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Subject[PATCH] [RSDL 6/6] sched: document rsdl cpu scheduler
Add comprehensive documentation of the RSDL cpu scheduler design.

Signed-off-by: Con Kolivas <>

Documentation/sched-design.txt | 273 ++++++++++++++++++++++++++++++++++++++++-
1 file changed, 267 insertions(+), 6 deletions(-)
Index: linux-2.6.20-rsdl/Documentation/sched-design.txt
--- linux-2.6.20-rsdl.orig/Documentation/sched-design.txt 2007-03-04 17:30:25.000000000 +1100
+++ linux-2.6.20-rsdl/Documentation/sched-design.txt 2007-03-04 17:30:31.000000000 +1100
@@ -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

@@ -163,3 +166,261 @@ certain code paths and data constructs.
code is smaller than the old one.

+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).
+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
+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
+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
+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
+The nice 19 task works out to
+So major epoch can take up a maximum of
+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 <>
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