				Block IO Controller
				===================
Overview
========
cgroup subsys "blkio" implements the block io controller. There seems to be
a need of various kinds of IO control policies (like proportional BW, max BW)
both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
Plan is to use the 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.

HOWTO
=====
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
	CONFIG_CFQ_GROUP_IOSCHED=y

- 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
===================================
CONFIG_CFQ_GROUP_IOSCHED
	- Enables group scheduling in CFQ. Currently only 1 level of group
	  creation is allowed.

CONFIG_DEBUG_CFQ_IOSCHED
	- 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.

CONFIG_BLK_CGROUP
	- Block IO controller. Selected by CONFIG_CFQ_GROUP_IOSCHED.

CONFIG_DEBUG_BLK_CGROUP
	- 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.io_service_bytes
	- Number of bytes transferred to/from the disk by the group. These
	  are further divided by the type of operation - read or write, sync
	  or async. First two fields specify the major and minor number of the
	  device, third field specifies the operation type and the fourth field
	  specifies the number of bytes.

- blkio.io_serviced
	- Number of IOs completed to/from the disk by the group. These
	  are further divided by the type of operation - read or write, sync
	  or async. First two fields specify the major and minor number of the
	  device, third field specifies the operation type and the fourth field
	  specifies the number of IOs.

- blkio.io_service_time
	- Total amount of time between request dispatch and request completion
	  for the IOs done by this cgroup. This is in nanoseconds to make it
	  meaningful for flash devices too. For devices with queue depth of 1,
	  this time represents the actual service time. When queue_depth > 1,
	  that is no longer true as requests may be served out of order. This
	  may cause the service time for a given IO to include the service time
	  of multiple IOs when served out of order which may result in total
	  io_service_time > actual time elapsed. This time is further divided by
	  the type of operation - read or write, sync or async. First two fields
	  specify the major and minor number of the device, third field
	  specifies the operation type and the fourth field specifies the
	  io_service_time in ns.

- blkio.io_wait_time
	- Total amount of time the IOs for this cgroup spent waiting in the
	  scheduler queues for service. This can be greater than the total time
	  elapsed since it is cumulative io_wait_time for all IOs. It is not a
	  measure of total time the cgroup spent waiting but rather a measure of
	  the wait_time for its individual IOs. For devices with queue_depth > 1
	  this metric does not include the time spent waiting for service once
	  the IO is dispatched to the device but till it actually gets serviced
	  (there might be a time lag here due to re-ordering of requests by the
	  device). This is in nanoseconds to make it meaningful for flash
	  devices too. This time is further divided by the type of operation -
	  read or write, sync or async. First two fields specify the major and
	  minor number of the device, third field specifies the operation type
	  and the fourth field specifies the io_wait_time in ns.

- 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.

- blkio.reset_stats
	- Writing an int to this file will result in resetting all the stats
	  for that cgroup.

CFQ sysfs tunable
=================
/sys/block/<disk>/queue/iosched/group_isolation

If group_isolation=1, it provides stronger isolation between groups at the
expense of throughput. By default group_isolation is 0. In general that
means that if group_isolation=0, expect fairness for sequential workload
only. Set group_isolation=1 to see fairness for random IO workload also.

Generally CFQ will put random seeky workload in sync-noidle category. CFQ
will disable idling on these queues and it does a collective idling on group
of such queues. Generally these are slow moving queues and if there is a
sync-noidle service tree in each group, that group gets exclusive access to
disk for certain period. That means it will bring the throughput down if
group does not have enough IO to drive deeper queue depths and utilize disk
capacity to the fullest in the slice allocated to it. But the flip side is
that even a random reader should get better latencies and overall throughput
if there are lots of sequential readers/sync-idle workload running in the
system.

If group_isolation=0, then CFQ automatically moves all the random seeky queues
in the root group. That means there will be no service differentiation for
that kind of workload. This leads to better throughput as we do collective
idling on root sync-noidle tree.

By default one should run with group_isolation=0. If that is not sufficient
and one wants stronger isolation between groups, then set group_isolation=1
but this will come at cost of reduced throughput.

What works
==========
- Currently only sync IO queues are support. All the buffered writes are
  still system wide and not per group. Hence we will not see service
  differentiation between buffered writes between groups.