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79 lines
2.7 KiB
79 lines
2.7 KiB
20 years ago
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Deadline IO scheduler tunables
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==============================
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This little file attempts to document how the deadline io scheduler works.
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In particular, it will clarify the meaning of the exposed tunables that may be
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of interest to power users.
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Each io queue has a set of io scheduler tunables associated with it. These
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tunables control how the io scheduler works. You can find these entries
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in:
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/sys/block/<device>/queue/iosched
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assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted,
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you can do so by typing:
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# mount none /sys -t sysfs
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********************************************************************************
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read_expire (in ms)
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-----------
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The goal of the deadline io scheduler is to attempt to guarentee a start
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service time for a request. As we focus mainly on read latencies, this is
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tunable. When a read request first enters the io scheduler, it is assigned
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a deadline that is the current time + the read_expire value in units of
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miliseconds.
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write_expire (in ms)
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-----------
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Similar to read_expire mentioned above, but for writes.
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fifo_batch
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----------
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When a read request expires its deadline, we must move some requests from
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the sorted io scheduler list to the block device dispatch queue. fifo_batch
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controls how many requests we move, based on the cost of each request. A
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request is either qualified as a seek or a stream. The io scheduler knows
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the last request that was serviced by the drive (or will be serviced right
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before this one). See seek_cost and stream_unit.
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write_starved (number of dispatches)
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-------------
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When we have to move requests from the io scheduler queue to the block
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device dispatch queue, we always give a preference to reads. However, we
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don't want to starve writes indefinitely either. So writes_starved controls
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how many times we give preference to reads over writes. When that has been
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done writes_starved number of times, we dispatch some writes based on the
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same criteria as reads.
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front_merges (bool)
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------------
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Sometimes it happens that a request enters the io scheduler that is contigious
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with a request that is already on the queue. Either it fits in the back of that
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request, or it fits at the front. That is called either a back merge candidate
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or a front merge candidate. Due to the way files are typically laid out,
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back merges are much more common than front merges. For some work loads, you
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may even know that it is a waste of time to spend any time attempting to
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front merge requests. Setting front_merges to 0 disables this functionality.
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Front merges may still occur due to the cached last_merge hint, but since
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that comes at basically 0 cost we leave that on. We simply disable the
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rbtree front sector lookup when the io scheduler merge function is called.
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Nov 11 2002, Jens Axboe <axboe@suse.de>
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