linux/Documentation/block/cfq-iosched.txt
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   1CFQ (Complete Fairness Queueing)
   2===============================
   3
   4The main aim of CFQ scheduler is to provide a fair allocation of the disk
   5I/O bandwidth for all the processes which requests an I/O operation.
   6
   7CFQ maintains the per process queue for the processes which request I/O
   8operation(syncronous requests). In case of asynchronous requests, all the
   9requests from all the processes are batched together according to their
  10process's I/O priority.
  11
  12CFQ ioscheduler tunables
  13========================
  14
  15slice_idle
  16----------
  17This specifies how long CFQ should idle for next request on certain cfq queues
  18(for sequential workloads) and service trees (for random workloads) before
  19queue is expired and CFQ selects next queue to dispatch from.
  20
  21By default slice_idle is a non-zero value. That means by default we idle on
  22queues/service trees. This can be very helpful on highly seeky media like
  23single spindle SATA/SAS disks where we can cut down on overall number of
  24seeks and see improved throughput.
  25
  26Setting slice_idle to 0 will remove all the idling on queues/service tree
  27level and one should see an overall improved throughput on faster storage
  28devices like multiple SATA/SAS disks in hardware RAID configuration. The down
  29side is that isolation provided from WRITES also goes down and notion of
  30IO priority becomes weaker.
  31
  32So depending on storage and workload, it might be useful to set slice_idle=0.
  33In general I think for SATA/SAS disks and software RAID of SATA/SAS disks
  34keeping slice_idle enabled should be useful. For any configurations where
  35there are multiple spindles behind single LUN (Host based hardware RAID
  36controller or for storage arrays), setting slice_idle=0 might end up in better
  37throughput and acceptable latencies.
  38
  39back_seek_max
  40-------------
  41This specifies, given in Kbytes, the maximum "distance" for backward seeking.
  42The distance is the amount of space from the current head location to the
  43sectors that are backward in terms of distance.
  44
  45This parameter allows the scheduler to anticipate requests in the "backward"
  46direction and consider them as being the "next" if they are within this
  47distance from the current head location.
  48
  49back_seek_penalty
  50-----------------
  51This parameter is used to compute the cost of backward seeking. If the
  52backward distance of request is just 1/back_seek_penalty from a "front"
  53request, then the seeking cost of two requests is considered equivalent.
  54
  55So scheduler will not bias toward one or the other request (otherwise scheduler
  56will bias toward front request). Default value of back_seek_penalty is 2.
  57
  58fifo_expire_async
  59-----------------
  60This parameter is used to set the timeout of asynchronous requests. Default
  61value of this is 248ms.
  62
  63fifo_expire_sync
  64----------------
  65This parameter is used to set the timeout of synchronous requests. Default
  66value of this is 124ms. In case to favor synchronous requests over asynchronous
  67one, this value should be decreased relative to fifo_expire_async.
  68
  69slice_async
  70-----------
  71This parameter is same as of slice_sync but for asynchronous queue. The
  72default value is 40ms.
  73
  74slice_async_rq
  75--------------
  76This parameter is used to limit the dispatching of asynchronous request to
  77device request queue in queue's slice time. The maximum number of request that
  78are allowed to be dispatched also depends upon the io priority. Default value
  79for this is 2.
  80
  81slice_sync
  82----------
  83When a queue is selected for execution, the queues IO requests are only
  84executed for a certain amount of time(time_slice) before switching to another
  85queue. This parameter is used to calculate the time slice of synchronous
  86queue.
  87
  88time_slice is computed using the below equation:-
  89time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
  90time_slice of synchronous queue, increase the value of slice_sync. Default
  91value is 100ms.
  92
  93quantum
  94-------
  95This specifies the number of request dispatched to the device queue. In a
  96queue's time slice, a request will not be dispatched if the number of request
  97in the device exceeds this parameter. This parameter is used for synchronous
  98request.
  99
 100In case of storage with several disk, this setting can limit the parallel
 101processing of request. Therefore, increasing the value can imporve the
 102performace although this can cause the latency of some I/O to increase due
 103to more number of requests.
 104
 105CFQ IOPS Mode for group scheduling
 106===================================
 107Basic CFQ design is to provide priority based time slices. Higher priority
 108process gets bigger time slice and lower priority process gets smaller time
 109slice. Measuring time becomes harder if storage is fast and supports NCQ and
 110it would be better to dispatch multiple requests from multiple cfq queues in
 111request queue at a time. In such scenario, it is not possible to measure time
 112consumed by single queue accurately.
 113
 114What is possible though is to measure number of requests dispatched from a
 115single queue and also allow dispatch from multiple cfq queue at the same time.
 116This effectively becomes the fairness in terms of IOPS (IO operations per
 117second).
 118
 119If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
 120to IOPS mode and starts providing fairness in terms of number of requests
 121dispatched. Note that this mode switching takes effect only for group
 122scheduling. For non-cgroup users nothing should change.
 123
 124CFQ IO scheduler Idling Theory
 125===============================
 126Idling on a queue is primarily about waiting for the next request to come
 127on same queue after completion of a request. In this process CFQ will not
 128dispatch requests from other cfq queues even if requests are pending there.
 129
 130The rationale behind idling is that it can cut down on number of seeks
 131on rotational media. For example, if a process is doing dependent
 132sequential reads (next read will come on only after completion of previous
 133one), then not dispatching request from other queue should help as we
 134did not move the disk head and kept on dispatching sequential IO from
 135one queue.
 136
 137CFQ has following service trees and various queues are put on these trees.
 138
 139        sync-idle       sync-noidle     async
 140
 141All cfq queues doing synchronous sequential IO go on to sync-idle tree.
 142On this tree we idle on each queue individually.
 143
 144All synchronous non-sequential queues go on sync-noidle tree. Also any
 145request which are marked with REQ_NOIDLE go on this service tree. On this
 146tree we do not idle on individual queues instead idle on the whole group
 147of queues or the tree. So if there are 4 queues waiting for IO to dispatch
 148we will idle only once last queue has dispatched the IO and there is
 149no more IO on this service tree.
 150
 151All async writes go on async service tree. There is no idling on async
 152queues.
 153
 154CFQ has some optimizations for SSDs and if it detects a non-rotational
 155media which can support higher queue depth (multiple requests at in
 156flight at a time), then it cuts down on idling of individual queues and
 157all the queues move to sync-noidle tree and only tree idle remains. This
 158tree idling provides isolation with buffered write queues on async tree.
 159
 160FAQ
 161===
 162Q1. Why to idle at all on queues marked with REQ_NOIDLE.
 163
 164A1. 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
 189Q2. When to specify REQ_NOIDLE
 190A2. 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.
 194
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