pass to accumulate some form of statistic on the task +before deciding on how much to give them preference. The shorter this period, +the more rapidly bursts of cpu ruin the interactive tasks behaviour. The +longer this period, the longer it takes for interactive tasks to get low +scheduling latencies and fair cpu. + +This design does not measure sleep time at all. Interactive tasks that sleep +often will wake up having consumed very little if any of their quota for +the current major priority rotation. The longer they have slept, the less +likely they are to even be on the current major priority rotation. Once +woken up, though, they get to use up a their full quota for that epoch, +whether part of a quota remains or a full quota. Overall, however, they +can still only run as much cpu time for that epoch as any other task of the +same nice level. This means that two tasks behaving completely differently +from fully cpu bound to waking/sleeping extremely frequently will still +get the same quota of cpu, but the latter will be using its quota for that +epoch in bursts rather than continuously. This guarantees that interactive +tasks get the same amount of cpu as cpu bound ones. + +The other requirement of interactive tasks is also to obtain low latencies +for when they are scheduled. Unlike fully cpu bound tasks and the maximum +latencies possible described in the modelling deadline behaviour section +above, tasks that sleep will wake up with quota available usually at the +current runqueue's priority_level or better. This means that the most latency +they are likely to see is one RR_INTERVAL, and often they will preempt the +current task if it is not of a sleeping nature. This then guarantees very +low latency for interactive tasks, and the lowest latencies for the least +cpu bound tasks. + + +Wed, 28 Feb 2007 +Con Kolivas _