Documentation/block/cfq-iosched.txt

   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 IOPS Mode for group scheduling
 106 ===================================
 107 Basic CFQ design is to provide priority based time slices. Higher priority
 108 process gets bigger time slice and lower priority process gets smaller time
 109 slice. Measuring time becomes harder if storage is fast and supports NCQ and
 110 it would be better to dispatch multiple requests from multiple cfq queues in
 111 request queue at a time. In such scenario, it is not possible to measure time
 112 consumed by single queue accurately.
 113
 114 What is possible though is to measure number of requests dispatched from a
 115 single queue and also allow dispatch from multiple cfq queue at the same time.
 116 This effectively becomes the fairness in terms of IOPS (IO operations per
 117 second).
 118
 119 If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
 120 to IOPS mode and starts providing fairness in terms of number of requests
 121 dispatched. Note that this mode switching takes effect only for group
 122 scheduling. For non-cgroup users nothing should change.
 123
 124 CFQ IO scheduler Idling Theory
 125 ===============================
 126 Idling on a queue is primarily about waiting for the next request to come
 127 on same queue after completion of a request. In this process CFQ will not
 128 dispatch requests from other cfq queues even if requests are pending there.
 129
 130 The rationale behind idling is that it can cut down on number of seeks
 131 on rotational media. For example, if a process is doing dependent
 132 sequential reads (next read will come on only after completion of previous
 133 one), then not dispatching request from other queue should help as we
 134 did not move the disk head and kept on dispatching sequential IO from
 135 one queue.
 136
 137 CFQ has following service trees and various queues are put on these trees.
 138
 139         sync-idle       sync-noidle     async
 140
 141 All cfq queues doing synchronous sequential IO go on to sync-idle tree.
 142 On this tree we idle on each queue individually.
 143
 144 All synchronous non-sequential queues go on sync-noidle tree. Also any
 145 request which are marked with REQ_NOIDLE go on this service tree. On this
 146 tree we do not idle on individual queues instead idle on the whole group
 147 of queues or the tree. So if there are 4 queues waiting for IO to dispatch
 148 we will idle only once last queue has dispatched the IO and there is
 149 no more IO on this service tree.
 150
 151 All async writes go on async service tree. There is no idling on async
 152 queues.
 153
 154 CFQ has some optimizations for SSDs and if it detects a non-rotational
 155 media which can support higher queue depth (multiple requests at in
 156 flight at a time), then it cuts down on idling of individual queues and
 157 all the queues move to sync-noidle tree and only tree idle remains. This
 158 tree idling provides isolation with buffered write queues on async tree.
 159
 160 FAQ
 161 ===
 162 Q1. Why to idle at all on queues marked with REQ_NOIDLE.
 163
 164 A1. We only do tree idle (all queues on sync-noidle tree) on queues marked
 165     with REQ_NOIDLE. This helps in providing isolation with all the sync-idle
 166     queues. Otherwise in presence of many sequential readers, other
 167     synchronous IO might not get fair share of disk.
 168
 169     For example, if there are 10 sequential readers doing IO and they get
 170     100ms each. If a REQ_NOIDLE request comes in, it will be scheduled
 171     roughly after 1 second. If after completion of REQ_NOIDLE request we
 172     do not idle, and after a couple of milli seconds a another REQ_NOIDLE
 173     request comes in, again it will be scheduled after 1second. Repeat it
 174     and notice how a workload can lose its disk share and suffer due to
 175     multiple sequential readers.
 176
 177     fsync can generate dependent IO where bunch of data is written in the
 178     context of fsync, and later some journaling data is written. Journaling
 179     data comes in only after fsync has finished its IO (atleast for ext4
 180     that seemed to be the case). Now if one decides not to idle on fsync
 181     thread due to REQ_NOIDLE, then next journaling write will not get
 182     scheduled for another second. A process doing small fsync, will suffer
 183     badly in presence of multiple sequential readers.
 184
 185     Hence doing tree idling on threads using REQ_NOIDLE flag on requests
 186     provides isolation from multiple sequential readers and at the same
 187     time we do not idle on individual threads.
 188
 189 Q2. When to specify REQ_NOIDLE
 190 A2. I would think whenever one is doing synchronous write and not expecting
 191     more writes to be dispatched from same context soon, should be able
 192     to specify REQ_NOIDLE on writes and that probably should work well for
 193     most of the cases.