168 lines
7.4 KiB
Plaintext
168 lines
7.4 KiB
Plaintext
CFS Bandwidth Control
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=====================
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[ This document only discusses CPU bandwidth control for SCHED_NORMAL.
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The SCHED_RT case is covered in Documentation/scheduler/sched-rt-group.txt ]
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CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the
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specification of the maximum CPU bandwidth available to a group or hierarchy.
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The bandwidth allowed for a group is specified using a quota and period. Within
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each given "period" (microseconds), a group is allowed to consume only up to
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"quota" microseconds of CPU time. When the CPU bandwidth consumption of a
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group exceeds this limit (for that period), the tasks belonging to its
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hierarchy will be throttled and are not allowed to run again until the next
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period.
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A group's unused runtime is globally tracked, being refreshed with quota units
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above at each period boundary. As threads consume this bandwidth it is
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transferred to cpu-local "silos" on a demand basis. The amount transferred
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within each of these updates is tunable and described as the "slice".
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Management
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----------
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Quota and period are managed within the cpu subsystem via cgroupfs.
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cpu.cfs_quota_us: the total available run-time within a period (in microseconds)
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cpu.cfs_period_us: the length of a period (in microseconds)
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cpu.stat: exports throttling statistics [explained further below]
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The default values are:
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cpu.cfs_period_us=100ms
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cpu.cfs_quota=-1
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A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
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bandwidth restriction in place, such a group is described as an unconstrained
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bandwidth group. This represents the traditional work-conserving behavior for
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CFS.
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Writing any (valid) positive value(s) will enact the specified bandwidth limit.
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The minimum quota allowed for the quota or period is 1ms. There is also an
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upper bound on the period length of 1s. Additional restrictions exist when
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bandwidth limits are used in a hierarchical fashion, these are explained in
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more detail below.
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Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
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and return the group to an unconstrained state once more.
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Any updates to a group's bandwidth specification will result in it becoming
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unthrottled if it is in a constrained state.
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System wide settings
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--------------------
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For efficiency run-time is transferred between the global pool and CPU local
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"silos" in a batch fashion. This greatly reduces global accounting pressure
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on large systems. The amount transferred each time such an update is required
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is described as the "slice".
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This is tunable via procfs:
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/proc/sys/kernel/sched_cfs_bandwidth_slice_us (default=5ms)
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Larger slice values will reduce transfer overheads, while smaller values allow
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for more fine-grained consumption.
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Statistics
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----------
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A group's bandwidth statistics are exported via 3 fields in cpu.stat.
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cpu.stat:
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- nr_periods: Number of enforcement intervals that have elapsed.
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- nr_throttled: Number of times the group has been throttled/limited.
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- throttled_time: The total time duration (in nanoseconds) for which entities
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of the group have been throttled.
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This interface is read-only.
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Hierarchical considerations
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---------------------------
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The interface enforces that an individual entity's bandwidth is always
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attainable, that is: max(c_i) <= C. However, over-subscription in the
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aggregate case is explicitly allowed to enable work-conserving semantics
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within a hierarchy.
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e.g. \Sum (c_i) may exceed C
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[ Where C is the parent's bandwidth, and c_i its children ]
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There are two ways in which a group may become throttled:
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a. it fully consumes its own quota within a period
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b. a parent's quota is fully consumed within its period
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In case b) above, even though the child may have runtime remaining it will not
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be allowed to until the parent's runtime is refreshed.
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CFS Bandwidth Quota Caveats
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---------------------------
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Once a slice is assigned to a cpu it does not expire. However all but 1ms of
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the slice may be returned to the global pool if all threads on that cpu become
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unrunnable. This is configured at compile time by the min_cfs_rq_runtime
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variable. This is a performance tweak that helps prevent added contention on
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the global lock.
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The fact that cpu-local slices do not expire results in some interesting corner
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cases that should be understood.
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For cgroup cpu constrained applications that are cpu limited this is a
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relatively moot point because they will naturally consume the entirety of their
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quota as well as the entirety of each cpu-local slice in each period. As a
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result it is expected that nr_periods roughly equal nr_throttled, and that
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cpuacct.usage will increase roughly equal to cfs_quota_us in each period.
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For highly-threaded, non-cpu bound applications this non-expiration nuance
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allows applications to briefly burst past their quota limits by the amount of
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unused slice on each cpu that the task group is running on (typically at most
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1ms per cpu or as defined by min_cfs_rq_runtime). This slight burst only
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applies if quota had been assigned to a cpu and then not fully used or returned
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in previous periods. This burst amount will not be transferred between cores.
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As a result, this mechanism still strictly limits the task group to quota
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average usage, albeit over a longer time window than a single period. This
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also limits the burst ability to no more than 1ms per cpu. This provides
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better more predictable user experience for highly threaded applications with
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small quota limits on high core count machines. It also eliminates the
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propensity to throttle these applications while simultanously using less than
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quota amounts of cpu. Another way to say this, is that by allowing the unused
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portion of a slice to remain valid across periods we have decreased the
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possibility of wastefully expiring quota on cpu-local silos that don't need a
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full slice's amount of cpu time.
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The interaction between cpu-bound and non-cpu-bound-interactive applications
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should also be considered, especially when single core usage hits 100%. If you
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gave each of these applications half of a cpu-core and they both got scheduled
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on the same CPU it is theoretically possible that the non-cpu bound application
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will use up to 1ms additional quota in some periods, thereby preventing the
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cpu-bound application from fully using its quota by that same amount. In these
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instances it will be up to the CFS algorithm (see sched-design-CFS.rst) to
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decide which application is chosen to run, as they will both be runnable and
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have remaining quota. This runtime discrepancy will be made up in the following
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periods when the interactive application idles.
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Examples
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--------
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1. Limit a group to 1 CPU worth of runtime.
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If period is 250ms and quota is also 250ms, the group will get
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1 CPU worth of runtime every 250ms.
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# echo 250000 > cpu.cfs_quota_us /* quota = 250ms */
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# echo 250000 > cpu.cfs_period_us /* period = 250ms */
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2. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine.
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With 500ms period and 1000ms quota, the group can get 2 CPUs worth of
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runtime every 500ms.
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# echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */
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# echo 500000 > cpu.cfs_period_us /* period = 500ms */
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The larger period here allows for increased burst capacity.
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3. Limit a group to 20% of 1 CPU.
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With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU.
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# echo 10000 > cpu.cfs_quota_us /* quota = 10ms */
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# echo 50000 > cpu.cfs_period_us /* period = 50ms */
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By using a small period here we are ensuring a consistent latency
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response at the expense of burst capacity.
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