Documentation/block/cfq-iosched.txt - maze/linux - Git at Google

 CFQ (Complete Fairness Queueing)
 ===============================

 The main aim of CFQ scheduler is to provide a fair allocation of the disk
 I/O bandwidth for all the processes which requests an I/O operation.

 CFQ maintains the per process queue for the processes which request I/O
 operation(synchronous requests). In case of asynchronous requests, all the
 requests from all the processes are batched together according to their
 process's I/O priority.

 CFQ ioscheduler tunables
 ========================

 slice_idle
 ----------
 This specifies how long CFQ should idle for next request on certain cfq queues
 (for sequential workloads) and service trees (for random workloads) before
 queue is expired and CFQ selects next queue to dispatch from.

 By default slice_idle is a non-zero value. That means by default we idle on
 queues/service trees. This can be very helpful on highly seeky media like
 single spindle SATA/SAS disks where we can cut down on overall number of
 seeks and see improved throughput.

 Setting slice_idle to 0 will remove all the idling on queues/service tree
 level and one should see an overall improved throughput on faster storage
 devices like multiple SATA/SAS disks in hardware RAID configuration. The down
 side is that isolation provided from WRITES also goes down and notion of
 IO priority becomes weaker.

 So depending on storage and workload, it might be useful to set slice_idle=0.
 In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
 keeping slice_idle enabled should be useful. For any configurations where
 there are multiple spindles behind single LUN (Host based hardware RAID
 controller or for storage arrays), setting slice_idle=0 might end up in better
 throughput and acceptable latencies.

 back_seek_max
 -------------
 This specifies, given in Kbytes, the maximum "distance" for backward seeking.
 The distance is the amount of space from the current head location to the
 sectors that are backward in terms of distance.

 This parameter allows the scheduler to anticipate requests in the "backward"
 direction and consider them as being the "next" if they are within this
 distance from the current head location.

 back_seek_penalty
 -----------------
 This parameter is used to compute the cost of backward seeking. If the
 backward distance of request is just 1/back_seek_penalty from a "front"
 request, then the seeking cost of two requests is considered equivalent.

 So scheduler will not bias toward one or the other request (otherwise scheduler
 will bias toward front request). Default value of back_seek_penalty is 2.

 fifo_expire_async
 -----------------
 This parameter is used to set the timeout of asynchronous requests. Default
 value of this is 248ms.

 fifo_expire_sync
 ----------------
 This parameter is used to set the timeout of synchronous requests. Default
 value of this is 124ms. In case to favor synchronous requests over asynchronous
 one, this value should be decreased relative to fifo_expire_async.

 group_idle
 -----------
 This parameter forces idling at the CFQ group level instead of CFQ
 queue level. This was introduced after after a bottleneck was observed
 in higher end storage due to idle on sequential queue and allow dispatch
 from a single queue. The idea with this parameter is that it can be run with
 slice_idle=0 and group_idle=8, so that idling does not happen on individual
 queues in the group but happens overall on the group and thus still keeps the
 IO controller working.
 Not idling on individual queues in the group will dispatch requests from
 multiple queues in the group at the same time and achieve higher throughput
 on higher end storage.

 Default value for this parameter is 8ms.

 latency
 -------
 This parameter is used to enable/disable the latency mode of the CFQ
 scheduler. If latency mode (called low_latency) is enabled, CFQ tries
 to recompute the slice time for each process based on the target_latency set
 for the system. This favors fairness over throughput. Disabling low
 latency (setting it to 0) ignores target latency, allowing each process in the
 system to get a full time slice.

 By default low latency mode is enabled.

 target_latency
 --------------
 This parameter is used to calculate the time slice for a process if cfq's
 latency mode is enabled. It will ensure that sync requests have an estimated
 latency. But if sequential workload is higher(e.g. sequential read),
 then to meet the latency constraints, throughput may decrease because of less
 time for each process to issue I/O request before the cfq queue is switched.

 Though this can be overcome by disabling the latency_mode, it may increase
 the read latency for some applications. This parameter allows for changing
 target_latency through the sysfs interface which can provide the balanced
 throughput and read latency.

 Default value for target_latency is 300ms.

 slice_async
 -----------
 This parameter is same as of slice_sync but for asynchronous queue. The
 default value is 40ms.

 slice_async_rq
 --------------
 This parameter is used to limit the dispatching of asynchronous request to
 device request queue in queue's slice time. The maximum number of request that
 are allowed to be dispatched also depends upon the io priority. Default value
 for this is 2.

 slice_sync
 ----------
 When a queue is selected for execution, the queues IO requests are only
 executed for a certain amount of time(time_slice) before switching to another
 queue. This parameter is used to calculate the time slice of synchronous
 queue.

 time_slice is computed using the below equation:-
 time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
 time_slice of synchronous queue, increase the value of slice_sync. Default
 value is 100ms.

 quantum
 -------
 This specifies the number of request dispatched to the device queue. In a
 queue's time slice, a request will not be dispatched if the number of request
 in the device exceeds this parameter. This parameter is used for synchronous
 request.

 In case of storage with several disk, this setting can limit the parallel
 processing of request. Therefore, increasing the value can improve the
 performance although this can cause the latency of some I/O to increase due
 to more number of requests.

 CFQ Group scheduling
 ====================

 CFQ supports blkio cgroup and has "blkio." prefixed files in each
 blkio cgroup directory. It is weight-based and there are four knobs
 for configuration - weight[_device] and leaf_weight[_device].
 Internal cgroup nodes (the ones with children) can also have tasks in
 them, so the former two configure how much proportion the cgroup as a
 whole is entitled to at its parent's level while the latter two
 configure how much proportion the tasks in the cgroup have compared to
 its direct children.

 Another way to think about it is assuming that each internal node has
 an implicit leaf child node which hosts all the tasks whose weight is
 configured by leaf_weight[_device]. Let's assume a blkio hierarchy
 composed of five cgroups - root, A, B, AA and AB - with the following
 weights where the names represent the hierarchy.

         weight leaf_weight
  root :  125    125
  A    :  500    750
  B    :  250    500
  AA   :  500    500
  AB   : 1000    500

 root never has a parent making its weight is meaningless. For backward
 compatibility, weight is always kept in sync with leaf_weight. B, AA
 and AB have no child and thus its tasks have no children cgroup to
 compete with. They always get 100% of what the cgroup won at the
 parent level. Considering only the weights which matter, the hierarchy
 looks like the following.

           root
        /    |   \
       A     B    leaf
      500   250   125
    /  |  \
   AA  AB  leaf
  500 1000 750

 If all cgroups have active IOs and competing with each other, disk
 time will be distributed like the following.

 Distribution below root. The total active weight at this level is
 A:500 + B:250 + C:125 = 875.

  root-leaf :   125 /  875      =~ 14%
  A         :   500 /  875      =~ 57%
  B(-leaf)  :   250 /  875      =~ 28%

 A has children and further distributes its 57% among the children and
 the implicit leaf node. The total active weight at this level is
 AA:500 + AB:1000 + A-leaf:750 = 2250.

  A-leaf    : ( 750 / 2250) * A =~ 19%
  AA(-leaf) : ( 500 / 2250) * A =~ 12%
  AB(-leaf) : (1000 / 2250) * A =~ 25%

 CFQ IOPS Mode for group scheduling
 ===================================
 Basic CFQ design is to provide priority based time slices. Higher priority
 process gets bigger time slice and lower priority process gets smaller time
 slice. Measuring time becomes harder if storage is fast and supports NCQ and
 it would be better to dispatch multiple requests from multiple cfq queues in
 request queue at a time. In such scenario, it is not possible to measure time
 consumed by single queue accurately.

 What is possible though is to measure number of requests dispatched from a
 single queue and also allow dispatch from multiple cfq queue at the same time.
 This effectively becomes the fairness in terms of IOPS (IO operations per
 second).

 If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
 to IOPS mode and starts providing fairness in terms of number of requests
 dispatched. Note that this mode switching takes effect only for group
 scheduling. For non-cgroup users nothing should change.

 CFQ IO scheduler Idling Theory
 ===============================
 Idling on a queue is primarily about waiting for the next request to come
 on same queue after completion of a request. In this process CFQ will not
 dispatch requests from other cfq queues even if requests are pending there.

 The rationale behind idling is that it can cut down on number of seeks
 on rotational media. For example, if a process is doing dependent
 sequential reads (next read will come on only after completion of previous
 one), then not dispatching request from other queue should help as we
 did not move the disk head and kept on dispatching sequential IO from
 one queue.

 CFQ has following service trees and various queues are put on these trees.

 	sync-idle	sync-noidle	async

 All cfq queues doing synchronous sequential IO go on to sync-idle tree.
 On this tree we idle on each queue individually.

 All synchronous non-sequential queues go on sync-noidle tree. Also any
 request which are marked with REQ_NOIDLE go on this service tree. On this
 tree we do not idle on individual queues instead idle on the whole group
 of queues or the tree. So if there are 4 queues waiting for IO to dispatch
 we will idle only once last queue has dispatched the IO and there is
 no more IO on this service tree.

 All async writes go on async service tree. There is no idling on async
 queues.

 CFQ has some optimizations for SSDs and if it detects a non-rotational
 media which can support higher queue depth (multiple requests at in
 flight at a time), then it cuts down on idling of individual queues and
 all the queues move to sync-noidle tree and only tree idle remains. This
 tree idling provides isolation with buffered write queues on async tree.

 FAQ
 ===
 Q1. Why to idle at all on queues marked with REQ_NOIDLE.

 A1. We only do tree idle (all queues on sync-noidle tree) on queues marked
     with REQ_NOIDLE. This helps in providing isolation with all the sync-idle
     queues. Otherwise in presence of many sequential readers, other
     synchronous IO might not get fair share of disk.

     For example, if there are 10 sequential readers doing IO and they get
     100ms each. If a REQ_NOIDLE request comes in, it will be scheduled
     roughly after 1 second. If after completion of REQ_NOIDLE request we
     do not idle, and after a couple of milli seconds a another REQ_NOIDLE
     request comes in, again it will be scheduled after 1second. Repeat it
     and notice how a workload can lose its disk share and suffer due to
     multiple sequential readers.

     fsync can generate dependent IO where bunch of data is written in the
     context of fsync, and later some journaling data is written. Journaling
     data comes in only after fsync has finished its IO (atleast for ext4
     that seemed to be the case). Now if one decides not to idle on fsync
     thread due to REQ_NOIDLE, then next journaling write will not get
     scheduled for another second. A process doing small fsync, will suffer
     badly in presence of multiple sequential readers.

     Hence doing tree idling on threads using REQ_NOIDLE flag on requests
     provides isolation from multiple sequential readers and at the same
     time we do not idle on individual threads.

 Q2. When to specify REQ_NOIDLE
 A2. I would think whenever one is doing synchronous write and not expecting
     more writes to be dispatched from same context soon, should be able
     to specify REQ_NOIDLE on writes and that probably should work well for
     most of the cases.
	CFQ (Complete Fairness Queueing)
	===============================

	The main aim of CFQ scheduler is to provide a fair allocation of the disk
	I/O bandwidth for all the processes which requests an I/O operation.

	CFQ maintains the per process queue for the processes which request I/O
	operation(synchronous requests). In case of asynchronous requests, all the
	requests from all the processes are batched together according to their
	process's I/O priority.

	CFQ ioscheduler tunables
	========================

	slice_idle
	----------
	This specifies how long CFQ should idle for next request on certain cfq queues
	(for sequential workloads) and service trees (for random workloads) before
	queue is expired and CFQ selects next queue to dispatch from.

	By default slice_idle is a non-zero value. That means by default we idle on
	queues/service trees. This can be very helpful on highly seeky media like
	single spindle SATA/SAS disks where we can cut down on overall number of
	seeks and see improved throughput.

	Setting slice_idle to 0 will remove all the idling on queues/service tree
	level and one should see an overall improved throughput on faster storage
	devices like multiple SATA/SAS disks in hardware RAID configuration. The down
	side is that isolation provided from WRITES also goes down and notion of
	IO priority becomes weaker.

	So depending on storage and workload, it might be useful to set slice_idle=0.
	In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
	keeping slice_idle enabled should be useful. For any configurations where
	there are multiple spindles behind single LUN (Host based hardware RAID
	controller or for storage arrays), setting slice_idle=0 might end up in better
	throughput and acceptable latencies.

	back_seek_max
	-------------
	This specifies, given in Kbytes, the maximum "distance" for backward seeking.
	The distance is the amount of space from the current head location to the
	sectors that are backward in terms of distance.

	This parameter allows the scheduler to anticipate requests in the "backward"
	direction and consider them as being the "next" if they are within this
	distance from the current head location.

	back_seek_penalty
	-----------------
	This parameter is used to compute the cost of backward seeking. If the
	backward distance of request is just 1/back_seek_penalty from a "front"
	request, then the seeking cost of two requests is considered equivalent.

	So scheduler will not bias toward one or the other request (otherwise scheduler
	will bias toward front request). Default value of back_seek_penalty is 2.

	fifo_expire_async
	-----------------
	This parameter is used to set the timeout of asynchronous requests. Default
	value of this is 248ms.

	fifo_expire_sync
	----------------
	This parameter is used to set the timeout of synchronous requests. Default
	value of this is 124ms. In case to favor synchronous requests over asynchronous
	one, this value should be decreased relative to fifo_expire_async.

	group_idle
	-----------
	This parameter forces idling at the CFQ group level instead of CFQ
	queue level. This was introduced after after a bottleneck was observed
	in higher end storage due to idle on sequential queue and allow dispatch
	from a single queue. The idea with this parameter is that it can be run with
	slice_idle=0 and group_idle=8, so that idling does not happen on individual
	queues in the group but happens overall on the group and thus still keeps the
	IO controller working.
	Not idling on individual queues in the group will dispatch requests from
	multiple queues in the group at the same time and achieve higher throughput
	on higher end storage.

	Default value for this parameter is 8ms.

	latency
	-------
	This parameter is used to enable/disable the latency mode of the CFQ
	scheduler. If latency mode (called low_latency) is enabled, CFQ tries
	to recompute the slice time for each process based on the target_latency set
	for the system. This favors fairness over throughput. Disabling low
	latency (setting it to 0) ignores target latency, allowing each process in the
	system to get a full time slice.

	By default low latency mode is enabled.

	target_latency
	--------------
	This parameter is used to calculate the time slice for a process if cfq's
	latency mode is enabled. It will ensure that sync requests have an estimated
	latency. But if sequential workload is higher(e.g. sequential read),
	then to meet the latency constraints, throughput may decrease because of less
	time for each process to issue I/O request before the cfq queue is switched.

	Though this can be overcome by disabling the latency_mode, it may increase
	the read latency for some applications. This parameter allows for changing
	target_latency through the sysfs interface which can provide the balanced
	throughput and read latency.

	Default value for target_latency is 300ms.

	slice_async
	-----------
	This parameter is same as of slice_sync but for asynchronous queue. The
	default value is 40ms.

	slice_async_rq
	--------------
	This parameter is used to limit the dispatching of asynchronous request to
	device request queue in queue's slice time. The maximum number of request that
	are allowed to be dispatched also depends upon the io priority. Default value
	for this is 2.

	slice_sync
	----------
	When a queue is selected for execution, the queues IO requests are only
	executed for a certain amount of time(time_slice) before switching to another
	queue. This parameter is used to calculate the time slice of synchronous
	queue.

	time_slice is computed using the below equation:-
	time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
	time_slice of synchronous queue, increase the value of slice_sync. Default
	value is 100ms.

	quantum
	-------
	This specifies the number of request dispatched to the device queue. In a
	queue's time slice, a request will not be dispatched if the number of request
	in the device exceeds this parameter. This parameter is used for synchronous
	request.

	In case of storage with several disk, this setting can limit the parallel
	processing of request. Therefore, increasing the value can improve the
	performance although this can cause the latency of some I/O to increase due
	to more number of requests.

	CFQ Group scheduling
	====================

	CFQ supports blkio cgroup and has "blkio." prefixed files in each
	blkio cgroup directory. It is weight-based and there are four knobs
	for configuration - weight[_device] and leaf_weight[_device].
	Internal cgroup nodes (the ones with children) can also have tasks in
	them, so the former two configure how much proportion the cgroup as a
	whole is entitled to at its parent's level while the latter two
	configure how much proportion the tasks in the cgroup have compared to
	its direct children.

	Another way to think about it is assuming that each internal node has
	an implicit leaf child node which hosts all the tasks whose weight is
	configured by leaf_weight[_device]. Let's assume a blkio hierarchy
	composed of five cgroups - root, A, B, AA and AB - with the following
	weights where the names represent the hierarchy.

	weight leaf_weight
	root : 125 125
	A : 500 750
	B : 250 500
	AA : 500 500
	AB : 1000 500

	root never has a parent making its weight is meaningless. For backward
	compatibility, weight is always kept in sync with leaf_weight. B, AA
	and AB have no child and thus its tasks have no children cgroup to
	compete with. They always get 100% of what the cgroup won at the
	parent level. Considering only the weights which matter, the hierarchy
	looks like the following.

	root
	/ \| \
	A B leaf
	500 250 125
	/ \| \
	AA AB leaf
	500 1000 750

	If all cgroups have active IOs and competing with each other, disk
	time will be distributed like the following.

	Distribution below root. The total active weight at this level is
	A:500 + B:250 + C:125 = 875.

	root-leaf : 125 / 875 =~ 14%
	A : 500 / 875 =~ 57%
	B(-leaf) : 250 / 875 =~ 28%

	A has children and further distributes its 57% among the children and
	the implicit leaf node. The total active weight at this level is
	AA:500 + AB:1000 + A-leaf:750 = 2250.

	A-leaf : ( 750 / 2250) * A =~ 19%
	AA(-leaf) : ( 500 / 2250) * A =~ 12%
	AB(-leaf) : (1000 / 2250) * A =~ 25%

	CFQ IOPS Mode for group scheduling
	===================================
	Basic CFQ design is to provide priority based time slices. Higher priority
	process gets bigger time slice and lower priority process gets smaller time
	slice. Measuring time becomes harder if storage is fast and supports NCQ and
	it would be better to dispatch multiple requests from multiple cfq queues in
	request queue at a time. In such scenario, it is not possible to measure time
	consumed by single queue accurately.

	What is possible though is to measure number of requests dispatched from a
	single queue and also allow dispatch from multiple cfq queue at the same time.
	This effectively becomes the fairness in terms of IOPS (IO operations per
	second).

	If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
	to IOPS mode and starts providing fairness in terms of number of requests
	dispatched. Note that this mode switching takes effect only for group
	scheduling. For non-cgroup users nothing should change.

	CFQ IO scheduler Idling Theory
	===============================
	Idling on a queue is primarily about waiting for the next request to come
	on same queue after completion of a request. In this process CFQ will not
	dispatch requests from other cfq queues even if requests are pending there.

	The rationale behind idling is that it can cut down on number of seeks
	on rotational media. For example, if a process is doing dependent
	sequential reads (next read will come on only after completion of previous
	one), then not dispatching request from other queue should help as we
	did not move the disk head and kept on dispatching sequential IO from
	one queue.

	CFQ has following service trees and various queues are put on these trees.

	sync-idle sync-noidle async

	All cfq queues doing synchronous sequential IO go on to sync-idle tree.
	On this tree we idle on each queue individually.

	All synchronous non-sequential queues go on sync-noidle tree. Also any
	request which are marked with REQ_NOIDLE go on this service tree. On this
	tree we do not idle on individual queues instead idle on the whole group
	of queues or the tree. So if there are 4 queues waiting for IO to dispatch
	we will idle only once last queue has dispatched the IO and there is
	no more IO on this service tree.

	All async writes go on async service tree. There is no idling on async
	queues.

	CFQ has some optimizations for SSDs and if it detects a non-rotational
	media which can support higher queue depth (multiple requests at in
	flight at a time), then it cuts down on idling of individual queues and
	all the queues move to sync-noidle tree and only tree idle remains. This
	tree idling provides isolation with buffered write queues on async tree.

	FAQ
	===
	Q1. Why to idle at all on queues marked with REQ_NOIDLE.

	A1. We only do tree idle (all queues on sync-noidle tree) on queues marked
	with REQ_NOIDLE. This helps in providing isolation with all the sync-idle
	queues. Otherwise in presence of many sequential readers, other
	synchronous IO might not get fair share of disk.

	For example, if there are 10 sequential readers doing IO and they get
	100ms each. If a REQ_NOIDLE request comes in, it will be scheduled
	roughly after 1 second. If after completion of REQ_NOIDLE request we
	do not idle, and after a couple of milli seconds a another REQ_NOIDLE
	request comes in, again it will be scheduled after 1second. Repeat it
	and notice how a workload can lose its disk share and suffer due to
	multiple sequential readers.

	fsync can generate dependent IO where bunch of data is written in the
	context of fsync, and later some journaling data is written. Journaling
	data comes in only after fsync has finished its IO (atleast for ext4
	that seemed to be the case). Now if one decides not to idle on fsync
	thread due to REQ_NOIDLE, then next journaling write will not get
	scheduled for another second. A process doing small fsync, will suffer
	badly in presence of multiple sequential readers.

	Hence doing tree idling on threads using REQ_NOIDLE flag on requests
	provides isolation from multiple sequential readers and at the same
	time we do not idle on individual threads.

	Q2. When to specify REQ_NOIDLE
	A2. I would think whenever one is doing synchronous write and not expecting
	more writes to be dispatched from same context soon, should be able
	to specify REQ_NOIDLE on writes and that probably should work well for
	most of the cases.