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-CFS Bandwidth Control
-=====================
-
-[ This document only discusses CPU bandwidth control for SCHED_NORMAL.
- The SCHED_RT case is covered in Documentation/scheduler/sched-rt-group.txt ]
-
-CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the
-specification of the maximum CPU bandwidth available to a group or hierarchy.
-
-The bandwidth allowed for a group is specified using a quota and period. Within
-each given "period" (microseconds), a group is allowed to consume only up to
-"quota" microseconds of CPU time. When the CPU bandwidth consumption of a
-group exceeds this limit (for that period), the tasks belonging to its
-hierarchy will be throttled and are not allowed to run again until the next
-period.
-
-A group's unused runtime is globally tracked, being refreshed with quota units
-above at each period boundary. As threads consume this bandwidth it is
-transferred to cpu-local "silos" on a demand basis. The amount transferred
-within each of these updates is tunable and described as the "slice".
-
-Management
-----------
-Quota and period are managed within the cpu subsystem via cgroupfs.
-
-cpu.cfs_quota_us: the total available run-time within a period (in microseconds)
-cpu.cfs_period_us: the length of a period (in microseconds)
-cpu.stat: exports throttling statistics [explained further below]
-
-The default values are:
- cpu.cfs_period_us=100ms
- cpu.cfs_quota=-1
-
-A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
-bandwidth restriction in place, such a group is described as an unconstrained
-bandwidth group. This represents the traditional work-conserving behavior for
-CFS.
-
-Writing any (valid) positive value(s) will enact the specified bandwidth limit.
-The minimum quota allowed for the quota or period is 1ms. There is also an
-upper bound on the period length of 1s. Additional restrictions exist when
-bandwidth limits are used in a hierarchical fashion, these are explained in
-more detail below.
-
-Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
-and return the group to an unconstrained state once more.
-
-Any updates to a group's bandwidth specification will result in it becoming
-unthrottled if it is in a constrained state.
-
-System wide settings
---------------------
-For efficiency run-time is transferred between the global pool and CPU local
-"silos" in a batch fashion. This greatly reduces global accounting pressure
-on large systems. The amount transferred each time such an update is required
-is described as the "slice".
-
-This is tunable via procfs:
- /proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms)
-
-Larger slice values will reduce transfer overheads, while smaller values allow
-for more fine-grained consumption.
-
-Statistics
-----------
-A group's bandwidth statistics are exported via 3 fields in cpu.stat.
-
-cpu.stat:
-- nr_periods: Number of enforcement intervals that have elapsed.
-- nr_throttled: Number of times the group has been throttled/limited.
-- throttled_time: The total time duration (in nanoseconds) for which entities
- of the group have been throttled.
-
-This interface is read-only.
-
-Hierarchical considerations
----------------------------
-The interface enforces that an individual entity's bandwidth is always
-attainable, that is: max(c_i) <= C. However, over-subscription in the
-aggregate case is explicitly allowed to enable work-conserving semantics
-within a hierarchy.
- e.g. \Sum (c_i) may exceed C
-[ Where C is the parent's bandwidth, and c_i its children ]
-
-
-There are two ways in which a group may become throttled:
- a. it fully consumes its own quota within a period
- b. a parent's quota is fully consumed within its period
-
-In case b) above, even though the child may have runtime remaining it will not
-be allowed to until the parent's runtime is refreshed.
-
-Examples
---------
-1. Limit a group to 1 CPU worth of runtime.
-
- If period is 250ms and quota is also 250ms, the group will get
- 1 CPU worth of runtime every 250ms.
-
- # echo 250000 > cpu.cfs_quota_us /* quota = 250ms */
- # echo 250000 > cpu.cfs_period_us /* period = 250ms */
-
-2. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine.
-
- With 500ms period and 1000ms quota, the group can get 2 CPUs worth of
- runtime every 500ms.
-
- # echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */
- # echo 500000 > cpu.cfs_period_us /* period = 500ms */
-
- The larger period here allows for increased burst capacity.
-
-3. Limit a group to 20% of 1 CPU.
-
- With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU.
-
- # echo 10000 > cpu.cfs_quota_us /* quota = 10ms */
- # echo 50000 > cpu.cfs_period_us /* period = 50ms */
-
- By using a small period here we are ensuring a consistent latency
- response at the expense of burst capacity.
-