Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 CFS 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 7 CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the 8 specification of the maximum CPU bandwidth available to a group or hierarchy. 9 10 The bandwidth allowed for a group is specified using a quota and period. Within 11 each 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 13 group exceeds this limit (for that period), the tasks belonging to its 14 hierarchy will be throttled and are not allowed to run again until the next 15 period. 16 17 A group's unused runtime is globally tracked, being refreshed with quota units 18 above at each period boundary. As threads consume this bandwidth it is 19 transferred to cpu-local "silos" on a demand basis. The amount transferred 20 within each of these updates is tunable and described as the "slice". 21 22 Management 23 ---------- 24 Quota and period are managed within the cpu subsystem via cgroupfs. 25 26 cpu.cfs_quota_us: the total available run-time within a period (in microseconds) 27 cpu.cfs_period_us: the length of a period (in microseconds) 28 cpu.stat: exports throttling statistics [explained further below] 29 30 The default values are: 31 cpu.cfs_period_us=100ms 32 cpu.cfs_quota=-1 33 34 A value of -1 for cpu.cfs_quota_us indicates that the group does not have any 35 bandwidth restriction in place, such a group is described as an unconstrained 36 bandwidth group. This represents the traditional work-conserving behavior for 37 CFS. 38 39 Writing any (valid) positive value(s) will enact the specified bandwidth limit. 40 The minimum quota allowed for the quota or period is 1ms. There is also an 41 upper bound on the period length of 1s. Additional restrictions exist when 42 bandwidth limits are used in a hierarchical fashion, these are explained in 43 more detail below. 44 45 Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit 46 and return the group to an unconstrained state once more. 47 48 Any updates to a group's bandwidth specification will result in it becoming 49 unthrottled if it is in a constrained state. 50 51 System wide settings 52 -------------------- 53 For efficiency run-time is transferred between the global pool and CPU local 54 "silos" in a batch fashion. This greatly reduces global accounting pressure 55 on large systems. The amount transferred each time such an update is required 56 is described as the "slice". 57 58 This is tunable via procfs: 59 /proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms) 60 61 Larger slice values will reduce transfer overheads, while smaller values allow 62 for more fine-grained consumption. 63 64 Statistics 65 ---------- 66 A group's bandwidth statistics are exported via 3 fields in cpu.stat. 67 68 cpu.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 74 This interface is read-only. 75 76 Hierarchical considerations 77 --------------------------- 78 The interface enforces that an individual entity's bandwidth is always 79 attainable, that is: max(c_i) <= C. However, over-subscription in the 80 aggregate case is explicitly allowed to enable work-conserving semantics 81 within 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 86 There 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 90 In case b) above, even though the child may have runtime remaining it will not 91 be allowed to until the parent's runtime is refreshed. 92 93 Examples 94 -------- 95 1. 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 */ 102 103 2. 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. 112 113 3. 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.