linux/Documentation/cgroups/blkio-controller.txt
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   1                                Block IO Controller
   2                                ===================
   3Overview
   4========
   5cgroup subsys "blkio" implements the block io controller. There seems to be
   6a need of various kinds of IO control policies (like proportional BW, max BW)
   7both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
   8Plan is to use the same cgroup based management interface for blkio controller
   9and based on user options switch IO policies in the background.
  10
  11Currently two IO control policies are implemented. First one is proportional
  12weight time based division of disk policy. It is implemented in CFQ. Hence
  13this policy takes effect only on leaf nodes when CFQ is being used. The second
  14one is throttling policy which can be used to specify upper IO rate limits
  15on devices. This policy is implemented in generic block layer and can be
  16used on leaf nodes as well as higher level logical devices like device mapper.
  17
  18HOWTO
  19=====
  20Proportional Weight division of bandwidth
  21-----------------------------------------
  22You can do a very simple testing of running two dd threads in two different
  23cgroups. Here is what you can do.
  24
  25- Enable Block IO controller
  26        CONFIG_BLK_CGROUP=y
  27
  28- Enable group scheduling in CFQ
  29        CONFIG_CFQ_GROUP_IOSCHED=y
  30
  31- Compile and boot into kernel and mount IO controller (blkio); see
  32  cgroups.txt, Why are cgroups needed?.
  33
  34        mount -t tmpfs cgroup_root /sys/fs/cgroup
  35        mkdir /sys/fs/cgroup/blkio
  36        mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
  37
  38- Create two cgroups
  39        mkdir -p /sys/fs/cgroup/blkio/test1/ /sys/fs/cgroup/blkio/test2
  40
  41- Set weights of group test1 and test2
  42        echo 1000 > /sys/fs/cgroup/blkio/test1/blkio.weight
  43        echo 500 > /sys/fs/cgroup/blkio/test2/blkio.weight
  44
  45- Create two same size files (say 512MB each) on same disk (file1, file2) and
  46  launch two dd threads in different cgroup to read those files.
  47
  48        sync
  49        echo 3 > /proc/sys/vm/drop_caches
  50
  51        dd if=/mnt/sdb/zerofile1 of=/dev/null &
  52        echo $! > /sys/fs/cgroup/blkio/test1/tasks
  53        cat /sys/fs/cgroup/blkio/test1/tasks
  54
  55        dd if=/mnt/sdb/zerofile2 of=/dev/null &
  56        echo $! > /sys/fs/cgroup/blkio/test2/tasks
  57        cat /sys/fs/cgroup/blkio/test2/tasks
  58
  59- At macro level, first dd should finish first. To get more precise data, keep
  60  on looking at (with the help of script), at blkio.disk_time and
  61  blkio.disk_sectors files of both test1 and test2 groups. This will tell how
  62  much disk time (in milli seconds), each group got and how many secotors each
  63  group dispatched to the disk. We provide fairness in terms of disk time, so
  64  ideally io.disk_time of cgroups should be in proportion to the weight.
  65
  66Throttling/Upper Limit policy
  67-----------------------------
  68- Enable Block IO controller
  69        CONFIG_BLK_CGROUP=y
  70
  71- Enable throttling in block layer
  72        CONFIG_BLK_DEV_THROTTLING=y
  73
  74- Mount blkio controller (see cgroups.txt, Why are cgroups needed?)
  75        mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
  76
  77- Specify a bandwidth rate on particular device for root group. The format
  78  for policy is "<major>:<minor>  <byes_per_second>".
  79
  80        echo "8:16  1048576" > /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device
  81
  82  Above will put a limit of 1MB/second on reads happening for root group
  83  on device having major/minor number 8:16.
  84
  85- Run dd to read a file and see if rate is throttled to 1MB/s or not.
  86
  87                # dd if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
  88                # iflag=direct
  89        1024+0 records in
  90        1024+0 records out
  91        4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s
  92
  93 Limits for writes can be put using blkio.throttle.write_bps_device file.
  94
  95Hierarchical Cgroups
  96====================
  97- Currently none of the IO control policy supports hierarchical groups. But
  98  cgroup interface does allow creation of hierarchical cgroups and internally
  99  IO policies treat them as flat hierarchy.
 100
 101  So this patch will allow creation of cgroup hierarchcy but at the backend
 102  everything will be treated as flat. So if somebody created a hierarchy like
 103  as follows.
 104
 105                        root
 106                        /  \
 107                     test1 test2
 108                        |
 109                     test3
 110
 111  CFQ and throttling will practically treat all groups at same level.
 112
 113                                pivot
 114                             /  /   \  \
 115                        root  test1 test2  test3
 116
 117  Down the line we can implement hierarchical accounting/control support
 118  and also introduce a new cgroup file "use_hierarchy" which will control
 119  whether cgroup hierarchy is viewed as flat or hierarchical by the policy..
 120  This is how memory controller also has implemented the things.
 121
 122Various user visible config options
 123===================================
 124CONFIG_BLK_CGROUP
 125        - Block IO controller.
 126
 127CONFIG_DEBUG_BLK_CGROUP
 128        - Debug help. Right now some additional stats file show up in cgroup
 129          if this option is enabled.
 130
 131CONFIG_CFQ_GROUP_IOSCHED
 132        - Enables group scheduling in CFQ. Currently only 1 level of group
 133          creation is allowed.
 134
 135CONFIG_BLK_DEV_THROTTLING
 136        - Enable block device throttling support in block layer.
 137
 138Details of cgroup files
 139=======================
 140Proportional weight policy files
 141--------------------------------
 142- blkio.weight
 143        - Specifies per cgroup weight. This is default weight of the group
 144          on all the devices until and unless overridden by per device rule.
 145          (See blkio.weight_device).
 146          Currently allowed range of weights is from 10 to 1000.
 147
 148- blkio.weight_device
 149        - One can specify per cgroup per device rules using this interface.
 150          These rules override the default value of group weight as specified
 151          by blkio.weight.
 152
 153          Following is the format.
 154
 155          # echo dev_maj:dev_minor weight > blkio.weight_device
 156          Configure weight=300 on /dev/sdb (8:16) in this cgroup
 157          # echo 8:16 300 > blkio.weight_device
 158          # cat blkio.weight_device
 159          dev     weight
 160          8:16    300
 161
 162          Configure weight=500 on /dev/sda (8:0) in this cgroup
 163          # echo 8:0 500 > blkio.weight_device
 164          # cat blkio.weight_device
 165          dev     weight
 166          8:0     500
 167          8:16    300
 168
 169          Remove specific weight for /dev/sda in this cgroup
 170          # echo 8:0 0 > blkio.weight_device
 171          # cat blkio.weight_device
 172          dev     weight
 173          8:16    300
 174
 175- blkio.time
 176        - disk time allocated to cgroup per device in milliseconds. First
 177          two fields specify the major and minor number of the device and
 178          third field specifies the disk time allocated to group in
 179          milliseconds.
 180
 181- blkio.sectors
 182        - number of sectors transferred to/from disk by the group. First
 183          two fields specify the major and minor number of the device and
 184          third field specifies the number of sectors transferred by the
 185          group to/from the device.
 186
 187- blkio.io_service_bytes
 188        - Number of bytes transferred to/from the disk by the group. These
 189          are further divided by the type of operation - read or write, sync
 190          or async. First two fields specify the major and minor number of the
 191          device, third field specifies the operation type and the fourth field
 192          specifies the number of bytes.
 193
 194- blkio.io_serviced
 195        - Number of IOs completed to/from the disk by the group. These
 196          are further divided by the type of operation - read or write, sync
 197          or async. First two fields specify the major and minor number of the
 198          device, third field specifies the operation type and the fourth field
 199          specifies the number of IOs.
 200
 201- blkio.io_service_time
 202        - Total amount of time between request dispatch and request completion
 203          for the IOs done by this cgroup. This is in nanoseconds to make it
 204          meaningful for flash devices too. For devices with queue depth of 1,
 205          this time represents the actual service time. When queue_depth > 1,
 206          that is no longer true as requests may be served out of order. This
 207          may cause the service time for a given IO to include the service time
 208          of multiple IOs when served out of order which may result in total
 209          io_service_time > actual time elapsed. This time is further divided by
 210          the type of operation - read or write, sync or async. First two fields
 211          specify the major and minor number of the device, third field
 212          specifies the operation type and the fourth field specifies the
 213          io_service_time in ns.
 214
 215- blkio.io_wait_time
 216        - Total amount of time the IOs for this cgroup spent waiting in the
 217          scheduler queues for service. This can be greater than the total time
 218          elapsed since it is cumulative io_wait_time for all IOs. It is not a
 219          measure of total time the cgroup spent waiting but rather a measure of
 220          the wait_time for its individual IOs. For devices with queue_depth > 1
 221          this metric does not include the time spent waiting for service once
 222          the IO is dispatched to the device but till it actually gets serviced
 223          (there might be a time lag here due to re-ordering of requests by the
 224          device). This is in nanoseconds to make it meaningful for flash
 225          devices too. This time is further divided by the type of operation -
 226          read or write, sync or async. First two fields specify the major and
 227          minor number of the device, third field specifies the operation type
 228          and the fourth field specifies the io_wait_time in ns.
 229
 230- blkio.io_merged
 231        - Total number of bios/requests merged into requests belonging to this
 232          cgroup. This is further divided by the type of operation - read or
 233          write, sync or async.
 234
 235- blkio.io_queued
 236        - Total number of requests queued up at any given instant for this
 237          cgroup. This is further divided by the type of operation - read or
 238          write, sync or async.
 239
 240- blkio.avg_queue_size
 241        - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 242          The average queue size for this cgroup over the entire time of this
 243          cgroup's existence. Queue size samples are taken each time one of the
 244          queues of this cgroup gets a timeslice.
 245
 246- blkio.group_wait_time
 247        - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 248          This is the amount of time the cgroup had to wait since it became busy
 249          (i.e., went from 0 to 1 request queued) to get a timeslice for one of
 250          its queues. This is different from the io_wait_time which is the
 251          cumulative total of the amount of time spent by each IO in that cgroup
 252          waiting in the scheduler queue. This is in nanoseconds. If this is
 253          read when the cgroup is in a waiting (for timeslice) state, the stat
 254          will only report the group_wait_time accumulated till the last time it
 255          got a timeslice and will not include the current delta.
 256
 257- blkio.empty_time
 258        - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 259          This is the amount of time a cgroup spends without any pending
 260          requests when not being served, i.e., it does not include any time
 261          spent idling for one of the queues of the cgroup. This is in
 262          nanoseconds. If this is read when the cgroup is in an empty state,
 263          the stat will only report the empty_time accumulated till the last
 264          time it had a pending request and will not include the current delta.
 265
 266- blkio.idle_time
 267        - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 268          This is the amount of time spent by the IO scheduler idling for a
 269          given cgroup in anticipation of a better request than the existing ones
 270          from other queues/cgroups. This is in nanoseconds. If this is read
 271          when the cgroup is in an idling state, the stat will only report the
 272          idle_time accumulated till the last idle period and will not include
 273          the current delta.
 274
 275- blkio.dequeue
 276        - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This
 277          gives the statistics about how many a times a group was dequeued
 278          from service tree of the device. First two fields specify the major
 279          and minor number of the device and third field specifies the number
 280          of times a group was dequeued from a particular device.
 281
 282Throttling/Upper limit policy files
 283-----------------------------------
 284- blkio.throttle.read_bps_device
 285        - Specifies upper limit on READ rate from the device. IO rate is
 286          specified in bytes per second. Rules are per device. Following is
 287          the format.
 288
 289  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.read_bps_device
 290
 291- blkio.throttle.write_bps_device
 292        - Specifies upper limit on WRITE rate to the device. IO rate is
 293          specified in bytes per second. Rules are per device. Following is
 294          the format.
 295
 296  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device
 297
 298- blkio.throttle.read_iops_device
 299        - Specifies upper limit on READ rate from the device. IO rate is
 300          specified in IO per second. Rules are per device. Following is
 301          the format.
 302
 303  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device
 304
 305- blkio.throttle.write_iops_device
 306        - Specifies upper limit on WRITE rate to the device. IO rate is
 307          specified in io per second. Rules are per device. Following is
 308          the format.
 309
 310  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device
 311
 312Note: If both BW and IOPS rules are specified for a device, then IO is
 313      subjected to both the constraints.
 314
 315- blkio.throttle.io_serviced
 316        - Number of IOs (bio) completed to/from the disk by the group (as
 317          seen by throttling policy). These are further divided by the type
 318          of operation - read or write, sync or async. First two fields specify
 319          the major and minor number of the device, third field specifies the
 320          operation type and the fourth field specifies the number of IOs.
 321
 322          blkio.io_serviced does accounting as seen by CFQ and counts are in
 323          number of requests (struct request). On the other hand,
 324          blkio.throttle.io_serviced counts number of IO in terms of number
 325          of bios as seen by throttling policy.  These bios can later be
 326          merged by elevator and total number of requests completed can be
 327          lesser.
 328
 329- blkio.throttle.io_service_bytes
 330        - Number of bytes transferred to/from the disk by the group. These
 331          are further divided by the type of operation - read or write, sync
 332          or async. First two fields specify the major and minor number of the
 333          device, third field specifies the operation type and the fourth field
 334          specifies the number of bytes.
 335
 336          These numbers should roughly be same as blkio.io_service_bytes as
 337          updated by CFQ. The difference between two is that
 338          blkio.io_service_bytes will not be updated if CFQ is not operating
 339          on request queue.
 340
 341Common files among various policies
 342-----------------------------------
 343- blkio.reset_stats
 344        - Writing an int to this file will result in resetting all the stats
 345          for that cgroup.
 346
 347CFQ sysfs tunable
 348=================
 349/sys/block/<disk>/queue/iosched/slice_idle
 350------------------------------------------
 351On a faster hardware CFQ can be slow, especially with sequential workload.
 352This happens because CFQ idles on a single queue and single queue might not
 353drive deeper request queue depths to keep the storage busy. In such scenarios
 354one can try setting slice_idle=0 and that would switch CFQ to IOPS
 355(IO operations per second) mode on NCQ supporting hardware.
 356
 357That means CFQ will not idle between cfq queues of a cfq group and hence be
 358able to driver higher queue depth and achieve better throughput. That also
 359means that cfq provides fairness among groups in terms of IOPS and not in
 360terms of disk time.
 361
 362/sys/block/<disk>/queue/iosched/group_idle
 363------------------------------------------
 364If one disables idling on individual cfq queues and cfq service trees by
 365setting slice_idle=0, group_idle kicks in. That means CFQ will still idle
 366on the group in an attempt to provide fairness among groups.
 367
 368By default group_idle is same as slice_idle and does not do anything if
 369slice_idle is enabled.
 370
 371One can experience an overall throughput drop if you have created multiple
 372groups and put applications in that group which are not driving enough
 373IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle
 374on individual groups and throughput should improve.
 375
 376What works
 377==========
 378- Currently only sync IO queues are support. All the buffered writes are
 379  still system wide and not per group. Hence we will not see service
 380  differentiation between buffered writes between groups.
 381
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