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