Commit 23c76983 authored by Alan D. Brunelle's avatar Alan D. Brunelle Committed by Jens Axboe

Some IO scheduler cleanup in Documentation/block

as-iosched.txt:
  o  Changed IO scheduler selection text to a reference to the
     switching-sched.txt file.

  o  Fixed typo: 'for up time...' -> 'for up to...'

  o  Added short description of the est_time file.

deadline-iosched.txt:
  o  Changed IO scheduler selection text to a reference to the
     switching-sched.txt file.

  o  Removed references to non-existent seek-cost and stream_unit.

  o  Fixed typo: 'write_starved' -> 'writes_starved'

switching-sched.txt:
  o  Added in boot-time argument to set the default IO scheduler. (From
     as-iosched.txt)

  o  Added in sysfs mount instructions. (From deadline-iosched.txt)
Signed-off-by: default avatarAlan D. Brunelle <Alan.Brunelle@hp.com>
Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent 26bbb29a
......@@ -20,15 +20,10 @@ actually has a head for each physical device in the logical RAID device.
However, setting the antic_expire (see tunable parameters below) produces
very similar behavior to the deadline IO scheduler.
Selecting IO schedulers
-----------------------
To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
'noop', 'as' and 'cfq' (the default) are also available. IO schedulers are
assigned globally at boot time only presently. It's also possible to change
the IO scheduler for a determined device on the fly, as described in
Documentation/block/switching-sched.txt.
Refer to Documentation/block/switching-sched.txt for information on
selecting an io scheduler on a per-device basis.
Anticipatory IO scheduler Policies
----------------------------------
......@@ -115,7 +110,7 @@ statistics (average think time, average seek distance) on the process
that submitted the just completed request are examined. If it seems
likely that that process will submit another request soon, and that
request is likely to be near the just completed request, then the IO
scheduler will stop dispatching more read requests for up time (antic_expire)
scheduler will stop dispatching more read requests for up to (antic_expire)
milliseconds, hoping that process will submit a new request near the one
that just completed. If such a request is made, then it is dispatched
immediately. If the antic_expire wait time expires, then the IO scheduler
......@@ -165,3 +160,13 @@ The parameters are:
for big seek time devices though not a linear correspondence - most
processes have only a few ms thinktime.
In addition to the tunables above there is a read-only file named est_time
which, when read, will show:
- The probability of a task exiting without a cooperating task
submitting an anticipated IO.
- The current mean think time.
- The seek distance used to determine if an incoming IO is better.
......@@ -5,16 +5,10 @@ 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.
Each io queue has a set of io scheduler tunables associated with it. These
tunables control how the io scheduler works. You can find these entries
in:
/sys/block/<device>/queue/iosched
assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted,
you can do so by typing:
# mount none /sys -t sysfs
Selecting IO schedulers
-----------------------
Refer to Documentation/block/switching-sched.txt for information on
selecting an io scheduler on a per-device basis.
********************************************************************************
......@@ -41,14 +35,11 @@ fifo_batch
When a read request expires its deadline, we must move some requests from
the sorted io scheduler list to the block device dispatch queue. fifo_batch
controls how many requests we move, based on the cost of each request. A
request is either qualified as a seek or a stream. The io scheduler knows
the last request that was serviced by the drive (or will be serviced right
before this one). See seek_cost and stream_unit.
controls how many requests we move.
write_starved (number of dispatches)
-------------
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
......
To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
'noop', 'as' and 'cfq' (the default) are also available. IO schedulers are
assigned globally at boot time only presently.
Each io queue has a set of io scheduler tunables associated with it. These
tunables control how the io scheduler works. You can find these entries
in:
/sys/block/<device>/queue/iosched
assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted,
you can do so by typing:
# mount none /sys -t sysfs
As of the Linux 2.6.10 kernel, it is now possible to change the
IO scheduler for a given block device on the fly (thus making it possible,
for instance, to set the CFQ scheduler for the system default, but
......@@ -20,3 +35,9 @@ noop anticipatory deadline [cfq]
# echo anticipatory > /sys/block/hda/queue/scheduler
# cat /sys/block/hda/queue/scheduler
noop [anticipatory] deadline cfq
Each io queue has a set of io scheduler tunables associated with it. These
tunables control how the io scheduler works. You can find these entries
in:
/sys/block/<device>/queue/iosched
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