				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.

Currently two IO control policies are implemented. First one is 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. The second
one is throttling policy which can be used to specify upper IO rate limits
on devices. This policy is implemented in generic block layer and can be
used on leaf nodes as well as higher level logical devices like device mapper.

HOWTO
=====
Proportional Weight division of bandwidth
-----------------------------------------
You can do a very simple testing of running two dd threads in two different
cgroups. Here is what you can do.

- Enable Block IO controller
	CONFIG_BLK_CGROUP=y

- Enable group scheduling in CFQ
	CONFIG_CFQ_GROUP_IOSCHED=y

- Compile and boot into kernel and mount IO controller (blkio); see
  cgroups.txt, Why are cgroups needed?.

	mount -t tmpfs cgroup_root /sys/fs/cgroup
	mkdir /sys/fs/cgroup/blkio
	mount -t cgroup -o blkio none /sys/fs/cgroup/blkio

- Create two cgroups
	mkdir -p /sys/fs/cgroup/blkio/test1/ /sys/fs/cgroup/blkio/test2

- Set weights of group test1 and test2
	echo 1000 > /sys/fs/cgroup/blkio/test1/blkio.weight
	echo 500 > /sys/fs/cgroup/blkio/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 $! > /sys/fs/cgroup/blkio/test1/tasks
	cat /sys/fs/cgroup/blkio/test1/tasks

	dd if=/mnt/sdb/zerofile2 of=/dev/null &
	echo $! > /sys/fs/cgroup/blkio/test2/tasks
	cat /sys/fs/cgroup/blkio/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.

Throttling/Upper Limit policy
-----------------------------
- Enable Block IO controller
	CONFIG_BLK_CGROUP=y

- Enable throttling in block layer
	CONFIG_BLK_DEV_THROTTLING=y

- Mount blkio controller (see cgroups.txt, Why are cgroups needed?)
        mount -t cgroup -o blkio none /sys/fs/cgroup/blkio

- Specify a bandwidth rate on particular device for root group. The format
  for policy is "<major>:<minor>  <bytes_per_second>".

        echo "8:16  1048576" > /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device

  Above will put a limit of 1MB/second on reads happening for root group
  on device having major/minor number 8:16.

- Run dd to read a file and see if rate is throttled to 1MB/s or not.

		# dd if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
		# iflag=direct
        1024+0 records in
        1024+0 records out
        4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s

 Limits for writes can be put using blkio.throttle.write_bps_device file.

Hierarchical Cgroups
====================

Both CFQ and throttling implement hierarchy support; however,
throttling's hierarchy support is enabled iff "sane_behavior" is
enabled from cgroup side, which currently is a development option and
not publicly available.

If somebody created a hierarchy like as follows.

			root
			/  \
		     test1 test2
			|
		     test3

CFQ by default and throttling with "sane_behavior" will handle the
hierarchy correctly.  For details on CFQ hierarchy support, refer to
Documentation/block/cfq-iosched.txt.  For throttling, all limits apply
to the whole subtree while all statistics are local to the IOs
directly generated by tasks in that cgroup.

Throttling without "sane_behavior" enabled from cgroup side will
practically treat all groups at same level as if it looks like the
following.

				pivot
			     /  /   \  \
			root  test1 test2  test3

Various user visible config options
===================================
CONFIG_BLK_CGROUP
	- Block IO controller.

CONFIG_DEBUG_BLK_CGROUP
	- Debug help. Right now some additional stats file show up in cgroup
	  if this option is enabled.

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

CONFIG_BLK_DEV_THROTTLING
	- Enable block device throttling support in block layer.

Details of cgroup files
=======================
Proportional weight policy files
--------------------------------
- blkio.weight
	- Specifies per cgroup weight. This is default weight of the group
	  on all the devices until and unless overridden by per device rule.
	  (See blkio.weight_device).
	  Currently allowed range of weights is from 10 to 1000.

- blkio.weight_device
	- One can specify per cgroup per device rules using this interface.
	  These rules override the default value of group weight as specified
	  by blkio.weight.

	  Following is the format.

	  # echo dev_maj:dev_minor weight > blkio.weight_device
	  Configure weight=300 on /dev/sdb (8:16) in this cgroup
	  # echo 8:16 300 > blkio.weight_device
	  # cat blkio.weight_device
	  dev     weight
	  8:16    300

	  Configure weight=500 on /dev/sda (8:0) in this cgroup
	  # echo 8:0 500 > blkio.weight_device
	  # cat blkio.weight_device
	  dev     weight
	  8:0     500
	  8:16    300

	  Remove specific weight for /dev/sda in this cgroup
	  # echo 8:0 0 > blkio.weight_device
	  # cat blkio.weight_device
	  dev     weight
	  8:16    300

- blkio.leaf_weight[_device]
	- Equivalents of blkio.weight[_device] for the purpose of
          deciding how much weight tasks in the given cgroup has while
          competing with the cgroup's child cgroups. For details,
          please refer to Documentation/block/cfq-iosched.txt.

- 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.io_merged
	- Total number of bios/requests merged into requests belonging to this
	  cgroup. This is further divided by the type of operation - read or
	  write, sync or async.

- blkio.io_queued
	- Total number of requests queued up at any given instant for this
	  cgroup. This is further divided by the type of operation - read or
	  write, sync or async.

- blkio.avg_queue_size
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
	  The average queue size for this cgroup over the entire time of this
	  cgroup's existence. Queue size samples are taken each time one of the
	  queues of this cgroup gets a timeslice.

- blkio.group_wait_time
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
	  This is the amount of time the cgroup had to wait since it became busy
	  (i.e., went from 0 to 1 request queued) to get a timeslice for one of
	  its queues. This is different from the io_wait_time which is the
	  cumulative total of the amount of time spent by each IO in that cgroup
	  waiting in the scheduler queue. This is in nanoseconds. If this is
	  read when the cgroup is in a waiting (for timeslice) state, the stat
	  will only report the group_wait_time accumulated till the last time it
	  got a timeslice and will not include the current delta.

- blkio.empty_time
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
	  This is the amount of time a cgroup spends without any pending
	  requests when not being served, i.e., it does not include any time
	  spent idling for one of the queues of the cgroup. This is in
	  nanoseconds. If this is read when the cgroup is in an empty state,
	  the stat will only report the empty_time accumulated till the last
	  time it had a pending request and will not include the current delta.

- blkio.idle_time
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
	  This is the amount of time spent by the IO scheduler idling for a
	  given cgroup in anticipation of a better request than the existing ones
	  from other queues/cgroups. This is in nanoseconds. If this is read
	  when the cgroup is in an idling state, the stat will only report the
	  idle_time accumulated till the last idle period and will not include
	  the current delta.

- blkio.dequeue
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=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.*_recursive
	- Recursive version of various stats. These files show the
          same information as their non-recursive counterparts but
          include stats from all the descendant cgroups.

Throttling/Upper limit policy files
-----------------------------------
- blkio.throttle.read_bps_device
	- Specifies upper limit on READ rate from the device. IO rate is
	  specified in bytes per second. Rules are per device. Following is
	  the format.

  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.read_bps_device

- blkio.throttle.write_bps_device
	- Specifies upper limit on WRITE rate to the device. IO rate is
	  specified in bytes per second. Rules are per device. Following is
	  the format.

  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device

- blkio.throttle.read_iops_device
	- Specifies upper limit on READ rate from the device. IO rate is
	  specified in IO per second. Rules are per device. Following is
	  the format.

  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device

- blkio.throttle.write_iops_device
	- Specifies upper limit on WRITE rate to the device. IO rate is
	  specified in io per second. Rules are per device. Following is
	  the format.

  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device

Note: If both BW and IOPS rules are specified for a device, then IO is
      subjected to both the constraints.

- blkio.throttle.io_serviced
	- Number of IOs (bio) completed to/from the disk by the group (as
	  seen by throttling policy). 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_serviced does accounting as seen by CFQ and counts are in
	  number of requests (struct request). On the other hand,
	  blkio.throttle.io_serviced counts number of IO in terms of number
	  of bios as seen by throttling policy.  These bios can later be
	  merged by elevator and total number of requests completed can be
	  lesser.

- blkio.throttle.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.

	  These numbers should roughly be same as blkio.io_service_bytes as
	  updated by CFQ. The difference between two is that
	  blkio.io_service_bytes will not be updated if CFQ is not operating
	  on request queue.

Common files among various policies
-----------------------------------
- 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/slice_idle
------------------------------------------
On a faster hardware CFQ can be slow, especially with sequential workload.
This happens because CFQ idles on a single queue and single queue might not
drive deeper request queue depths to keep the storage busy. In such scenarios
one can try setting slice_idle=0 and that would switch CFQ to IOPS
(IO operations per second) mode on NCQ supporting hardware.

That means CFQ will not idle between cfq queues of a cfq group and hence be
able to driver higher queue depth and achieve better throughput. That also
means that cfq provides fairness among groups in terms of IOPS and not in
terms of disk time.

/sys/block/<disk>/queue/iosched/group_idle
------------------------------------------
If one disables idling on individual cfq queues and cfq service trees by
setting slice_idle=0, group_idle kicks in. That means CFQ will still idle
on the group in an attempt to provide fairness among groups.

By default group_idle is same as slice_idle and does not do anything if
slice_idle is enabled.

One can experience an overall throughput drop if you have created multiple
groups and put applications in that group which are not driving enough
IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle
on individual groups and throughput should improve.

Writeback
=========

Page cache is dirtied through buffered writes and shared mmaps and
written asynchronously to the backing filesystem by the writeback
mechanism.  Writeback sits between the memory and IO domains and
regulates the proportion of dirty memory by balancing dirtying and
write IOs.

On traditional cgroup hierarchies, relationships between different
controllers cannot be established making it impossible for writeback
to operate accounting for cgroup resource restrictions and all
writeback IOs are attributed to the root cgroup.

If both the blkio and memory controllers are used on the v2 hierarchy
and the filesystem supports cgroup writeback, writeback operations
correctly follow the resource restrictions imposed by both memory and
blkio controllers.

Writeback examines both system-wide and per-cgroup dirty memory status
and enforces the more restrictive of the two.  Also, writeback control
parameters which are absolute values - vm.dirty_bytes and
vm.dirty_background_bytes - are distributed across cgroups according
to their current writeback bandwidth.

There's a peculiarity stemming from the discrepancy in ownership
granularity between memory controller and writeback.  While memory
controller tracks ownership per page, writeback operates on inode
basis.  cgroup writeback bridges the gap by tracking ownership by
inode but migrating ownership if too many foreign pages, pages which
don't match the current inode ownership, have been encountered while
writing back the inode.

This is a conscious design choice as writeback operations are
inherently tied to inodes making strictly following page ownership
complicated and inefficient.  The only use case which suffers from
this compromise is multiple cgroups concurrently dirtying disjoint
regions of the same inode, which is an unlikely use case and decided
to be unsupported.  Note that as memory controller assigns page
ownership on the first use and doesn't update it until the page is
released, even if cgroup writeback strictly follows page ownership,
multiple cgroups dirtying overlapping areas wouldn't work as expected.
In general, write-sharing an inode across multiple cgroups is not well
supported.

Filesystem support for cgroup writeback
---------------------------------------

A filesystem can make writeback IOs cgroup-aware by updating
address_space_operations->writepage[s]() to annotate bio's using the
following two functions.

* wbc_init_bio(@wbc, @bio)

  Should be called for each bio carrying writeback data and associates
  the bio with the inode's owner cgroup.  Can be called anytime
  between bio allocation and submission.

* wbc_account_io(@wbc, @page, @bytes)

  Should be called for each data segment being written out.  While
  this function doesn't care exactly when it's called during the
  writeback session, it's the easiest and most natural to call it as
  data segments are added to a bio.

With writeback bio's annotated, cgroup support can be enabled per
super_block by setting MS_CGROUPWB in ->s_flags.  This allows for
selective disabling of cgroup writeback support which is helpful when
certain filesystem features, e.g. journaled data mode, are
incompatible.

wbc_init_bio() binds the specified bio to its cgroup.  Depending on
the configuration, the bio may be executed at a lower priority and if
the writeback session is holding shared resources, e.g. a journal
entry, may lead to priority inversion.  There is no one easy solution
for the problem.  Filesystems can try to work around specific problem
cases by skipping wbc_init_bio() or using bio_associate_blkcg()
directly.