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1 CFQ (Complete Fairness Queueing)
2 ===============================
4 The main aim of CFQ scheduler is to provide a fair allocation of the disk
5 I/O bandwidth for all the processes which requests an I/O operation.
7 CFQ maintains the per process queue for the processes which request I/O
8 operation(synchronous requests). In case of asynchronous requests, all the
9 requests from all the processes are batched together according to their
10 process's I/O priority.
12 CFQ ioscheduler tunables
13 ========================
15 slice_idle
16 ----------
17 This specifies how long CFQ should idle for next request on certain cfq queues
18 (for sequential workloads) and service trees (for random workloads) before
19 queue is expired and CFQ selects next queue to dispatch from.
21 By default slice_idle is a non-zero value. That means by default we idle on
22 queues/service trees. This can be very helpful on highly seeky media like
23 single spindle SATA/SAS disks where we can cut down on overall number of
24 seeks and see improved throughput.
26 Setting slice_idle to 0 will remove all the idling on queues/service tree
27 level and one should see an overall improved throughput on faster storage
28 devices like multiple SATA/SAS disks in hardware RAID configuration. The down
29 side is that isolation provided from WRITES also goes down and notion of
30 IO priority becomes weaker.
32 So depending on storage and workload, it might be useful to set slice_idle=0.
33 In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
34 keeping slice_idle enabled should be useful. For any configurations where
35 there are multiple spindles behind single LUN (Host based hardware RAID
36 controller or for storage arrays), setting slice_idle=0 might end up in better
37 throughput and acceptable latencies.
39 back_seek_max
40 -------------
41 This specifies, given in Kbytes, the maximum "distance" for backward seeking.
42 The distance is the amount of space from the current head location to the
43 sectors that are backward in terms of distance.
45 This parameter allows the scheduler to anticipate requests in the "backward"
46 direction and consider them as being the "next" if they are within this
47 distance from the current head location.
49 back_seek_penalty
50 -----------------
51 This parameter is used to compute the cost of backward seeking. If the
52 backward distance of request is just 1/back_seek_penalty from a "front"
53 request, then the seeking cost of two requests is considered equivalent.
55 So scheduler will not bias toward one or the other request (otherwise scheduler
56 will bias toward front request). Default value of back_seek_penalty is 2.
58 fifo_expire_async
59 -----------------
60 This parameter is used to set the timeout of asynchronous requests. Default
61 value of this is 248ms.
63 fifo_expire_sync
64 ----------------
65 This parameter is used to set the timeout of synchronous requests. Default
66 value of this is 124ms. In case to favor synchronous requests over asynchronous
67 one, this value should be decreased relative to fifo_expire_async.
69 group_idle
70 -----------
71 This parameter forces idling at the CFQ group level instead of CFQ
72 queue level. This was introduced after a bottleneck was observed
73 in higher end storage due to idle on sequential queue and allow dispatch
74 from a single queue. The idea with this parameter is that it can be run with
75 slice_idle=0 and group_idle=8, so that idling does not happen on individual
76 queues in the group but happens overall on the group and thus still keeps the
77 IO controller working.
78 Not idling on individual queues in the group will dispatch requests from
79 multiple queues in the group at the same time and achieve higher throughput
80 on higher end storage.
82 Default value for this parameter is 8ms.
84 low_latency
85 -----------
86 This parameter is used to enable/disable the low latency mode of the CFQ
87 scheduler. If enabled, CFQ tries to recompute the slice time for each process
88 based on the target_latency set for the system. This favors fairness over
89 throughput. Disabling low latency (setting it to 0) ignores target latency,
90 allowing each process in the system to get a full time slice.
92 By default low latency mode is enabled.
94 target_latency
95 --------------
96 This parameter is used to calculate the time slice for a process if cfq's
97 latency mode is enabled. It will ensure that sync requests have an estimated
98 latency. But if sequential workload is higher(e.g. sequential read),
99 then to meet the latency constraints, throughput may decrease because of less
100 time for each process to issue I/O request before the cfq queue is switched.
102 Though this can be overcome by disabling the latency_mode, it may increase
103 the read latency for some applications. This parameter allows for changing
104 target_latency through the sysfs interface which can provide the balanced
105 throughput and read latency.
107 Default value for target_latency is 300ms.
109 slice_async
110 -----------
111 This parameter is same as of slice_sync but for asynchronous queue. The
112 default value is 40ms.
114 slice_async_rq
115 --------------
116 This parameter is used to limit the dispatching of asynchronous request to
117 device request queue in queue's slice time. The maximum number of request that
118 are allowed to be dispatched also depends upon the io priority. Default value
119 for this is 2.
121 slice_sync
122 ----------
123 When a queue is selected for execution, the queues IO requests are only
124 executed for a certain amount of time(time_slice) before switching to another
125 queue. This parameter is used to calculate the time slice of synchronous
126 queue.
128 time_slice is computed using the below equation:-
129 time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
130 time_slice of synchronous queue, increase the value of slice_sync. Default
131 value is 100ms.
133 quantum
134 -------
135 This specifies the number of request dispatched to the device queue. In a
136 queue's time slice, a request will not be dispatched if the number of request
137 in the device exceeds this parameter. This parameter is used for synchronous
138 request.
140 In case of storage with several disk, this setting can limit the parallel
141 processing of request. Therefore, increasing the value can improve the
142 performance although this can cause the latency of some I/O to increase due
143 to more number of requests.
145 CFQ Group scheduling
146 ====================
148 CFQ supports blkio cgroup and has "blkio." prefixed files in each
149 blkio cgroup directory. It is weight-based and there are four knobs
150 for configuration - weight[_device] and leaf_weight[_device].
151 Internal cgroup nodes (the ones with children) can also have tasks in
152 them, so the former two configure how much proportion the cgroup as a
153 whole is entitled to at its parent's level while the latter two
154 configure how much proportion the tasks in the cgroup have compared to
155 its direct children.
157 Another way to think about it is assuming that each internal node has
158 an implicit leaf child node which hosts all the tasks whose weight is
159 configured by leaf_weight[_device]. Let's assume a blkio hierarchy
160 composed of five cgroups - root, A, B, AA and AB - with the following
161 weights where the names represent the hierarchy.
163 weight leaf_weight
164 root : 125 125
165 A : 500 750
166 B : 250 500
167 AA : 500 500
168 AB : 1000 500
170 root never has a parent making its weight is meaningless. For backward
171 compatibility, weight is always kept in sync with leaf_weight. B, AA
172 and AB have no child and thus its tasks have no children cgroup to
173 compete with. They always get 100% of what the cgroup won at the
174 parent level. Considering only the weights which matter, the hierarchy
175 looks like the following.
177 root
178 / | \
179 A B leaf
180 500 250 125
181 / | \
182 AA AB leaf
183 500 1000 750
185 If all cgroups have active IOs and competing with each other, disk
186 time will be distributed like the following.
188 Distribution below root. The total active weight at this level is
189 A:500 + B:250 + C:125 = 875.
191 root-leaf : 125 / 875 =~ 14%
192 A : 500 / 875 =~ 57%
193 B(-leaf) : 250 / 875 =~ 28%
195 A has children and further distributes its 57% among the children and
196 the implicit leaf node. The total active weight at this level is
197 AA:500 + AB:1000 + A-leaf:750 = 2250.
199 A-leaf : ( 750 / 2250) * A =~ 19%
200 AA(-leaf) : ( 500 / 2250) * A =~ 12%
201 AB(-leaf) : (1000 / 2250) * A =~ 25%
203 CFQ IOPS Mode for group scheduling
204 ===================================
205 Basic CFQ design is to provide priority based time slices. Higher priority
206 process gets bigger time slice and lower priority process gets smaller time
207 slice. Measuring time becomes harder if storage is fast and supports NCQ and
208 it would be better to dispatch multiple requests from multiple cfq queues in
209 request queue at a time. In such scenario, it is not possible to measure time
210 consumed by single queue accurately.
212 What is possible though is to measure number of requests dispatched from a
213 single queue and also allow dispatch from multiple cfq queue at the same time.
214 This effectively becomes the fairness in terms of IOPS (IO operations per
215 second).
217 If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
218 to IOPS mode and starts providing fairness in terms of number of requests
219 dispatched. Note that this mode switching takes effect only for group
220 scheduling. For non-cgroup users nothing should change.
222 CFQ IO scheduler Idling Theory
223 ===============================
224 Idling on a queue is primarily about waiting for the next request to come
225 on same queue after completion of a request. In this process CFQ will not
226 dispatch requests from other cfq queues even if requests are pending there.
228 The rationale behind idling is that it can cut down on number of seeks
229 on rotational media. For example, if a process is doing dependent
230 sequential reads (next read will come on only after completion of previous
231 one), then not dispatching request from other queue should help as we
232 did not move the disk head and kept on dispatching sequential IO from
233 one queue.
235 CFQ has following service trees and various queues are put on these trees.
237 sync-idle sync-noidle async
239 All cfq queues doing synchronous sequential IO go on to sync-idle tree.
240 On this tree we idle on each queue individually.
242 All synchronous non-sequential queues go on sync-noidle tree. Also any
243 synchronous write request which is not marked with REQ_IDLE goes on this
244 service tree. On this tree we do not idle on individual queues instead idle
245 on the whole group of queues or the tree. So if there are 4 queues waiting
246 for IO to dispatch we will idle only once last queue has dispatched the IO
247 and there is no more IO on this service tree.
249 All async writes go on async service tree. There is no idling on async
250 queues.
252 CFQ has some optimizations for SSDs and if it detects a non-rotational
253 media which can support higher queue depth (multiple requests at in
254 flight at a time), then it cuts down on idling of individual queues and
255 all the queues move to sync-noidle tree and only tree idle remains. This
256 tree idling provides isolation with buffered write queues on async tree.
258 FAQ
259 ===
260 Q1. Why to idle at all on queues not marked with REQ_IDLE.
262 A1. We only do tree idle (all queues on sync-noidle tree) on queues not marked
263 with REQ_IDLE. This helps in providing isolation with all the sync-idle
264 queues. Otherwise in presence of many sequential readers, other
265 synchronous IO might not get fair share of disk.
267 For example, if there are 10 sequential readers doing IO and they get
268 100ms each. If a !REQ_IDLE request comes in, it will be scheduled
269 roughly after 1 second. If after completion of !REQ_IDLE request we
270 do not idle, and after a couple of milli seconds a another !REQ_IDLE
271 request comes in, again it will be scheduled after 1second. Repeat it
272 and notice how a workload can lose its disk share and suffer due to
273 multiple sequential readers.
275 fsync can generate dependent IO where bunch of data is written in the
276 context of fsync, and later some journaling data is written. Journaling
277 data comes in only after fsync has finished its IO (atleast for ext4
278 that seemed to be the case). Now if one decides not to idle on fsync
279 thread due to !REQ_IDLE, then next journaling write will not get
280 scheduled for another second. A process doing small fsync, will suffer
281 badly in presence of multiple sequential readers.
283 Hence doing tree idling on threads using !REQ_IDLE flag on requests
284 provides isolation from multiple sequential readers and at the same
285 time we do not idle on individual threads.
287 Q2. When to specify REQ_IDLE
288 A2. I would think whenever one is doing synchronous write and expecting
289 more writes to be dispatched from same context soon, should be able
290 to specify REQ_IDLE on writes and that probably should work well for
291 most of the cases.