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 group is allowed to consume only up to 13"quota" microseconds of CPU time. When the CPU bandwidth consumption of a 14group exceeds this limit (for that period), the tasks belonging to its 15hierarchy will be throttled and are not allowed to run again until the next 16period. 17 18A group's unused runtime is globally tracked, being refreshed with quota units 19above at each period boundary. As threads consume this bandwidth it is 20transferred to cpu-local "silos" on a demand basis. The amount transferred 21within each of these updates is tunable and described as the "slice". 22 23Management 24---------- 25Quota and period are managed within the cpu subsystem via cgroupfs. 26 27cpu.cfs_quota_us: the total available run-time within a period (in microseconds) 28cpu.cfs_period_us: the length of a period (in microseconds) 29cpu.stat: exports throttling statistics [explained further below] 30 31The default values are:: 32 33 cpu.cfs_period_us=100ms 34 cpu.cfs_quota=-1 35 36A value of -1 for cpu.cfs_quota_us indicates that the group does not have any 37bandwidth restriction in place, such a group is described as an unconstrained 38bandwidth group. This represents the traditional work-conserving behavior for 39CFS. 40 41Writing any (valid) positive value(s) will enact the specified bandwidth limit. 42The minimum quota allowed for the quota or period is 1ms. There is also an 43upper bound on the period length of 1s. Additional restrictions exist when 44bandwidth limits are used in a hierarchical fashion, these are explained in 45more detail below. 46 47Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit 48and return the group to an unconstrained state once more. 49 50Any updates to a group's bandwidth specification will result in it becoming 51unthrottled if it is in a constrained state. 52 53System wide settings 54-------------------- 55For efficiency run-time is transferred between the global pool and CPU local 56"silos" in a batch fashion. This greatly reduces global accounting pressure 57on large systems. The amount transferred each time such an update is required 58is described as the "slice". 59 60This is tunable via procfs:: 61 62 /proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms) 63 64Larger slice values will reduce transfer overheads, while smaller values allow 65for more fine-grained consumption. 66 67Statistics 68---------- 69A group's bandwidth statistics are exported via 3 fields in cpu.stat. 70 71cpu.stat: 72 73- nr_periods: Number of enforcement intervals that have elapsed. 74- nr_throttled: Number of times the group has been throttled/limited. 75- throttled_time: The total time duration (in nanoseconds) for which entities 76 of the group have been throttled. 77 78This interface is read-only. 79 80Hierarchical considerations 81--------------------------- 82The interface enforces that an individual entity's bandwidth is always 83attainable, that is: max(c_i) <= C. However, over-subscription in the 84aggregate case is explicitly allowed to enable work-conserving semantics 85within a hierarchy: 86 87 e.g. \Sum (c_i) may exceed C 88 89[ Where C is the parent's bandwidth, and c_i its children ] 90 91 92There are two ways in which a group may become throttled: 93 94 a. it fully consumes its own quota within a period 95 b. a parent's quota is fully consumed within its period 96 97In case b) above, even though the child may have runtime remaining it will not 98be allowed to until the parent's runtime is refreshed. 99 100Examples 101-------- 1021. Limit a group to 1 CPU worth of runtime:: 103 104 If period is 250ms and quota is also 250ms, the group will get 105 1 CPU worth of runtime every 250ms. 106 107 # echo 250000 > cpu.cfs_quota_us /* quota = 250ms */ 108 # echo 250000 > cpu.cfs_period_us /* period = 250ms */ 109 1102. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine 111 112 With 500ms period and 1000ms quota, the group can get 2 CPUs worth of 113 runtime every 500ms:: 114 115 # echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */ 116 # echo 500000 > cpu.cfs_period_us /* period = 500ms */ 117 118 The larger period here allows for increased burst capacity. 119 1203. Limit a group to 20% of 1 CPU. 121 122 With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU:: 123 124 # echo 10000 > cpu.cfs_quota_us /* quota = 10ms */ 125 # echo 50000 > cpu.cfs_period_us /* period = 50ms */ 126 127 By using a small period here we are ensuring a consistent latency 128 response at the expense of burst capacity. 129