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Documentation / scheduler / sched-design-CFS.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	                      =============
2	                      CFS Scheduler
3	                      =============
4	
5	
6	1.  OVERVIEW
7	
8	CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
9	scheduler implemented by Ingo Molnar and merged in Linux 2.6.23.  It is the
10	replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
11	code.
12	
13	80% of CFS's design can be summed up in a single sentence: CFS basically models
14	an "ideal, precise multi-tasking CPU" on real hardware.
15	
16	"Ideal multi-tasking CPU" is a (non-existent  :-)) CPU that has 100% physical
17	power and which can run each task at precise equal speed, in parallel, each at
18	1/nr_running speed.  For example: if there are 2 tasks running, then it runs
19	each at 50% physical power --- i.e., actually in parallel.
20	
21	On real hardware, we can run only a single task at once, so we have to
22	introduce the concept of "virtual runtime."  The virtual runtime of a task
23	specifies when its next timeslice would start execution on the ideal
24	multi-tasking CPU described above.  In practice, the virtual runtime of a task
25	is its actual runtime normalized to the total number of running tasks.
26	
27	
28	
29	2.  FEW IMPLEMENTATION DETAILS
30	
31	In CFS the virtual runtime is expressed and tracked via the per-task
32	p->se.vruntime (nanosec-unit) value.  This way, it's possible to accurately
33	timestamp and measure the "expected CPU time" a task should have gotten.
34	
35	[ small detail: on "ideal" hardware, at any time all tasks would have the same
36	  p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
37	  would ever get "out of balance" from the "ideal" share of CPU time.  ]
38	
39	CFS's task picking logic is based on this p->se.vruntime value and it is thus
40	very simple: it always tries to run the task with the smallest p->se.vruntime
41	value (i.e., the task which executed least so far).  CFS always tries to split
42	up CPU time between runnable tasks as close to "ideal multitasking hardware" as
43	possible.
44	
45	Most of the rest of CFS's design just falls out of this really simple concept,
46	with a few add-on embellishments like nice levels, multiprocessing and various
47	algorithm variants to recognize sleepers.
48	
49	
50	
51	3.  THE RBTREE
52	
53	CFS's design is quite radical: it does not use the old data structures for the
54	runqueues, but it uses a time-ordered rbtree to build a "timeline" of future
55	task execution, and thus has no "array switch" artifacts (by which both the
56	previous vanilla scheduler and RSDL/SD are affected).
57	
58	CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
59	increasing value tracking the smallest vruntime among all tasks in the
60	runqueue.  The total amount of work done by the system is tracked using
61	min_vruntime; that value is used to place newly activated entities on the left
62	side of the tree as much as possible.
63	
64	The total number of running tasks in the runqueue is accounted through the
65	rq->cfs.load value, which is the sum of the weights of the tasks queued on the
66	runqueue.
67	
68	CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
69	p->se.vruntime key. CFS picks the "leftmost" task from this tree and sticks to it.
70	As the system progresses forwards, the executed tasks are put into the tree
71	more and more to the right --- slowly but surely giving a chance for every task
72	to become the "leftmost task" and thus get on the CPU within a deterministic
73	amount of time.
74	
75	Summing up, CFS works like this: it runs a task a bit, and when the task
76	schedules (or a scheduler tick happens) the task's CPU usage is "accounted
77	for": the (small) time it just spent using the physical CPU is added to
78	p->se.vruntime.  Once p->se.vruntime gets high enough so that another task
79	becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a
80	small amount of "granularity" distance relative to the leftmost task so that we
81	do not over-schedule tasks and trash the cache), then the new leftmost task is
82	picked and the current task is preempted.
83	
84	
85	
86	4.  SOME FEATURES OF CFS
87	
88	CFS uses nanosecond granularity accounting and does not rely on any jiffies or
89	other HZ detail.  Thus the CFS scheduler has no notion of "timeslices" in the
90	way the previous scheduler had, and has no heuristics whatsoever.  There is
91	only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):
92	
93	   /proc/sys/kernel/sched_min_granularity_ns
94	
95	which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
96	"server" (i.e., good batching) workloads.  It defaults to a setting suitable
97	for desktop workloads.  SCHED_BATCH is handled by the CFS scheduler module too.
98	
99	Due to its design, the CFS scheduler is not prone to any of the "attacks" that
100	exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
101	chew.c, ring-test.c, massive_intr.c all work fine and do not impact
102	interactivity and produce the expected behavior.
103	
104	The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
105	than the previous vanilla scheduler: both types of workloads are isolated much
106	more aggressively.
107	
108	SMP load-balancing has been reworked/sanitized: the runqueue-walking
109	assumptions are gone from the load-balancing code now, and iterators of the
110	scheduling modules are used.  The balancing code got quite a bit simpler as a
111	result.
112	
113	
114	
115	5. Scheduling policies
116	
117	CFS implements three scheduling policies:
118	
119	  - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
120	    policy that is used for regular tasks.
121	
122	  - SCHED_BATCH: Does not preempt nearly as often as regular tasks
123	    would, thereby allowing tasks to run longer and make better use of
124	    caches but at the cost of interactivity. This is well suited for
125	    batch jobs.
126	
127	  - SCHED_IDLE: This is even weaker than nice 19, but its not a true
128	    idle timer scheduler in order to avoid to get into priority
129	    inversion problems which would deadlock the machine.
130	
131	SCHED_FIFO/_RR are implemented in sched/rt.c and are as specified by
132	POSIX.
133	
134	The command chrt from util-linux-ng 2.13.1.1 can set all of these except
135	SCHED_IDLE.
136	
137	
138	
139	6.  SCHEDULING CLASSES
140	
141	The new CFS scheduler has been designed in such a way to introduce "Scheduling
142	Classes," an extensible hierarchy of scheduler modules.  These modules
143	encapsulate scheduling policy details and are handled by the scheduler core
144	without the core code assuming too much about them.
145	
146	sched/fair.c implements the CFS scheduler described above.
147	
148	sched/rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
149	the previous vanilla scheduler did.  It uses 100 runqueues (for all 100 RT
150	priority levels, instead of 140 in the previous scheduler) and it needs no
151	expired array.
152	
153	Scheduling classes are implemented through the sched_class structure, which
154	contains hooks to functions that must be called whenever an interesting event
155	occurs.
156	
157	This is the (partial) list of the hooks:
158	
159	 - enqueue_task(...)
160	
161	   Called when a task enters a runnable state.
162	   It puts the scheduling entity (task) into the red-black tree and
163	   increments the nr_running variable.
164	
165	 - dequeue_task(...)
166	
167	   When a task is no longer runnable, this function is called to keep the
168	   corresponding scheduling entity out of the red-black tree.  It decrements
169	   the nr_running variable.
170	
171	 - yield_task(...)
172	
173	   This function is basically just a dequeue followed by an enqueue, unless the
174	   compat_yield sysctl is turned on; in that case, it places the scheduling
175	   entity at the right-most end of the red-black tree.
176	
177	 - check_preempt_curr(...)
178	
179	   This function checks if a task that entered the runnable state should
180	   preempt the currently running task.
181	
182	 - pick_next_task(...)
183	
184	   This function chooses the most appropriate task eligible to run next.
185	
186	 - set_curr_task(...)
187	
188	   This function is called when a task changes its scheduling class or changes
189	   its task group.
190	
191	 - task_tick(...)
192	
193	   This function is mostly called from time tick functions; it might lead to
194	   process switch.  This drives the running preemption.
195	
196	
197	
198	
199	7.  GROUP SCHEDULER EXTENSIONS TO CFS
200	
201	Normally, the scheduler operates on individual tasks and strives to provide
202	fair CPU time to each task.  Sometimes, it may be desirable to group tasks and
203	provide fair CPU time to each such task group.  For example, it may be
204	desirable to first provide fair CPU time to each user on the system and then to
205	each task belonging to a user.
206	
207	CONFIG_CGROUP_SCHED strives to achieve exactly that.  It lets tasks to be
208	grouped and divides CPU time fairly among such groups.
209	
210	CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
211	SCHED_RR) tasks.
212	
213	CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
214	SCHED_BATCH) tasks.
215	
216	   These options need CONFIG_CGROUPS to be defined, and let the administrator
217	   create arbitrary groups of tasks, using the "cgroup" pseudo filesystem.  See
218	   Documentation/cgroups/cgroups.txt for more information about this filesystem.
219	
220	When CONFIG_FAIR_GROUP_SCHED is defined, a "cpu.shares" file is created for each
221	group created using the pseudo filesystem.  See example steps below to create
222	task groups and modify their CPU share using the "cgroups" pseudo filesystem.
223	
224		# mount -t tmpfs cgroup_root /sys/fs/cgroup
225		# mkdir /sys/fs/cgroup/cpu
226		# mount -t cgroup -ocpu none /sys/fs/cgroup/cpu
227		# cd /sys/fs/cgroup/cpu
228	
229		# mkdir multimedia	# create "multimedia" group of tasks
230		# mkdir browser		# create "browser" group of tasks
231	
232		# #Configure the multimedia group to receive twice the CPU bandwidth
233		# #that of browser group
234	
235		# echo 2048 > multimedia/cpu.shares
236		# echo 1024 > browser/cpu.shares
237	
238		# firefox &	# Launch firefox and move it to "browser" group
239		# echo <firefox_pid> > browser/tasks
240	
241		# #Launch gmplayer (or your favourite movie player)
242		# echo <movie_player_pid> > multimedia/tasks
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