linux/Documentation/block/deadline-iosched.txt
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   1Deadline IO scheduler tunables
   2==============================
   3
   4This little file attempts to document how the deadline io scheduler works.
   5In particular, it will clarify the meaning of the exposed tunables that may be
   6of interest to power users.
   7
   8Selecting IO schedulers
   9-----------------------
  10Refer to Documentation/block/switching-sched.txt for information on
  11selecting an io scheduler on a per-device basis.
  12
  13
  14********************************************************************************
  15
  16
  17read_expire     (in ms)
  18-----------
  19
  20The goal of the deadline io scheduler is to attempt to guarantee a start
  21service time for a request. As we focus mainly on read latencies, this is
  22tunable. When a read request first enters the io scheduler, it is assigned
  23a deadline that is the current time + the read_expire value in units of
  24milliseconds.
  25
  26
  27write_expire    (in ms)
  28-----------
  29
  30Similar to read_expire mentioned above, but for writes.
  31
  32
  33fifo_batch      (number of requests)
  34----------
  35
  36Requests are grouped into ``batches'' of a particular data direction (read or
  37write) which are serviced in increasing sector order.  To limit extra seeking,
  38deadline expiries are only checked between batches.  fifo_batch controls the
  39maximum number of requests per batch.
  40
  41This parameter tunes the balance between per-request latency and aggregate
  42throughput.  When low latency is the primary concern, smaller is better (where
  43a value of 1 yields first-come first-served behaviour).  Increasing fifo_batch
  44generally improves throughput, at the cost of latency variation.
  45
  46
  47writes_starved  (number of dispatches)
  48--------------
  49
  50When we have to move requests from the io scheduler queue to the block
  51device dispatch queue, we always give a preference to reads. However, we
  52don't want to starve writes indefinitely either. So writes_starved controls
  53how many times we give preference to reads over writes. When that has been
  54done writes_starved number of times, we dispatch some writes based on the
  55same criteria as reads.
  56
  57
  58front_merges    (bool)
  59------------
  60
  61Sometimes it happens that a request enters the io scheduler that is contiguous
  62with a request that is already on the queue. Either it fits in the back of that
  63request, or it fits at the front. That is called either a back merge candidate
  64or a front merge candidate. Due to the way files are typically laid out,
  65back merges are much more common than front merges. For some work loads, you
  66may even know that it is a waste of time to spend any time attempting to
  67front merge requests. Setting front_merges to 0 disables this functionality.
  68Front merges may still occur due to the cached last_merge hint, but since
  69that comes at basically 0 cost we leave that on. We simply disable the
  70rbtree front sector lookup when the io scheduler merge function is called.
  71
  72
  73Nov 11 2002, Jens Axboe <jens.axboe@oracle.com>
  74
  75
  76
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