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 ]
8 CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the
9 specification of the maximum CPU bandwidth available to a group or hierarchy.
11 The bandwidth allowed for a group is specified using a quota and period. Within
12 each given "period" (microseconds), a task group is allocated up to "quota"
13 microseconds of CPU time. That quota is assigned to per-cpu run queues in
14 slices as threads in the cgroup become runnable. Once all quota has been
15 assigned any additional requests for quota will result in those threads being
16 throttled. Throttled threads will not be able to run again until the next
17 period when the quota is replenished.
19 A group's unassigned quota is globally tracked, being refreshed back to
20 cfs_quota units at each period boundary. As threads consume this bandwidth it
21 is transferred to cpu-local "silos" on a demand basis. The amount transferred
22 within each of these updates is tunable and described as the "slice".
26 Quota and period are managed within the cpu subsystem via cgroupfs.
28 cpu.cfs_quota_us: the total available run-time within a period (in microseconds)
29 cpu.cfs_period_us: the length of a period (in microseconds)
30 cpu.stat: exports throttling statistics [explained further below]
32 The default values are::
34 cpu.cfs_period_us=100ms
37 A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
38 bandwidth restriction in place, such a group is described as an unconstrained
39 bandwidth group. This represents the traditional work-conserving behavior for
42 Writing any (valid) positive value(s) will enact the specified bandwidth limit.
43 The minimum quota allowed for the quota or period is 1ms. There is also an
44 upper bound on the period length of 1s. Additional restrictions exist when
45 bandwidth limits are used in a hierarchical fashion, these are explained in
48 Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
49 and return the group to an unconstrained state once more.
51 Any updates to a group's bandwidth specification will result in it becoming
52 unthrottled if it is in a constrained state.
56 For efficiency run-time is transferred between the global pool and CPU local
57 "silos" in a batch fashion. This greatly reduces global accounting pressure
58 on large systems. The amount transferred each time such an update is required
59 is described as the "slice".
61 This is tunable via procfs::
63 /proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms)
65 Larger slice values will reduce transfer overheads, while smaller values allow
66 for more fine-grained consumption.
70 A group's bandwidth statistics are exported via 3 fields in cpu.stat.
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.
79 This interface is read-only.
81 Hierarchical considerations
82 ---------------------------
83 The interface enforces that an individual entity's bandwidth is always
84 attainable, that is: max(c_i) <= C. However, over-subscription in the
85 aggregate case is explicitly allowed to enable work-conserving semantics
88 e.g. \Sum (c_i) may exceed C
90 [ Where C is the parent's bandwidth, and c_i its children ]
93 There are two ways in which a group may become throttled:
95 a. it fully consumes its own quota within a period
96 b. a parent's quota is fully consumed within its period
98 In case b) above, even though the child may have runtime remaining it will not
99 be allowed to until the parent's runtime is refreshed.
101 CFS Bandwidth Quota Caveats
102 ---------------------------
103 Once a slice is assigned to a cpu it does not expire. However all but 1ms of
104 the slice may be returned to the global pool if all threads on that cpu become
105 unrunnable. This is configured at compile time by the min_cfs_rq_runtime
106 variable. This is a performance tweak that helps prevent added contention on
109 The fact that cpu-local slices do not expire results in some interesting corner
110 cases that should be understood.
112 For cgroup cpu constrained applications that are cpu limited this is a
113 relatively moot point because they will naturally consume the entirety of their
114 quota as well as the entirety of each cpu-local slice in each period. As a
115 result it is expected that nr_periods roughly equal nr_throttled, and that
116 cpuacct.usage will increase roughly equal to cfs_quota_us in each period.
118 For highly-threaded, non-cpu bound applications this non-expiration nuance
119 allows applications to briefly burst past their quota limits by the amount of
120 unused slice on each cpu that the task group is running on (typically at most
121 1ms per cpu or as defined by min_cfs_rq_runtime). This slight burst only
122 applies if quota had been assigned to a cpu and then not fully used or returned
123 in previous periods. This burst amount will not be transferred between cores.
124 As a result, this mechanism still strictly limits the task group to quota
125 average usage, albeit over a longer time window than a single period. This
126 also limits the burst ability to no more than 1ms per cpu. This provides
127 better more predictable user experience for highly threaded applications with
128 small quota limits on high core count machines. It also eliminates the
129 propensity to throttle these applications while simultanously using less than
130 quota amounts of cpu. Another way to say this, is that by allowing the unused
131 portion of a slice to remain valid across periods we have decreased the
132 possibility of wastefully expiring quota on cpu-local silos that don't need a
133 full slice's amount of cpu time.
135 The interaction between cpu-bound and non-cpu-bound-interactive applications
136 should also be considered, especially when single core usage hits 100%. If you
137 gave each of these applications half of a cpu-core and they both got scheduled
138 on the same CPU it is theoretically possible that the non-cpu bound application
139 will use up to 1ms additional quota in some periods, thereby preventing the
140 cpu-bound application from fully using its quota by that same amount. In these
141 instances it will be up to the CFS algorithm (see sched-design-CFS.rst) to
142 decide which application is chosen to run, as they will both be runnable and
143 have remaining quota. This runtime discrepancy will be made up in the following
144 periods when the interactive application idles.
148 1. Limit a group to 1 CPU worth of runtime::
150 If period is 250ms and quota is also 250ms, the group will get
151 1 CPU worth of runtime every 250ms.
153 # echo 250000 > cpu.cfs_quota_us /* quota = 250ms */
154 # echo 250000 > cpu.cfs_period_us /* period = 250ms */
156 2. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine
158 With 500ms period and 1000ms quota, the group can get 2 CPUs worth of
159 runtime every 500ms::
161 # echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */
162 # echo 500000 > cpu.cfs_period_us /* period = 500ms */
164 The larger period here allows for increased burst capacity.
166 3. Limit a group to 20% of 1 CPU.
168 With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU::
170 # echo 10000 > cpu.cfs_quota_us /* quota = 10ms */
171 # echo 50000 > cpu.cfs_period_us /* period = 50ms */
173 By using a small period here we are ensuring a consistent latency
174 response at the expense of burst capacity.