linux/Documentation/scheduler/sched-bwc.txt
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   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
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