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Diffstat (limited to 'Documentation/scheduler/sched-bwc.rst')
| -rw-r--r-- | Documentation/scheduler/sched-bwc.rst | 74 |
1 files changed, 60 insertions, 14 deletions
diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst index 3a9064219656..9801d6b284b1 100644 --- a/Documentation/scheduler/sched-bwc.rst +++ b/Documentation/scheduler/sched-bwc.rst @@ -9,15 +9,16 @@ CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the specification of the maximum CPU bandwidth available to a group or hierarchy. The bandwidth allowed for a group is specified using a quota and period. Within -each given "period" (microseconds), a group is allowed to consume only up to -"quota" microseconds of CPU time. When the CPU bandwidth consumption of a -group exceeds this limit (for that period), the tasks belonging to its -hierarchy will be throttled and are not allowed to run again until the next -period. - -A group's unused runtime is globally tracked, being refreshed with quota units -above at each period boundary. As threads consume this bandwidth it is -transferred to cpu-local "silos" on a demand basis. The amount transferred +each given "period" (microseconds), a task group is allocated up to "quota" +microseconds of CPU time. That quota is assigned to per-cpu run queues in +slices as threads in the cgroup become runnable. Once all quota has been +assigned any additional requests for quota will result in those threads being +throttled. Throttled threads will not be able to run again until the next +period when the quota is replenished. + +A group's unassigned quota is globally tracked, being refreshed back to +cfs_quota units at each period boundary. As threads consume this bandwidth it +is transferred to cpu-local "silos" on a demand basis. The amount transferred within each of these updates is tunable and described as the "slice". Management @@ -35,12 +36,12 @@ The default values are:: A value of -1 for cpu.cfs_quota_us indicates that the group does not have any bandwidth restriction in place, such a group is described as an unconstrained -bandwidth group. This represents the traditional work-conserving behavior for +bandwidth group. This represents the traditional work-conserving behavior for CFS. Writing any (valid) positive value(s) will enact the specified bandwidth limit. -The minimum quota allowed for the quota or period is 1ms. There is also an -upper bound on the period length of 1s. Additional restrictions exist when +The minimum quota allowed for the quota or period is 1ms. There is also an +upper bound on the period length of 1s. Additional restrictions exist when bandwidth limits are used in a hierarchical fashion, these are explained in more detail below. @@ -53,8 +54,8 @@ unthrottled if it is in a constrained state. System wide settings -------------------- For efficiency run-time is transferred between the global pool and CPU local -"silos" in a batch fashion. This greatly reduces global accounting pressure -on large systems. The amount transferred each time such an update is required +"silos" in a batch fashion. This greatly reduces global accounting pressure +on large systems. The amount transferred each time such an update is required is described as the "slice". This is tunable via procfs:: @@ -97,6 +98,51 @@ There are two ways in which a group may become throttled: In case b) above, even though the child may have runtime remaining it will not be allowed to until the parent's runtime is refreshed. +CFS Bandwidth Quota Caveats +--------------------------- +Once a slice is assigned to a cpu it does not expire. However all but 1ms of +the slice may be returned to the global pool if all threads on that cpu become +unrunnable. This is configured at compile time by the min_cfs_rq_runtime +variable. This is a performance tweak that helps prevent added contention on +the global lock. + +The fact that cpu-local slices do not expire results in some interesting corner +cases that should be understood. + +For cgroup cpu constrained applications that are cpu limited this is a +relatively moot point because they will naturally consume the entirety of their +quota as well as the entirety of each cpu-local slice in each period. As a +result it is expected that nr_periods roughly equal nr_throttled, and that +cpuacct.usage will increase roughly equal to cfs_quota_us in each period. + +For highly-threaded, non-cpu bound applications this non-expiration nuance +allows applications to briefly burst past their quota limits by the amount of +unused slice on each cpu that the task group is running on (typically at most +1ms per cpu or as defined by min_cfs_rq_runtime). This slight burst only +applies if quota had been assigned to a cpu and then not fully used or returned +in previous periods. This burst amount will not be transferred between cores. +As a result, this mechanism still strictly limits the task group to quota +average usage, albeit over a longer time window than a single period. This +also limits the burst ability to no more than 1ms per cpu. This provides +better more predictable user experience for highly threaded applications with +small quota limits on high core count machines. It also eliminates the +propensity to throttle these applications while simultanously using less than +quota amounts of cpu. Another way to say this, is that by allowing the unused +portion of a slice to remain valid across periods we have decreased the +possibility of wastefully expiring quota on cpu-local silos that don't need a +full slice's amount of cpu time. + +The interaction between cpu-bound and non-cpu-bound-interactive applications +should also be considered, especially when single core usage hits 100%. If you +gave each of these applications half of a cpu-core and they both got scheduled +on the same CPU it is theoretically possible that the non-cpu bound application +will use up to 1ms additional quota in some periods, thereby preventing the +cpu-bound application from fully using its quota by that same amount. In these +instances it will be up to the CFS algorithm (see sched-design-CFS.rst) to +decide which application is chosen to run, as they will both be runnable and +have remaining quota. This runtime discrepancy will be made up in the following +periods when the interactive application idles. + Examples -------- 1. Limit a group to 1 CPU worth of runtime:: |