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Documentation / scheduler / sched-bwc.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	CFS Bandwidth Control
2	=====================
3	
4	[ This document only discusses CPU bandwidth control for SCHED_NORMAL.
5	  The SCHED_RT case is covered in Documentation/scheduler/sched-rt-group.txt ]
6	
7	CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the
8	specification of the maximum CPU bandwidth available to a group or hierarchy.
9	
10	The bandwidth allowed for a group is specified using a quota and period. Within
11	each given "period" (microseconds), a group is allowed to consume only up to
12	"quota" microseconds of CPU time.  When the CPU bandwidth consumption of a
13	group exceeds this limit (for that period), the tasks belonging to its
14	hierarchy will be throttled and are not allowed to run again until the next
15	period.
16	
17	A group's unused runtime is globally tracked, being refreshed with quota units
18	above at each period boundary.  As threads consume this bandwidth it is
19	transferred to cpu-local "silos" on a demand basis.  The amount transferred
20	within each of these updates is tunable and described as the "slice".
21	
22	Management
23	----------
24	Quota and period are managed within the cpu subsystem via cgroupfs.
25	
26	cpu.cfs_quota_us: the total available run-time within a period (in microseconds)
27	cpu.cfs_period_us: the length of a period (in microseconds)
28	cpu.stat: exports throttling statistics [explained further below]
29	
30	The default values are:
31		cpu.cfs_period_us=100ms
32		cpu.cfs_quota=-1
33	
34	A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
35	bandwidth restriction in place, such a group is described as an unconstrained
36	bandwidth group.  This represents the traditional work-conserving behavior for
37	CFS.
38	
39	Writing any (valid) positive value(s) will enact the specified bandwidth limit.
40	The minimum quota allowed for the quota or period is 1ms.  There is also an
41	upper bound on the period length of 1s.  Additional restrictions exist when
42	bandwidth limits are used in a hierarchical fashion, these are explained in
43	more detail below.
44	
45	Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
46	and return the group to an unconstrained state once more.
47	
48	Any updates to a group's bandwidth specification will result in it becoming
49	unthrottled if it is in a constrained state.
50	
51	System wide settings
52	--------------------
53	For efficiency run-time is transferred between the global pool and CPU local
54	"silos" in a batch fashion.  This greatly reduces global accounting pressure
55	on large systems.  The amount transferred each time such an update is required
56	is described as the "slice".
57	
58	This is tunable via procfs:
59		/proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms)
60	
61	Larger slice values will reduce transfer overheads, while smaller values allow
62	for more fine-grained consumption.
63	
64	Statistics
65	----------
66	A group's bandwidth statistics are exported via 3 fields in cpu.stat.
67	
68	cpu.stat:
69	- nr_periods: Number of enforcement intervals that have elapsed.
70	- nr_throttled: Number of times the group has been throttled/limited.
71	- throttled_time: The total time duration (in nanoseconds) for which entities
72	  of the group have been throttled.
73	
74	This interface is read-only.
75	
76	Hierarchical considerations
77	---------------------------
78	The interface enforces that an individual entity's bandwidth is always
79	attainable, that is: max(c_i) <= C. However, over-subscription in the
80	aggregate case is explicitly allowed to enable work-conserving semantics
81	within a hierarchy.
82	  e.g. \Sum (c_i) may exceed C
83	[ Where C is the parent's bandwidth, and c_i its children ]
84	
85	
86	There are two ways in which a group may become throttled:
87		a. it fully consumes its own quota within a period
88		b. a parent's quota is fully consumed within its period
89	
90	In case b) above, even though the child may have runtime remaining it will not
91	be allowed to until the parent's runtime is refreshed.
92	
93	Examples
94	--------
95	1. Limit a group to 1 CPU worth of runtime.
96	
97		If period is 250ms and quota is also 250ms, the group will get
98		1 CPU worth of runtime every 250ms.
99	
100		# echo 250000 > cpu.cfs_quota_us /* quota = 250ms */
101		# echo 250000 > cpu.cfs_period_us /* period = 250ms */
102	
103	2. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine.
104	
105		With 500ms period and 1000ms quota, the group can get 2 CPUs worth of
106		runtime every 500ms.
107	
108		# echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */
109		# echo 500000 > cpu.cfs_period_us /* period = 500ms */
110	
111		The larger period here allows for increased burst capacity.
112	
113	3. Limit a group to 20% of 1 CPU.
114	
115		With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU.
116	
117		# echo 10000 > cpu.cfs_quota_us /* quota = 10ms */
118		# echo 50000 > cpu.cfs_period_us /* period = 50ms */
119	
120		By using a small period here we are ensuring a consistent latency
121		response at the expense of burst capacity.
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