Kernel-3.10.0-957.el7_blkio-controller

            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 “: “.

    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 “: “ > /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 “: “ > /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 “: “ > /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 “: “ > /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//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//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.

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.