1=====================
2CFS Bandwidth Control
3=====================
4
5[ This document only discusses CPU bandwidth control for SCHED_NORMAL.
6  The SCHED_RT case is covered in Documentation/scheduler/sched-rt-group.rst ]
7
8CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the
9specification of the maximum CPU bandwidth available to a group or hierarchy.
10
11The bandwidth allowed for a group is specified using a quota and period. Within
12each given "period" (microseconds), a task group is allocated up to "quota"
13microseconds of CPU time. That quota is assigned to per-cpu run queues in
14slices as threads in the cgroup become runnable. Once all quota has been
15assigned any additional requests for quota will result in those threads being
16throttled. Throttled threads will not be able to run again until the next
17period when the quota is replenished.
18
19A group's unassigned quota is globally tracked, being refreshed back to
20cfs_quota units at each period boundary. As threads consume this bandwidth it
21is transferred to cpu-local "silos" on a demand basis. The amount transferred
22within each of these updates is tunable and described as the "slice".
23
24Management
25----------
26Quota and period are managed within the cpu subsystem via cgroupfs.
27
28cpu.cfs_quota_us: the total available run-time within a period (in microseconds)
29cpu.cfs_period_us: the length of a period (in microseconds)
30cpu.stat: exports throttling statistics [explained further below]
31
32The default values are::
33
34	cpu.cfs_period_us=100ms
35	cpu.cfs_quota=-1
36
37A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
38bandwidth restriction in place, such a group is described as an unconstrained
39bandwidth group. This represents the traditional work-conserving behavior for
40CFS.
41
42Writing any (valid) positive value(s) will enact the specified bandwidth limit.
43The minimum quota allowed for the quota or period is 1ms. There is also an
44upper bound on the period length of 1s. Additional restrictions exist when
45bandwidth limits are used in a hierarchical fashion, these are explained in
46more detail below.
47
48Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
49and return the group to an unconstrained state once more.
50
51Any updates to a group's bandwidth specification will result in it becoming
52unthrottled if it is in a constrained state.
53
54System wide settings
55--------------------
56For efficiency run-time is transferred between the global pool and CPU local
57"silos" in a batch fashion. This greatly reduces global accounting pressure
58on large systems. The amount transferred each time such an update is required
59is described as the "slice".
60
61This is tunable via procfs::
62
63	/proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms)
64
65Larger slice values will reduce transfer overheads, while smaller values allow
66for more fine-grained consumption.
67
68Statistics
69----------
70A group's bandwidth statistics are exported via 3 fields in cpu.stat.
71
72cpu.stat:
73
74- nr_periods: Number of enforcement intervals that have elapsed.
75- nr_throttled: Number of times the group has been throttled/limited.
76- throttled_time: The total time duration (in nanoseconds) for which entities
77  of the group have been throttled.
78
79This interface is read-only.
80
81Hierarchical considerations
82---------------------------
83The interface enforces that an individual entity's bandwidth is always
84attainable, that is: max(c_i) <= C. However, over-subscription in the
85aggregate case is explicitly allowed to enable work-conserving semantics
86within a hierarchy:
87
88  e.g. \Sum (c_i) may exceed C
89
90[ Where C is the parent's bandwidth, and c_i its children ]
91
92
93There are two ways in which a group may become throttled:
94
95	a. it fully consumes its own quota within a period
96	b. a parent's quota is fully consumed within its period
97
98In case b) above, even though the child may have runtime remaining it will not
99be allowed to until the parent's runtime is refreshed.
100
101CFS Bandwidth Quota Caveats
102---------------------------
103Once a slice is assigned to a cpu it does not expire.  However all but 1ms of
104the slice may be returned to the global pool if all threads on that cpu become
105unrunnable. This is configured at compile time by the min_cfs_rq_runtime
106variable. This is a performance tweak that helps prevent added contention on
107the global lock.
108
109The fact that cpu-local slices do not expire results in some interesting corner
110cases that should be understood.
111
112For cgroup cpu constrained applications that are cpu limited this is a
113relatively moot point because they will naturally consume the entirety of their
114quota as well as the entirety of each cpu-local slice in each period. As a
115result it is expected that nr_periods roughly equal nr_throttled, and that
116cpuacct.usage will increase roughly equal to cfs_quota_us in each period.
117
118For highly-threaded, non-cpu bound applications this non-expiration nuance
119allows applications to briefly burst past their quota limits by the amount of
120unused slice on each cpu that the task group is running on (typically at most
1211ms per cpu or as defined by min_cfs_rq_runtime).  This slight burst only
122applies if quota had been assigned to a cpu and then not fully used or returned
123in previous periods. This burst amount will not be transferred between cores.
124As a result, this mechanism still strictly limits the task group to quota
125average usage, albeit over a longer time window than a single period.  This
126also limits the burst ability to no more than 1ms per cpu.  This provides
127better more predictable user experience for highly threaded applications with
128small quota limits on high core count machines. It also eliminates the
129propensity to throttle these applications while simultanously using less than
130quota amounts of cpu. Another way to say this, is that by allowing the unused
131portion of a slice to remain valid across periods we have decreased the
132possibility of wastefully expiring quota on cpu-local silos that don't need a
133full slice's amount of cpu time.
134
135The interaction between cpu-bound and non-cpu-bound-interactive applications
136should also be considered, especially when single core usage hits 100%. If you
137gave each of these applications half of a cpu-core and they both got scheduled
138on the same CPU it is theoretically possible that the non-cpu bound application
139will use up to 1ms additional quota in some periods, thereby preventing the
140cpu-bound application from fully using its quota by that same amount. In these
141instances it will be up to the CFS algorithm (see sched-design-CFS.rst) to
142decide which application is chosen to run, as they will both be runnable and
143have remaining quota. This runtime discrepancy will be made up in the following
144periods when the interactive application idles.
145
146Examples
147--------
1481. Limit a group to 1 CPU worth of runtime::
149
150	If period is 250ms and quota is also 250ms, the group will get
151	1 CPU worth of runtime every 250ms.
152
153	# echo 250000 > cpu.cfs_quota_us /* quota = 250ms */
154	# echo 250000 > cpu.cfs_period_us /* period = 250ms */
155
1562. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine
157
158   With 500ms period and 1000ms quota, the group can get 2 CPUs worth of
159   runtime every 500ms::
160
161	# echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */
162	# echo 500000 > cpu.cfs_period_us /* period = 500ms */
163
164	The larger period here allows for increased burst capacity.
165
1663. Limit a group to 20% of 1 CPU.
167
168   With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU::
169
170	# echo 10000 > cpu.cfs_quota_us /* quota = 10ms */
171	# echo 50000 > cpu.cfs_period_us /* period = 50ms */
172
173   By using a small period here we are ensuring a consistent latency
174   response at the expense of burst capacity.
175