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- Deadline IO scheduler tunables
- ==============================
- This little file attempts to document how the deadline io scheduler works.
- In particular, it will clarify the meaning of the exposed tunables that may be
- of interest to power users.
- Selecting IO schedulers
- -----------------------
- Refer to Documentation/block/switching-sched.txt for information on
- selecting an io scheduler on a per-device basis.
- ********************************************************************************
- read_expire (in ms)
- -----------
- The goal of the deadline io scheduler is to attempt to guarantee a start
- service time for a request. As we focus mainly on read latencies, this is
- tunable. When a read request first enters the io scheduler, it is assigned
- a deadline that is the current time + the read_expire value in units of
- milliseconds.
- write_expire (in ms)
- -----------
- Similar to read_expire mentioned above, but for writes.
- fifo_batch (number of requests)
- ----------
- Requests are grouped into ``batches'' of a particular data direction (read or
- write) which are serviced in increasing sector order. To limit extra seeking,
- deadline expiries are only checked between batches. fifo_batch controls the
- maximum number of requests per batch.
- This parameter tunes the balance between per-request latency and aggregate
- throughput. When low latency is the primary concern, smaller is better (where
- a value of 1 yields first-come first-served behaviour). Increasing fifo_batch
- generally improves throughput, at the cost of latency variation.
- writes_starved (number of dispatches)
- --------------
- When we have to move requests from the io scheduler queue to the block
- device dispatch queue, we always give a preference to reads. However, we
- don't want to starve writes indefinitely either. So writes_starved controls
- how many times we give preference to reads over writes. When that has been
- done writes_starved number of times, we dispatch some writes based on the
- same criteria as reads.
- front_merges (bool)
- ------------
- Sometimes it happens that a request enters the io scheduler that is contiguous
- with a request that is already on the queue. Either it fits in the back of that
- request, or it fits at the front. That is called either a back merge candidate
- or a front merge candidate. Due to the way files are typically laid out,
- back merges are much more common than front merges. For some work loads, you
- may even know that it is a waste of time to spend any time attempting to
- front merge requests. Setting front_merges to 0 disables this functionality.
- Front merges may still occur due to the cached last_merge hint, but since
- that comes at basically 0 cost we leave that on. We simply disable the
- rbtree front sector lookup when the io scheduler merge function is called.
- Nov 11 2002, Jens Axboe <jens.axboe@oracle.com>
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