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| -rw-r--r-- | Documentation/scheduler/sched-bwc.txt | 167 |
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diff --git a/Documentation/scheduler/sched-bwc.txt b/Documentation/scheduler/sched-bwc.txt new file mode 100644 index 000000000..de583fbbf --- /dev/null +++ b/Documentation/scheduler/sched-bwc.txt @@ -0,0 +1,167 @@ +CFS Bandwidth Control +===================== + +[ This document only discusses CPU bandwidth control for SCHED_NORMAL. + The SCHED_RT case is covered in Documentation/scheduler/sched-rt-group.txt ] + +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 +within each of these updates is tunable and described as the "slice". + +Management +---------- +Quota and period are managed within the cpu subsystem via cgroupfs. + +cpu.cfs_quota_us: the total available run-time within a period (in microseconds) +cpu.cfs_period_us: the length of a period (in microseconds) +cpu.stat: exports throttling statistics [explained further below] + +The default values are: + cpu.cfs_period_us=100ms + cpu.cfs_quota=-1 + +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 +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 +bandwidth limits are used in a hierarchical fashion, these are explained in +more detail below. + +Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit +and return the group to an unconstrained state once more. + +Any updates to a group's bandwidth specification will result in it becoming +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 +is described as the "slice". + +This is tunable via procfs: + /proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms) + +Larger slice values will reduce transfer overheads, while smaller values allow +for more fine-grained consumption. + +Statistics +---------- +A group's bandwidth statistics are exported via 3 fields in cpu.stat. + +cpu.stat: +- nr_periods: Number of enforcement intervals that have elapsed. +- nr_throttled: Number of times the group has been throttled/limited. +- throttled_time: The total time duration (in nanoseconds) for which entities + of the group have been throttled. + +This interface is read-only. + +Hierarchical considerations +--------------------------- +The interface enforces that an individual entity's bandwidth is always +attainable, that is: max(c_i) <= C. However, over-subscription in the +aggregate case is explicitly allowed to enable work-conserving semantics +within a hierarchy. + e.g. \Sum (c_i) may exceed C +[ Where C is the parent's bandwidth, and c_i its children ] + + +There are two ways in which a group may become throttled: + a. it fully consumes its own quota within a period + b. a parent's quota is fully consumed within its period + +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. + + If period is 250ms and quota is also 250ms, the group will get + 1 CPU worth of runtime every 250ms. + + # echo 250000 > cpu.cfs_quota_us /* quota = 250ms */ + # echo 250000 > cpu.cfs_period_us /* period = 250ms */ + +2. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine. + + With 500ms period and 1000ms quota, the group can get 2 CPUs worth of + runtime every 500ms. + + # echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */ + # echo 500000 > cpu.cfs_period_us /* period = 500ms */ + + The larger period here allows for increased burst capacity. + +3. Limit a group to 20% of 1 CPU. + + With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU. + + # echo 10000 > cpu.cfs_quota_us /* quota = 10ms */ + # echo 50000 > cpu.cfs_period_us /* period = 50ms */ + + By using a small period here we are ensuring a consistent latency + response at the expense of burst capacity. + |