168 lines
7.4 KiB
Plaintext
Executable file
168 lines
7.4 KiB
Plaintext
Executable file
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.
|
|
|