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    Subject[PATCH 16/16] blkio: Documentation
    Signed-off-by: Vivek Goyal <>
    Documentation/cgroups/blkio-controller.txt | 100 ++++++++++++++++++++++++++++
    1 files changed, 100 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/cgroups/blkio-controller.txt

    diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
    new file mode 100644
    index 0000000..6fb772f
    --- /dev/null
    +++ b/Documentation/cgroups/blkio-controller.txt
    @@ -0,0 +1,100 @@
    + Block IO Controller
    + ===================
    +cgroup subsys "blkio" implements the block io controller. There seems to be
    +a need of various kind of IO control policies (like proportional BW, max BW)
    +both at leaf nodes as well as at intermediate nodes in storage hierarchy. Plan
    +is to use same cgroup based management interface for blkio controller and
    +based on user options switch IO policies in the background.
    +In the first phase, this patchset implements proportional weight time based
    +division of disk policy. It is implemented in CFQ. Hence this policy takes
    +effect only on leaf nodes when CFQ is being used.
    +You can do a very simple testing of running two dd threads in two different
    +cgroups. Here is what you can do.
    +- Enable group scheduling in CFQ
    +- Compile and boot into kernel and mount IO controller (blkio).
    + mount -t cgroup -o blkio none /cgroup
    +- Create two cgroups
    + mkdir -p /cgroup/test1/ /cgroup/test2
    +- Set weights of group test1 and test2
    + echo 1000 > /cgroup/test1/blkio.weight
    + echo 500 > /cgroup/test2/blkio.weight
    +- Create two same size files (say 512MB each) on same disk (file1, file2) and
    + launch two dd threads in different cgroup to read those files.
    + sync
    + echo 3 > /proc/sys/vm/drop_caches
    + dd if=/mnt/sdb/zerofile1 of=/dev/null &
    + echo $! > /cgroup/test1/tasks
    + cat /cgroup/test1/tasks
    + dd if=/mnt/sdb/zerofile2 of=/dev/null &
    + echo $! > /cgroup/test2/tasks
    + cat /cgroup/test2/tasks
    +- At macro level, first dd should finish first. To get more precise data, keep
    + on looking at (with the help of script), at blkio.disk_time and
    + blkio.disk_sectors files of both test1 and test2 groups. This will tell how
    + much disk time (in milli seconds), each group got and how many secotors each
    + group dispatched to the disk. We provide fairness in terms of disk time, so
    + ideally io.disk_time of cgroups should be in proportion to the weight.
    +Various user visible config options
    + - Enables group scheduling in CFQ. Currently only 1 level of group
    + creation is allowed.
    + - Enables some debugging messages in blktrace. Also creates extra
    + cgroup file blkio.dequeue.
    +Config options selected automatically
    +These config options are not user visible and are selected/deselected
    +automatically based on IO scheduler configuration.
    + - Block IO controller. Selected by CONFIG_CFQ_GROUP_IOSCHED.
    + - Debug help. Selected by CONFIG_DEBUG_CFQ_IOSCHED.
    +Details of cgroup files
    +- blkio.weight
    + - Specifies per cgroup weight.
    + Currently allowed range of weights is from 100 to 1000.
    +- blkio.time
    + - disk time allocated to cgroup per device in milliseconds. First
    + two fields specify the major and minor number of the device and
    + third field specifies the disk time allocated to group in
    + milliseconds.
    +- blkio.sectors
    + - number of sectors transferred to/from disk by the group. First
    + two fields specify the major and minor number of the device and
    + third field specifies the number of sectors transferred by the
    + group to/from the device.
    +- blkio.dequeue
    + - Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y. This
    + gives the statistics about how many a times a group was dequeued
    + from service tree of the device. First two fields specify the major
    + and minor number of the device and third field specifies the number
    + of times a group was dequeued from a particular device.

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