+++ /dev/null
- 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.
-
-
-Goal
-====
-
-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.
-
-
-Design
-======
-
-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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