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+ Goals, Design and Implementation of the
+ new ultra-scalable O(1) scheduler
+ 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.
+The main goal of the new scheduler is to keep all the good things we know
+and love about the current Linux scheduler:
+ - good interactive performance even during high load: if the user
+ types or clicks then the system must react instantly and must execute
+ the user tasks smoothly, even during considerable background load.
+ - good scheduling/wakeup performance with 1-2 runnable processes.
+ - fairness: no process should stay without any timeslice for any
+ unreasonable amount of time. No process should get an unjustly high
+ amount of CPU time.
+ - priorities: less important tasks can be started with lower priority,
+ more important tasks with higher priority.
+ - SMP efficiency: no CPU should stay idle if there is work to do.
+ - SMP affinity: processes which run on one CPU should stay affine to
+ that CPU. Processes should not bounce between CPUs too frequently.
+ - plus additional scheduler features: RT scheduling, CPU binding.
+and the goal is also to add a few new things:
+ - fully O(1) scheduling. Are you tired of the recalculation loop
+ blowing the L1 cache away every now and then? Do you think the goodness
+ loop is taking a bit too long to finish if there are lots of runnable
+ processes? This new scheduler takes no prisoners: wakeup(), schedule(),
+ the timer interrupt are all O(1) algorithms. There is no recalculation
+ loop. There is no goodness loop either.
+ - 'perfect' SMP scalability. With the new scheduler there is no 'big'
+ runqueue_lock anymore - it's all per-CPU runqueues and locks - two
+ tasks on two separate CPUs can wake up, schedule and context-switch
+ completely in parallel, without any interlocking. All
+ scheduling-relevant data is structured for maximum scalability.
+ - better SMP affinity. The old scheduler has a particular weakness that
+ causes the random bouncing of tasks between CPUs if/when higher
+ priority/interactive tasks, this was observed and reported by many
+ people. The reason is that the timeslice recalculation loop first needs
+ every currently running task to consume its timeslice. But when this
+ happens on eg. an 8-way system, then this property starves an
+ increasing number of CPUs from executing any process. Once the last
+ task that has a timeslice left has finished using up that timeslice,
+ the recalculation loop is triggered and other CPUs can start executing
+ tasks again - after having idled around for a number of timer ticks.
+ The more CPUs, the worse this effect.
+ Furthermore, this same effect causes the bouncing effect as well:
+ whenever there is such a 'timeslice squeeze' of the global runqueue,
+ idle processors start executing tasks which are not affine to that CPU.
+ (because the affine tasks have finished off their timeslices already.)
+ The new scheduler solves this problem by distributing timeslices on a
+ per-CPU basis, without having any global synchronization or
+ recalculation.
+ - batch scheduling. A significant proportion of computing-intensive tasks
+ benefit from batch-scheduling, where timeslices are long and processes
+ are roundrobin scheduled. The new scheduler does such batch-scheduling
+ of the lowest priority tasks - so nice +19 jobs will get
+ 'batch-scheduled' automatically. With this scheduler, nice +19 jobs are
+ in essence SCHED_IDLE, from an interactiveness point of view.
+ - handle extreme loads more smoothly, without breakdown and scheduling
+ storms.
+ - O(1) RT scheduling. For those RT folks who are paranoid about the
+ O(nr_running) property of the goodness loop and the recalculation loop.
+ - run fork()ed children before the parent. Andrea has pointed out the
+ advantages of this a few months ago, but patches for this feature
+ do not work with the old scheduler as well as they should,
+ because idle processes often steal the new child before the fork()ing
+ CPU gets to execute it.
+The core of the new scheduler contains the following mechanisms:
+ - *two* priority-ordered 'priority arrays' per CPU. There is an 'active'
+ array and an 'expired' array. The active array contains all tasks that
+ are affine to this CPU and have timeslices left. The expired array
+ contains all tasks which have used up their timeslices - but this array
+ is kept sorted as well. The active and expired array is not accessed
+ directly, it's accessed through two pointers in the per-CPU runqueue
+ structure. If all active tasks are used up then we 'switch' the two
+ pointers and from now on the ready-to-go (former-) expired array is the
+ active array - and the empty active array serves as the new collector
+ for expired tasks.
+ - there is a 64-bit bitmap cache for array indices. Finding the highest
+ priority task is thus a matter of two x86 BSFL bit-search instructions.
+the split-array solution enables us to have an arbitrary number of active
+and expired tasks, and the recalculation of timeslices can be done
+immediately when the timeslice expires. Because the arrays are always
+access through the pointers in the runqueue, switching the two arrays can
+be done very quickly.
+this is a hybride priority-list approach coupled with roundrobin
+scheduling and the array-switch method of distributing timeslices.
+ - there is a per-task 'load estimator'.
+one of the toughest things to get right is good interactive feel during
+heavy system load. While playing with various scheduler variants i found
+that the best interactive feel is achieved not by 'boosting' interactive
+tasks, but by 'punishing' tasks that want to use more CPU time than there
+is available. This method is also much easier to do in an O(1) fashion.
+to establish the actual 'load' the task contributes to the system, a
+complex-looking but pretty accurate method is used: there is a 4-entry
+'history' ringbuffer of the task's activities during the last 4 seconds.
+This ringbuffer is operated without much overhead. The entries tell the
+scheduler a pretty accurate load-history of the task: has it used up more
+CPU time or less during the past N seconds. [the size '4' and the interval
+of 4x 1 seconds was found by lots of experimentation - this part is
+flexible and can be changed in both directions.]
+the penalty a task gets for generating more load than the CPU can handle
+is a priority decrease - there is a maximum amount to this penalty
+relative to their static priority, so even fully CPU-bound tasks will
+observe each other's priorities, and will share the CPU accordingly.
+the SMP load-balancer can be extended/switched with additional parallel
+computing and cache hierarchy concepts: NUMA scheduling, multi-core CPUs
+can be supported easily by changing the load-balancer. Right now it's
+tuned for my SMP systems.
+i skipped the prev->mm == next->mm advantage - no workload i know of shows
+any sensitivity to this. It can be added back by sacrificing O(1)
+schedule() [the current and one-lower priority list can be searched for a
+that->mm == current->mm condition], but costs a fair number of cycles
+during a number of important workloads, so i wanted to avoid this as much
+as possible.
+- the SMP idle-task startup code was still racy and the new scheduler
+triggered this. So i streamlined the idle-setup code a bit. We do not call
+into schedule() before all processors have started up fully and all idle
+threads are in place.
+- the patch also cleans up a number of aspects of sched.c - moves code
+into other areas of the kernel where it's appropriate, and simplifies
+certain code paths and data constructs. As a result, the new scheduler's
+code is smaller than the old one.
+ Ingo

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