1CFS 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
7CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the
8specification of the maximum CPU bandwidth available to a group or hierarchy.
9
10The bandwidth allowed for a group is specified using a quota and period. Within
11each 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
13group exceeds this limit (for that period), the tasks belonging to its
14hierarchy will be throttled and are not allowed to run again until the next
15period.
16
17A group's unused runtime is globally tracked, being refreshed with quota units
18above at each period boundary.  As threads consume this bandwidth it is
19transferred to cpu-local "silos" on a demand basis.  The amount transferred
20within each of these updates is tunable and described as the "slice".
21
22Management
23----------
24Quota and period are managed within the cpu subsystem via cgroupfs.
25
26cpu.cfs_quota_us: the total available run-time within a period (in microseconds)
27cpu.cfs_period_us: the length of a period (in microseconds)
28cpu.stat: exports throttling statistics [explained further below]
29
30The default values are:
31	cpu.cfs_period_us=100ms
32	cpu.cfs_quota=-1
33
34A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
35bandwidth restriction in place, such a group is described as an unconstrained
36bandwidth group.  This represents the traditional work-conserving behavior for
37CFS.
38
39Writing any (valid) positive value(s) will enact the specified bandwidth limit.
40The minimum quota allowed for the quota or period is 1ms.  There is also an
41upper bound on the period length of 1s.  Additional restrictions exist when
42bandwidth limits are used in a hierarchical fashion, these are explained in
43more detail below.
44
45Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
46and return the group to an unconstrained state once more.
47
48Any updates to a group's bandwidth specification will result in it becoming
49unthrottled if it is in a constrained state.
50
51System wide settings
52--------------------
53For efficiency run-time is transferred between the global pool and CPU local
54"silos" in a batch fashion.  This greatly reduces global accounting pressure
55on large systems.  The amount transferred each time such an update is required
56is described as the "slice".
57
58This is tunable via procfs:
59	/proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms)
60
61Larger slice values will reduce transfer overheads, while smaller values allow
62for more fine-grained consumption.
63
64Statistics
65----------
66A group's bandwidth statistics are exported via 3 fields in cpu.stat.
67
68cpu.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
74This interface is read-only.
75
76Hierarchical considerations
77---------------------------
78The interface enforces that an individual entity's bandwidth is always
79attainable, that is: max(c_i) <= C. However, over-subscription in the
80aggregate case is explicitly allowed to enable work-conserving semantics
81within 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
86There 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
90In case b) above, even though the child may have runtime remaining it will not
91be allowed to until the parent's runtime is refreshed.
92
93Examples
94--------
951. 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
1032. 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
1133. 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.
122
123