Documentation/scheduler/sched-design-CFS.txt - kernel/skids - Git at Google

                       =============
                       CFS Scheduler
                       =============


 1.  OVERVIEW

 CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
 scheduler implemented by Ingo Molnar and merged in Linux 2.6.23.  It is the
 replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
 code.

 80% of CFS's design can be summed up in a single sentence: CFS basically models
 an "ideal, precise multi-tasking CPU" on real hardware.

 "Ideal multi-tasking CPU" is a (non-existent  :-)) CPU that has 100% physical
 power and which can run each task at precise equal speed, in parallel, each at
 1/nr_running speed.  For example: if there are 2 tasks running, then it runs
 each at 50% physical power --- i.e., actually in parallel.

 On real hardware, we can run only a single task at once, so we have to
 introduce the concept of "virtual runtime."  The virtual runtime of a task
 specifies when its next timeslice would start execution on the ideal
 multi-tasking CPU described above.  In practice, the virtual runtime of a task
 is its actual runtime normalized to the total number of running tasks.


 2.  FEW IMPLEMENTATION DETAILS

 In CFS the virtual runtime is expressed and tracked via the per-task
 p->se.vruntime (nanosec-unit) value.  This way, it's possible to accurately
 timestamp and measure the "expected CPU time" a task should have gotten.

 [ small detail: on "ideal" hardware, at any time all tasks would have the same
   p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
   would ever get "out of balance" from the "ideal" share of CPU time.  ]

 CFS's task picking logic is based on this p->se.vruntime value and it is thus
 very simple: it always tries to run the task with the smallest p->se.vruntime
 value (i.e., the task which executed least so far).  CFS always tries to split
 up CPU time between runnable tasks as close to "ideal multitasking hardware" as
 possible.

 Most of the rest of CFS's design just falls out of this really simple concept,
 with a few add-on embellishments like nice levels, multiprocessing and various
 algorithm variants to recognize sleepers.


 3.  THE RBTREE

 CFS's design is quite radical: it does not use the old data structures for the
 runqueues, but it uses a time-ordered rbtree to build a "timeline" of future
 task execution, and thus has no "array switch" artifacts (by which both the
 previous vanilla scheduler and RSDL/SD are affected).

 CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
 increasing value tracking the smallest vruntime among all tasks in the
 runqueue.  The total amount of work done by the system is tracked using
 min_vruntime; that value is used to place newly activated entities on the left
 side of the tree as much as possible.

 The total number of running tasks in the runqueue is accounted through the
 rq->cfs.load value, which is the sum of the weights of the tasks queued on the
 runqueue.

 CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
 p->se.vruntime key (there is a subtraction using rq->cfs.min_vruntime to
 account for possible wraparounds).  CFS picks the "leftmost" task from this
 tree and sticks to it.
 As the system progresses forwards, the executed tasks are put into the tree
 more and more to the right --- slowly but surely giving a chance for every task
 to become the "leftmost task" and thus get on the CPU within a deterministic
 amount of time.

 Summing up, CFS works like this: it runs a task a bit, and when the task
 schedules (or a scheduler tick happens) the task's CPU usage is "accounted
 for": the (small) time it just spent using the physical CPU is added to
 p->se.vruntime.  Once p->se.vruntime gets high enough so that another task
 becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a
 small amount of "granularity" distance relative to the leftmost task so that we
 do not over-schedule tasks and trash the cache), then the new leftmost task is
 picked and the current task is preempted.


 4.  SOME FEATURES OF CFS

 CFS uses nanosecond granularity accounting and does not rely on any jiffies or
 other HZ detail.  Thus the CFS scheduler has no notion of "timeslices" in the
 way the previous scheduler had, and has no heuristics whatsoever.  There is
 only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):

    /proc/sys/kernel/sched_min_granularity_ns

 which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
 "server" (i.e., good batching) workloads.  It defaults to a setting suitable
 for desktop workloads.  SCHED_BATCH is handled by the CFS scheduler module too.

 Due to its design, the CFS scheduler is not prone to any of the "attacks" that
 exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
 chew.c, ring-test.c, massive_intr.c all work fine and do not impact
 interactivity and produce the expected behavior.

 The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
 than the previous vanilla scheduler: both types of workloads are isolated much
 more aggressively.

 SMP load-balancing has been reworked/sanitized: the runqueue-walking
 assumptions are gone from the load-balancing code now, and iterators of the
 scheduling modules are used.  The balancing code got quite a bit simpler as a
 result.


 5. Scheduling policies

 CFS implements three scheduling policies:

   - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
     policy that is used for regular tasks.

   - SCHED_BATCH: Does not preempt nearly as often as regular tasks
     would, thereby allowing tasks to run longer and make better use of
     caches but at the cost of interactivity. This is well suited for
     batch jobs.

   - SCHED_IDLE: This is even weaker than nice 19, but its not a true
     idle timer scheduler in order to avoid to get into priority
     inversion problems which would deadlock the machine.

 SCHED_FIFO/_RR are implemented in sched_rt.c and are as specified by
 POSIX.

 The command chrt from util-linux-ng 2.13.1.1 can set all of these except
 SCHED_IDLE.


 6.  SCHEDULING CLASSES

 The new CFS scheduler has been designed in such a way to introduce "Scheduling
 Classes," an extensible hierarchy of scheduler modules.  These modules
 encapsulate scheduling policy details and are handled by the scheduler core
 without the core code assuming too much about them.

 sched_fair.c implements the CFS scheduler described above.

 sched_rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
 the previous vanilla scheduler did.  It uses 100 runqueues (for all 100 RT
 priority levels, instead of 140 in the previous scheduler) and it needs no
 expired array.

 Scheduling classes are implemented through the sched_class structure, which
 contains hooks to functions that must be called whenever an interesting event
 occurs.

 This is the (partial) list of the hooks:

  - enqueue_task(...)

    Called when a task enters a runnable state.
    It puts the scheduling entity (task) into the red-black tree and
    increments the nr_running variable.

  - dequeue_tree(...)

    When a task is no longer runnable, this function is called to keep the
    corresponding scheduling entity out of the red-black tree.  It decrements
    the nr_running variable.

  - yield_task(...)

    This function is basically just a dequeue followed by an enqueue, unless the
    compat_yield sysctl is turned on; in that case, it places the scheduling
    entity at the right-most end of the red-black tree.

  - check_preempt_curr(...)

    This function checks if a task that entered the runnable state should
    preempt the currently running task.

  - pick_next_task(...)

    This function chooses the most appropriate task eligible to run next.

  - set_curr_task(...)

    This function is called when a task changes its scheduling class or changes
    its task group.

  - task_tick(...)

    This function is mostly called from time tick functions; it might lead to
    process switch.  This drives the running preemption.

  - task_new(...)

    The core scheduler gives the scheduling module an opportunity to manage new
    task startup.  The CFS scheduling module uses it for group scheduling, while
    the scheduling module for a real-time task does not use it.


 7.  GROUP SCHEDULER EXTENSIONS TO CFS

 Normally, the scheduler operates on individual tasks and strives to provide
 fair CPU time to each task.  Sometimes, it may be desirable to group tasks and
 provide fair CPU time to each such task group.  For example, it may be
 desirable to first provide fair CPU time to each user on the system and then to
 each task belonging to a user.

 CONFIG_CGROUP_SCHED strives to achieve exactly that.  It lets tasks to be
 grouped and divides CPU time fairly among such groups.

 CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
 SCHED_RR) tasks.

 CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
 SCHED_BATCH) tasks.

    These options need CONFIG_CGROUPS to be defined, and let the administrator
    create arbitrary groups of tasks, using the "cgroup" pseudo filesystem.  See
    Documentation/cgroups/cgroups.txt for more information about this filesystem.

 When CONFIG_FAIR_GROUP_SCHED is defined, a "cpu.shares" file is created for each
 group created using the pseudo filesystem.  See example steps below to create
 task groups and modify their CPU share using the "cgroups" pseudo filesystem.

 	# mkdir /dev/cpuctl
 	# mount -t cgroup -ocpu none /dev/cpuctl
 	# cd /dev/cpuctl

 	# mkdir multimedia	# create "multimedia" group of tasks
 	# mkdir browser		# create "browser" group of tasks

 	# #Configure the multimedia group to receive twice the CPU bandwidth
 	# #that of browser group

 	# echo 2048 > multimedia/cpu.shares
 	# echo 1024 > browser/cpu.shares

 	# firefox &	# Launch firefox and move it to "browser" group
 	# echo <firefox_pid> > browser/tasks

 	# #Launch gmplayer (or your favourite movie player)
 	# echo <movie_player_pid> > multimedia/tasks
	=============
	CFS Scheduler
	=============


	1. OVERVIEW

	CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
	scheduler implemented by Ingo Molnar and merged in Linux 2.6.23. It is the
	replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
	code.

	80% of CFS's design can be summed up in a single sentence: CFS basically models
	an "ideal, precise multi-tasking CPU" on real hardware.

	"Ideal multi-tasking CPU" is a (non-existent :-)) CPU that has 100% physical
	power and which can run each task at precise equal speed, in parallel, each at
	1/nr_running speed. For example: if there are 2 tasks running, then it runs
	each at 50% physical power --- i.e., actually in parallel.

	On real hardware, we can run only a single task at once, so we have to
	introduce the concept of "virtual runtime." The virtual runtime of a task
	specifies when its next timeslice would start execution on the ideal
	multi-tasking CPU described above. In practice, the virtual runtime of a task
	is its actual runtime normalized to the total number of running tasks.



	2. FEW IMPLEMENTATION DETAILS

	In CFS the virtual runtime is expressed and tracked via the per-task
	p->se.vruntime (nanosec-unit) value. This way, it's possible to accurately
	timestamp and measure the "expected CPU time" a task should have gotten.

	[ small detail: on "ideal" hardware, at any time all tasks would have the same
	p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
	would ever get "out of balance" from the "ideal" share of CPU time. ]

	CFS's task picking logic is based on this p->se.vruntime value and it is thus
	very simple: it always tries to run the task with the smallest p->se.vruntime
	value (i.e., the task which executed least so far). CFS always tries to split
	up CPU time between runnable tasks as close to "ideal multitasking hardware" as
	possible.

	Most of the rest of CFS's design just falls out of this really simple concept,
	with a few add-on embellishments like nice levels, multiprocessing and various
	algorithm variants to recognize sleepers.



	3. THE RBTREE

	CFS's design is quite radical: it does not use the old data structures for the
	runqueues, but it uses a time-ordered rbtree to build a "timeline" of future
	task execution, and thus has no "array switch" artifacts (by which both the
	previous vanilla scheduler and RSDL/SD are affected).

	CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
	increasing value tracking the smallest vruntime among all tasks in the
	runqueue. The total amount of work done by the system is tracked using
	min_vruntime; that value is used to place newly activated entities on the left
	side of the tree as much as possible.

	The total number of running tasks in the runqueue is accounted through the
	rq->cfs.load value, which is the sum of the weights of the tasks queued on the
	runqueue.

	CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
	p->se.vruntime key (there is a subtraction using rq->cfs.min_vruntime to
	account for possible wraparounds). CFS picks the "leftmost" task from this
	tree and sticks to it.
	As the system progresses forwards, the executed tasks are put into the tree
	more and more to the right --- slowly but surely giving a chance for every task
	to become the "leftmost task" and thus get on the CPU within a deterministic
	amount of time.

	Summing up, CFS works like this: it runs a task a bit, and when the task
	schedules (or a scheduler tick happens) the task's CPU usage is "accounted
	for": the (small) time it just spent using the physical CPU is added to
	p->se.vruntime. Once p->se.vruntime gets high enough so that another task
	becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a
	small amount of "granularity" distance relative to the leftmost task so that we
	do not over-schedule tasks and trash the cache), then the new leftmost task is
	picked and the current task is preempted.



	4. SOME FEATURES OF CFS

	CFS uses nanosecond granularity accounting and does not rely on any jiffies or
	other HZ detail. Thus the CFS scheduler has no notion of "timeslices" in the
	way the previous scheduler had, and has no heuristics whatsoever. There is
	only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):

	/proc/sys/kernel/sched_min_granularity_ns

	which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
	"server" (i.e., good batching) workloads. It defaults to a setting suitable
	for desktop workloads. SCHED_BATCH is handled by the CFS scheduler module too.

	Due to its design, the CFS scheduler is not prone to any of the "attacks" that
	exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
	chew.c, ring-test.c, massive_intr.c all work fine and do not impact
	interactivity and produce the expected behavior.

	The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
	than the previous vanilla scheduler: both types of workloads are isolated much
	more aggressively.

	SMP load-balancing has been reworked/sanitized: the runqueue-walking
	assumptions are gone from the load-balancing code now, and iterators of the
	scheduling modules are used. The balancing code got quite a bit simpler as a
	result.



	5. Scheduling policies

	CFS implements three scheduling policies:

	- SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
	policy that is used for regular tasks.

	- SCHED_BATCH: Does not preempt nearly as often as regular tasks
	would, thereby allowing tasks to run longer and make better use of
	caches but at the cost of interactivity. This is well suited for
	batch jobs.

	- SCHED_IDLE: This is even weaker than nice 19, but its not a true
	idle timer scheduler in order to avoid to get into priority
	inversion problems which would deadlock the machine.

	SCHED_FIFO/_RR are implemented in sched_rt.c and are as specified by
	POSIX.

	The command chrt from util-linux-ng 2.13.1.1 can set all of these except
	SCHED_IDLE.



	6. SCHEDULING CLASSES

	The new CFS scheduler has been designed in such a way to introduce "Scheduling
	Classes," an extensible hierarchy of scheduler modules. These modules
	encapsulate scheduling policy details and are handled by the scheduler core
	without the core code assuming too much about them.

	sched_fair.c implements the CFS scheduler described above.

	sched_rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
	the previous vanilla scheduler did. It uses 100 runqueues (for all 100 RT
	priority levels, instead of 140 in the previous scheduler) and it needs no
	expired array.

	Scheduling classes are implemented through the sched_class structure, which
	contains hooks to functions that must be called whenever an interesting event
	occurs.

	This is the (partial) list of the hooks:

	- enqueue_task(...)

	Called when a task enters a runnable state.
	It puts the scheduling entity (task) into the red-black tree and
	increments the nr_running variable.

	- dequeue_tree(...)

	When a task is no longer runnable, this function is called to keep the
	corresponding scheduling entity out of the red-black tree. It decrements
	the nr_running variable.

	- yield_task(...)

	This function is basically just a dequeue followed by an enqueue, unless the
	compat_yield sysctl is turned on; in that case, it places the scheduling
	entity at the right-most end of the red-black tree.

	- check_preempt_curr(...)

	This function checks if a task that entered the runnable state should
	preempt the currently running task.

	- pick_next_task(...)

	This function chooses the most appropriate task eligible to run next.

	- set_curr_task(...)

	This function is called when a task changes its scheduling class or changes
	its task group.

	- task_tick(...)

	This function is mostly called from time tick functions; it might lead to
	process switch. This drives the running preemption.

	- task_new(...)

	The core scheduler gives the scheduling module an opportunity to manage new
	task startup. The CFS scheduling module uses it for group scheduling, while
	the scheduling module for a real-time task does not use it.



	7. GROUP SCHEDULER EXTENSIONS TO CFS

	Normally, the scheduler operates on individual tasks and strives to provide
	fair CPU time to each task. Sometimes, it may be desirable to group tasks and
	provide fair CPU time to each such task group. For example, it may be
	desirable to first provide fair CPU time to each user on the system and then to
	each task belonging to a user.

	CONFIG_CGROUP_SCHED strives to achieve exactly that. It lets tasks to be
	grouped and divides CPU time fairly among such groups.

	CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
	SCHED_RR) tasks.

	CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
	SCHED_BATCH) tasks.

	These options need CONFIG_CGROUPS to be defined, and let the administrator
	create arbitrary groups of tasks, using the "cgroup" pseudo filesystem. See
	Documentation/cgroups/cgroups.txt for more information about this filesystem.

	When CONFIG_FAIR_GROUP_SCHED is defined, a "cpu.shares" file is created for each
	group created using the pseudo filesystem. See example steps below to create
	task groups and modify their CPU share using the "cgroups" pseudo filesystem.

	# mkdir /dev/cpuctl
	# mount -t cgroup -ocpu none /dev/cpuctl
	# cd /dev/cpuctl

	# mkdir multimedia # create "multimedia" group of tasks
	# mkdir browser # create "browser" group of tasks

	# #Configure the multimedia group to receive twice the CPU bandwidth
	# #that of browser group

	# echo 2048 > multimedia/cpu.shares
	# echo 1024 > browser/cpu.shares

	# firefox & # Launch firefox and move it to "browser" group
	# echo <firefox_pid> > browser/tasks

	# #Launch gmplayer (or your favourite movie player)
	# echo <movie_player_pid> > multimedia/tasks