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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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