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Hi Con It would be nice if you could rebase this patch to latest git or at least to 2.6.21-rc3. Regards, Fabio On 3/4/07, Con Kolivas <kernel@kolivas.org> wrote: > This message is to announce the first general public release of the "Rotating > Staircase DeadLine" cpu scheduler. > > Based on previous work from the staircase cpu scheduler I set out to design, > from scratch, a new scheduling policy design which satisfies every > requirement for SCHED_NORMAL (otherwise known as SCHED_OTHER) task management. > > Available for download are: > > A full rollup of the patch for 2.6.20: > http://ck.kolivas.org/patches/staircase-deadline/sched-rsdl-0.26.patch > > Split patches for 2.6.20(which will follow this email): > http://ck.kolivas.org/patches/staircase-deadline/split-out/ > > The readme (which will also constitute the rest of this email): > http://ck.kolivas.org/patches/staircase-deadline/rsdl_scheduler.readme > > > The following readme is also included as documentation in > Documentation/sched-design.txt > > > 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. > > Sunday, 4th March 2007 > Con Kolivas > > -- > -ck > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | |||||||||
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