1===================== 2CFS Bandwidth Control 3===================== 4 5.. note:: 6 This document only discusses CPU bandwidth control for SCHED_NORMAL. 7 The SCHED_RT case is covered in Documentation/scheduler/sched-rt-group.rst 8 9CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the 10specification of the maximum CPU bandwidth available to a group or hierarchy. 11 12The bandwidth allowed for a group is specified using a quota and period. Within 13each given "period" (microseconds), a task group is allocated up to "quota" 14microseconds of CPU time. That quota is assigned to per-cpu run queues in 15slices as threads in the cgroup become runnable. Once all quota has been 16assigned any additional requests for quota will result in those threads being 17throttled. Throttled threads will not be able to run again until the next 18period when the quota is replenished. 19 20A group's unassigned quota is globally tracked, being refreshed back to 21cfs_quota units at each period boundary. As threads consume this bandwidth it 22is transferred to cpu-local "silos" on a demand basis. The amount transferred 23within each of these updates is tunable and described as the "slice". 24 25Management 26---------- 27Quota and period are managed within the cpu subsystem via cgroupfs. 28 29.. note:: 30 The cgroupfs files described in this section are only applicable 31 to cgroup v1. For cgroup v2, see 32 :ref:`Documentation/admin-guide/cgroupv2.rst <cgroup-v2-cpu>`. 33 34- cpu.cfs_quota_us: the total available run-time within a period (in 35 microseconds) 36- cpu.cfs_period_us: the length of a period (in microseconds) 37- cpu.stat: exports throttling statistics [explained further below] 38 39The default values are:: 40 41 cpu.cfs_period_us=100ms 42 cpu.cfs_quota=-1 43 44A value of -1 for cpu.cfs_quota_us indicates that the group does not have any 45bandwidth restriction in place, such a group is described as an unconstrained 46bandwidth group. This represents the traditional work-conserving behavior for 47CFS. 48 49Writing any (valid) positive value(s) will enact the specified bandwidth limit. 50The minimum quota allowed for the quota or period is 1ms. There is also an 51upper bound on the period length of 1s. Additional restrictions exist when 52bandwidth limits are used in a hierarchical fashion, these are explained in 53more detail below. 54 55Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit 56and return the group to an unconstrained state once more. 57 58Any updates to a group's bandwidth specification will result in it becoming 59unthrottled if it is in a constrained state. 60 61System wide settings 62-------------------- 63For efficiency run-time is transferred between the global pool and CPU local 64"silos" in a batch fashion. This greatly reduces global accounting pressure 65on large systems. The amount transferred each time such an update is required 66is described as the "slice". 67 68This is tunable via procfs:: 69 70 /proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms) 71 72Larger slice values will reduce transfer overheads, while smaller values allow 73for more fine-grained consumption. 74 75Statistics 76---------- 77A group's bandwidth statistics are exported via 3 fields in cpu.stat. 78 79cpu.stat: 80 81- nr_periods: Number of enforcement intervals that have elapsed. 82- nr_throttled: Number of times the group has been throttled/limited. 83- throttled_time: The total time duration (in nanoseconds) for which entities 84 of the group have been throttled. 85 86This interface is read-only. 87 88Hierarchical considerations 89--------------------------- 90The interface enforces that an individual entity's bandwidth is always 91attainable, that is: max(c_i) <= C. However, over-subscription in the 92aggregate case is explicitly allowed to enable work-conserving semantics 93within a hierarchy: 94 95 e.g. \Sum (c_i) may exceed C 96 97[ Where C is the parent's bandwidth, and c_i its children ] 98 99 100There are two ways in which a group may become throttled: 101 102 a. it fully consumes its own quota within a period 103 b. a parent's quota is fully consumed within its period 104 105In case b) above, even though the child may have runtime remaining it will not 106be allowed to until the parent's runtime is refreshed. 107 108CFS Bandwidth Quota Caveats 109--------------------------- 110Once a slice is assigned to a cpu it does not expire. However all but 1ms of 111the slice may be returned to the global pool if all threads on that cpu become 112unrunnable. This is configured at compile time by the min_cfs_rq_runtime 113variable. This is a performance tweak that helps prevent added contention on 114the global lock. 115 116The fact that cpu-local slices do not expire results in some interesting corner 117cases that should be understood. 118 119For cgroup cpu constrained applications that are cpu limited this is a 120relatively moot point because they will naturally consume the entirety of their 121quota as well as the entirety of each cpu-local slice in each period. As a 122result it is expected that nr_periods roughly equal nr_throttled, and that 123cpuacct.usage will increase roughly equal to cfs_quota_us in each period. 124 125For highly-threaded, non-cpu bound applications this non-expiration nuance 126allows applications to briefly burst past their quota limits by the amount of 127unused slice on each cpu that the task group is running on (typically at most 1281ms per cpu or as defined by min_cfs_rq_runtime). This slight burst only 129applies if quota had been assigned to a cpu and then not fully used or returned 130in previous periods. This burst amount will not be transferred between cores. 131As a result, this mechanism still strictly limits the task group to quota 132average usage, albeit over a longer time window than a single period. This 133also limits the burst ability to no more than 1ms per cpu. This provides 134better more predictable user experience for highly threaded applications with 135small quota limits on high core count machines. It also eliminates the 136propensity to throttle these applications while simultanously using less than 137quota amounts of cpu. Another way to say this, is that by allowing the unused 138portion of a slice to remain valid across periods we have decreased the 139possibility of wastefully expiring quota on cpu-local silos that don't need a 140full slice's amount of cpu time. 141 142The interaction between cpu-bound and non-cpu-bound-interactive applications 143should also be considered, especially when single core usage hits 100%. If you 144gave each of these applications half of a cpu-core and they both got scheduled 145on the same CPU it is theoretically possible that the non-cpu bound application 146will use up to 1ms additional quota in some periods, thereby preventing the 147cpu-bound application from fully using its quota by that same amount. In these 148instances it will be up to the CFS algorithm (see sched-design-CFS.rst) to 149decide which application is chosen to run, as they will both be runnable and 150have remaining quota. This runtime discrepancy will be made up in the following 151periods when the interactive application idles. 152 153Examples 154-------- 1551. Limit a group to 1 CPU worth of runtime:: 156 157 If period is 250ms and quota is also 250ms, the group will get 158 1 CPU worth of runtime every 250ms. 159 160 # echo 250000 > cpu.cfs_quota_us /* quota = 250ms */ 161 # echo 250000 > cpu.cfs_period_us /* period = 250ms */ 162 1632. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine 164 165 With 500ms period and 1000ms quota, the group can get 2 CPUs worth of 166 runtime every 500ms:: 167 168 # echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */ 169 # echo 500000 > cpu.cfs_period_us /* period = 500ms */ 170 171 The larger period here allows for increased burst capacity. 172 1733. Limit a group to 20% of 1 CPU. 174 175 With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU:: 176 177 # echo 10000 > cpu.cfs_quota_us /* quota = 10ms */ 178 # echo 50000 > cpu.cfs_period_us /* period = 50ms */ 179 180 By using a small period here we are ensuring a consistent latency 181 response at the expense of burst capacity. 182