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.rst ]

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:

1. it fully consumes its own quota within a period
2. 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.

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.