Based on kernel version 3.9. Page generated on 2013-05-02 22:55 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(syncronous 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 slice_async 70 ----------- 71 This parameter is same as of slice_sync but for asynchronous queue. The 72 default value is 40ms. 73 74 slice_async_rq 75 -------------- 76 This parameter is used to limit the dispatching of asynchronous request to 77 device request queue in queue's slice time. The maximum number of request that 78 are allowed to be dispatched also depends upon the io priority. Default value 79 for this is 2. 80 81 slice_sync 82 ---------- 83 When a queue is selected for execution, the queues IO requests are only 84 executed for a certain amount of time(time_slice) before switching to another 85 queue. This parameter is used to calculate the time slice of synchronous 86 queue. 87 88 time_slice is computed using the below equation:- 89 time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the 90 time_slice of synchronous queue, increase the value of slice_sync. Default 91 value is 100ms. 92 93 quantum 94 ------- 95 This specifies the number of request dispatched to the device queue. In a 96 queue's time slice, a request will not be dispatched if the number of request 97 in the device exceeds this parameter. This parameter is used for synchronous 98 request. 99 100 In case of storage with several disk, this setting can limit the parallel 101 processing of request. Therefore, increasing the value can imporve the 102 performace although this can cause the latency of some I/O to increase due 103 to more number of requests. 104 105 CFQ Group scheduling 106 ==================== 107 108 CFQ supports blkio cgroup and has "blkio." prefixed files in each 109 blkio cgroup directory. It is weight-based and there are four knobs 110 for configuration - weight[_device] and leaf_weight[_device]. 111 Internal cgroup nodes (the ones with children) can also have tasks in 112 them, so the former two configure how much proportion the cgroup as a 113 whole is entitled to at its parent's level while the latter two 114 configure how much proportion the tasks in the cgroup have compared to 115 its direct children. 116 117 Another way to think about it is assuming that each internal node has 118 an implicit leaf child node which hosts all the tasks whose weight is 119 configured by leaf_weight[_device]. Let's assume a blkio hierarchy 120 composed of five cgroups - root, A, B, AA and AB - with the following 121 weights where the names represent the hierarchy. 122 123 weight leaf_weight 124 root : 125 125 125 A : 500 750 126 B : 250 500 127 AA : 500 500 128 AB : 1000 500 129 130 root never has a parent making its weight is meaningless. For backward 131 compatibility, weight is always kept in sync with leaf_weight. B, AA 132 and AB have no child and thus its tasks have no children cgroup to 133 compete with. They always get 100% of what the cgroup won at the 134 parent level. Considering only the weights which matter, the hierarchy 135 looks like the following. 136 137 root 138 / | \ 139 A B leaf 140 500 250 125 141 / | \ 142 AA AB leaf 143 500 1000 750 144 145 If all cgroups have active IOs and competing with each other, disk 146 time will be distributed like the following. 147 148 Distribution below root. The total active weight at this level is 149 A:500 + B:250 + C:125 = 875. 150 151 root-leaf : 125 / 875 =~ 14% 152 A : 500 / 875 =~ 57% 153 B(-leaf) : 250 / 875 =~ 28% 154 155 A has children and further distributes its 57% among the children and 156 the implicit leaf node. The total active weight at this level is 157 AA:500 + AB:1000 + A-leaf:750 = 2250. 158 159 A-leaf : ( 750 / 2250) * A =~ 19% 160 AA(-leaf) : ( 500 / 2250) * A =~ 12% 161 AB(-leaf) : (1000 / 2250) * A =~ 25% 162 163 CFQ IOPS Mode for group scheduling 164 =================================== 165 Basic CFQ design is to provide priority based time slices. Higher priority 166 process gets bigger time slice and lower priority process gets smaller time 167 slice. Measuring time becomes harder if storage is fast and supports NCQ and 168 it would be better to dispatch multiple requests from multiple cfq queues in 169 request queue at a time. In such scenario, it is not possible to measure time 170 consumed by single queue accurately. 171 172 What is possible though is to measure number of requests dispatched from a 173 single queue and also allow dispatch from multiple cfq queue at the same time. 174 This effectively becomes the fairness in terms of IOPS (IO operations per 175 second). 176 177 If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches 178 to IOPS mode and starts providing fairness in terms of number of requests 179 dispatched. Note that this mode switching takes effect only for group 180 scheduling. For non-cgroup users nothing should change. 181 182 CFQ IO scheduler Idling Theory 183 =============================== 184 Idling on a queue is primarily about waiting for the next request to come 185 on same queue after completion of a request. In this process CFQ will not 186 dispatch requests from other cfq queues even if requests are pending there. 187 188 The rationale behind idling is that it can cut down on number of seeks 189 on rotational media. For example, if a process is doing dependent 190 sequential reads (next read will come on only after completion of previous 191 one), then not dispatching request from other queue should help as we 192 did not move the disk head and kept on dispatching sequential IO from 193 one queue. 194 195 CFQ has following service trees and various queues are put on these trees. 196 197 sync-idle sync-noidle async 198 199 All cfq queues doing synchronous sequential IO go on to sync-idle tree. 200 On this tree we idle on each queue individually. 201 202 All synchronous non-sequential queues go on sync-noidle tree. Also any 203 request which are marked with REQ_NOIDLE go on this service tree. On this 204 tree we do not idle on individual queues instead idle on the whole group 205 of queues or the tree. So if there are 4 queues waiting for IO to dispatch 206 we will idle only once last queue has dispatched the IO and there is 207 no more IO on this service tree. 208 209 All async writes go on async service tree. There is no idling on async 210 queues. 211 212 CFQ has some optimizations for SSDs and if it detects a non-rotational 213 media which can support higher queue depth (multiple requests at in 214 flight at a time), then it cuts down on idling of individual queues and 215 all the queues move to sync-noidle tree and only tree idle remains. This 216 tree idling provides isolation with buffered write queues on async tree. 217 218 FAQ 219 === 220 Q1. Why to idle at all on queues marked with REQ_NOIDLE. 221 222 A1. We only do tree idle (all queues on sync-noidle tree) on queues marked 223 with REQ_NOIDLE. This helps in providing isolation with all the sync-idle 224 queues. Otherwise in presence of many sequential readers, other 225 synchronous IO might not get fair share of disk. 226 227 For example, if there are 10 sequential readers doing IO and they get 228 100ms each. If a REQ_NOIDLE request comes in, it will be scheduled 229 roughly after 1 second. If after completion of REQ_NOIDLE request we 230 do not idle, and after a couple of milli seconds a another REQ_NOIDLE 231 request comes in, again it will be scheduled after 1second. Repeat it 232 and notice how a workload can lose its disk share and suffer due to 233 multiple sequential readers. 234 235 fsync can generate dependent IO where bunch of data is written in the 236 context of fsync, and later some journaling data is written. Journaling 237 data comes in only after fsync has finished its IO (atleast for ext4 238 that seemed to be the case). Now if one decides not to idle on fsync 239 thread due to REQ_NOIDLE, then next journaling write will not get 240 scheduled for another second. A process doing small fsync, will suffer 241 badly in presence of multiple sequential readers. 242 243 Hence doing tree idling on threads using REQ_NOIDLE flag on requests 244 provides isolation from multiple sequential readers and at the same 245 time we do not idle on individual threads. 246 247 Q2. When to specify REQ_NOIDLE 248 A2. I would think whenever one is doing synchronous write and not expecting 249 more writes to be dispatched from same context soon, should be able 250 to specify REQ_NOIDLE on writes and that probably should work well for 251 most of the cases.