Kernel-2.6.32-573.12.1.el6_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.

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 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).

  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.

Hierarchical Cgroups

• Currently none of the IO control policy supports hierarhical groups. But
  cgroup interface does allow creation of hierarhical cgroups and internally
  IO policies treat them as flat hierarchy.

  So this patch will allow creation of cgroup hierarhcy but at the backend
  everything will be treated as flat. So if somebody created a hierarchy like
  as follows.

        root
        /  \
         test1 test2
        |
         test3
  

  CFQ and throttling will practically treat all groups at same level.

            pivot
             /  |   \  \
        root  test1 test2  test3
  

  Down the line we can implement hierarchical accounting/control support
  and also introduce a new cgroup file “use_hierarchy” which will control
  whether cgroup hierarchy is viewed as flat or hierarchical by the policy..
  This is how memory controller also has implemented the things.

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.

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.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 exising 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.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/group_isolation

If group_isolation=1, it provides stronger isolation between groups at the
expense of throughput. By default group_isolation is 1. 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.

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.