Based on kernel version 3.19. Page generated on 2015-02-13 21:16 EST.
1 CFQ (Complete Fairness Queueing) 2 =============================== 3 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. 6 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. 11 12 CFQ ioscheduler tunables 13 ======================== 14 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. 20 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. 25 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. 31 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. 38 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. 44 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. 48 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. 54 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. 57 58 fifo_expire_async 59 ----------------- 60 This parameter is used to set the timeout of asynchronous requests. Default 61 value of this is 248ms. 62 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. 68 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. 81 82 Default value for this parameter is 8ms. 83 84 latency 85 ------- 86 This parameter is used to enable/disable the latency mode of the CFQ 87 scheduler. If latency mode (called low_latency) is enabled, CFQ tries 88 to recompute the slice time for each process based on the target_latency set 89 for the system. This favors fairness over throughput. Disabling low 90 latency (setting it to 0) ignores target latency, allowing each process in the 91 system to get a full time slice. 92 93 By default low latency mode is enabled. 94 95 target_latency 96 -------------- 97 This parameter is used to calculate the time slice for a process if cfq's 98 latency mode is enabled. It will ensure that sync requests have an estimated 99 latency. But if sequential workload is higher(e.g. sequential read), 100 then to meet the latency constraints, throughput may decrease because of less 101 time for each process to issue I/O request before the cfq queue is switched. 102 103 Though this can be overcome by disabling the latency_mode, it may increase 104 the read latency for some applications. This parameter allows for changing 105 target_latency through the sysfs interface which can provide the balanced 106 throughput and read latency. 107 108 Default value for target_latency is 300ms. 109 110 slice_async 111 ----------- 112 This parameter is same as of slice_sync but for asynchronous queue. The 113 default value is 40ms. 114 115 slice_async_rq 116 -------------- 117 This parameter is used to limit the dispatching of asynchronous request to 118 device request queue in queue's slice time. The maximum number of request that 119 are allowed to be dispatched also depends upon the io priority. Default value 120 for this is 2. 121 122 slice_sync 123 ---------- 124 When a queue is selected for execution, the queues IO requests are only 125 executed for a certain amount of time(time_slice) before switching to another 126 queue. This parameter is used to calculate the time slice of synchronous 127 queue. 128 129 time_slice is computed using the below equation:- 130 time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the 131 time_slice of synchronous queue, increase the value of slice_sync. Default 132 value is 100ms. 133 134 quantum 135 ------- 136 This specifies the number of request dispatched to the device queue. In a 137 queue's time slice, a request will not be dispatched if the number of request 138 in the device exceeds this parameter. This parameter is used for synchronous 139 request. 140 141 In case of storage with several disk, this setting can limit the parallel 142 processing of request. Therefore, increasing the value can improve the 143 performance although this can cause the latency of some I/O to increase due 144 to more number of requests. 145 146 CFQ Group scheduling 147 ==================== 148 149 CFQ supports blkio cgroup and has "blkio." prefixed files in each 150 blkio cgroup directory. It is weight-based and there are four knobs 151 for configuration - weight[_device] and leaf_weight[_device]. 152 Internal cgroup nodes (the ones with children) can also have tasks in 153 them, so the former two configure how much proportion the cgroup as a 154 whole is entitled to at its parent's level while the latter two 155 configure how much proportion the tasks in the cgroup have compared to 156 its direct children. 157 158 Another way to think about it is assuming that each internal node has 159 an implicit leaf child node which hosts all the tasks whose weight is 160 configured by leaf_weight[_device]. Let's assume a blkio hierarchy 161 composed of five cgroups - root, A, B, AA and AB - with the following 162 weights where the names represent the hierarchy. 163 164 weight leaf_weight 165 root : 125 125 166 A : 500 750 167 B : 250 500 168 AA : 500 500 169 AB : 1000 500 170 171 root never has a parent making its weight is meaningless. For backward 172 compatibility, weight is always kept in sync with leaf_weight. B, AA 173 and AB have no child and thus its tasks have no children cgroup to 174 compete with. They always get 100% of what the cgroup won at the 175 parent level. Considering only the weights which matter, the hierarchy 176 looks like the following. 177 178 root 179 / | \ 180 A B leaf 181 500 250 125 182 / | \ 183 AA AB leaf 184 500 1000 750 185 186 If all cgroups have active IOs and competing with each other, disk 187 time will be distributed like the following. 188 189 Distribution below root. The total active weight at this level is 190 A:500 + B:250 + C:125 = 875. 191 192 root-leaf : 125 / 875 =~ 14% 193 A : 500 / 875 =~ 57% 194 B(-leaf) : 250 / 875 =~ 28% 195 196 A has children and further distributes its 57% among the children and 197 the implicit leaf node. The total active weight at this level is 198 AA:500 + AB:1000 + A-leaf:750 = 2250. 199 200 A-leaf : ( 750 / 2250) * A =~ 19% 201 AA(-leaf) : ( 500 / 2250) * A =~ 12% 202 AB(-leaf) : (1000 / 2250) * A =~ 25% 203 204 CFQ IOPS Mode for group scheduling 205 =================================== 206 Basic CFQ design is to provide priority based time slices. Higher priority 207 process gets bigger time slice and lower priority process gets smaller time 208 slice. Measuring time becomes harder if storage is fast and supports NCQ and 209 it would be better to dispatch multiple requests from multiple cfq queues in 210 request queue at a time. In such scenario, it is not possible to measure time 211 consumed by single queue accurately. 212 213 What is possible though is to measure number of requests dispatched from a 214 single queue and also allow dispatch from multiple cfq queue at the same time. 215 This effectively becomes the fairness in terms of IOPS (IO operations per 216 second). 217 218 If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches 219 to IOPS mode and starts providing fairness in terms of number of requests 220 dispatched. Note that this mode switching takes effect only for group 221 scheduling. For non-cgroup users nothing should change. 222 223 CFQ IO scheduler Idling Theory 224 =============================== 225 Idling on a queue is primarily about waiting for the next request to come 226 on same queue after completion of a request. In this process CFQ will not 227 dispatch requests from other cfq queues even if requests are pending there. 228 229 The rationale behind idling is that it can cut down on number of seeks 230 on rotational media. For example, if a process is doing dependent 231 sequential reads (next read will come on only after completion of previous 232 one), then not dispatching request from other queue should help as we 233 did not move the disk head and kept on dispatching sequential IO from 234 one queue. 235 236 CFQ has following service trees and various queues are put on these trees. 237 238 sync-idle sync-noidle async 239 240 All cfq queues doing synchronous sequential IO go on to sync-idle tree. 241 On this tree we idle on each queue individually. 242 243 All synchronous non-sequential queues go on sync-noidle tree. Also any 244 request which are marked with REQ_NOIDLE go on this service tree. On this 245 tree we do not idle on individual queues instead idle on the whole group 246 of queues or the tree. So if there are 4 queues waiting for IO to dispatch 247 we will idle only once last queue has dispatched the IO and there is 248 no more IO on this service tree. 249 250 All async writes go on async service tree. There is no idling on async 251 queues. 252 253 CFQ has some optimizations for SSDs and if it detects a non-rotational 254 media which can support higher queue depth (multiple requests at in 255 flight at a time), then it cuts down on idling of individual queues and 256 all the queues move to sync-noidle tree and only tree idle remains. This 257 tree idling provides isolation with buffered write queues on async tree. 258 259 FAQ 260 === 261 Q1. Why to idle at all on queues marked with REQ_NOIDLE. 262 263 A1. We only do tree idle (all queues on sync-noidle tree) on queues marked 264 with REQ_NOIDLE. This helps in providing isolation with all the sync-idle 265 queues. Otherwise in presence of many sequential readers, other 266 synchronous IO might not get fair share of disk. 267 268 For example, if there are 10 sequential readers doing IO and they get 269 100ms each. If a REQ_NOIDLE request comes in, it will be scheduled 270 roughly after 1 second. If after completion of REQ_NOIDLE request we 271 do not idle, and after a couple of milli seconds a another REQ_NOIDLE 272 request comes in, again it will be scheduled after 1second. Repeat it 273 and notice how a workload can lose its disk share and suffer due to 274 multiple sequential readers. 275 276 fsync can generate dependent IO where bunch of data is written in the 277 context of fsync, and later some journaling data is written. Journaling 278 data comes in only after fsync has finished its IO (atleast for ext4 279 that seemed to be the case). Now if one decides not to idle on fsync 280 thread due to REQ_NOIDLE, then next journaling write will not get 281 scheduled for another second. A process doing small fsync, will suffer 282 badly in presence of multiple sequential readers. 283 284 Hence doing tree idling on threads using REQ_NOIDLE flag on requests 285 provides isolation from multiple sequential readers and at the same 286 time we do not idle on individual threads. 287 288 Q2. When to specify REQ_NOIDLE 289 A2. I would think whenever one is doing synchronous write and not expecting 290 more writes to be dispatched from same context soon, should be able 291 to specify REQ_NOIDLE on writes and that probably should work well for 292 most of the cases.