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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.

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