cachepc-linux

sched-bwc.rst (11047B)

=====================
CFS Bandwidth Control
=====================

.. note::
   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 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".

Burst feature
-------------
This feature borrows time now against our future underrun, at the cost of
increased interference against the other system users. All nicely bounded.

Traditional (UP-EDF) bandwidth control is something like:

  (U = \Sum u_i) <= 1

This guaranteeds both that every deadline is met and that the system is
stable. After all, if U were > 1, then for every second of walltime,
we'd have to run more than a second of program time, and obviously miss
our deadline, but the next deadline will be further out still, there is
never time to catch up, unbounded fail.

The burst feature observes that a workload doesn't always executes the full
quota; this enables one to describe u_i as a statistical distribution.

For example, have u_i = {x,e}_i, where x is the p(95) and x+e p(100)
(the traditional WCET). This effectively allows u to be smaller,
increasing the efficiency (we can pack more tasks in the system), but at
the cost of missing deadlines when all the odds line up. However, it
does maintain stability, since every overrun must be paired with an
underrun as long as our x is above the average.

That is, suppose we have 2 tasks, both specify a p(95) value, then we
have a p(95)*p(95) = 90.25% chance both tasks are within their quota and
everything is good. At the same time we have a p(5)p(5) = 0.25% chance
both tasks will exceed their quota at the same time (guaranteed deadline
fail). Somewhere in between there's a threshold where one exceeds and
the other doesn't underrun enough to compensate; this depends on the
specific CDFs.

At the same time, we can say that the worst case deadline miss, will be
\Sum e_i; that is, there is a bounded tardiness (under the assumption
that x+e is indeed WCET).

The interferenece when using burst is valued by the possibilities for
missing the deadline and the average WCET. Test results showed that when
there many cgroups or CPU is under utilized, the interference is
limited. More details are shown in:
https://lore.kernel.org/lkml/5371BD36-55AE-4F71-B9D7-B86DC32E3D2B@linux.alibaba.com/

Management
----------
Quota, period and burst are managed within the cpu subsystem via cgroupfs.

.. note::
   The cgroupfs files described in this section are only applicable
   to cgroup v1. For cgroup v2, see
   :ref:`Documentation/admin-guide/cgroup-v2.rst <cgroup-v2-cpu>`.

- cpu.cfs_quota_us: run-time replenished within a period (in microseconds)
- cpu.cfs_period_us: the length of a period (in microseconds)
- cpu.stat: exports throttling statistics [explained further below]
- cpu.cfs_burst_us: the maximum accumulated run-time (in microseconds)

The default values are::

	cpu.cfs_period_us=100ms
	cpu.cfs_quota_us=-1
	cpu.cfs_burst_us=0

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) no smaller than cpu.cfs_burst_us 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.

A value of 0 for cpu.cfs_burst_us indicates that the group can not accumulate
any unused bandwidth. It makes the traditional bandwidth control behavior for
CFS unchanged. Writing any (valid) positive value(s) no larger than
cpu.cfs_quota_us into cpu.cfs_burst_us will enact the cap on unused bandwidth
accumulation.

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 5 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.
- nr_bursts: Number of periods burst occurs.
- burst_time: Cumulative wall-time (in nanoseconds) that any CPUs has used
  above quota in respective periods.

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.

4. Limit a group to 40% of 1 CPU, and allow accumulate up to 20% of 1 CPU
   additionally, in case accumulation has been done.

   With 50ms period, 20ms quota will be equivalent to 40% of 1 CPU.
   And 10ms burst will be equivalent to 20% of 1 CPU::

	# echo 20000 > cpu.cfs_quota_us /* quota = 20ms */
	# echo 50000 > cpu.cfs_period_us /* period = 50ms */
	# echo 10000 > cpu.cfs_burst_us /* burst = 10ms */

   Larger buffer setting (no larger than quota) allows greater burst capacity.