Documentation/scheduler/sched-bwc.txt - maze/linux - Git at Google

 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.

 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.
	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.

	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.