Documentation/cgroups/blkio-controller.txt

   1                                 Block IO Controller
   2                                 ===================
   3 Overview
   4 ========
   5 cgroup subsys "blkio" implements the block io controller. There seems to be
   6 a need of various kinds of IO control policies (like proportional BW, max BW)
   7 both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
   8 Plan is to use the same cgroup based management interface for blkio controller
   9 and based on user options switch IO policies in the background.
  10
  11 In the first phase, this patchset implements proportional weight time based
  12 division of disk policy. It is implemented in CFQ. Hence this policy takes
  13 effect only on leaf nodes when CFQ is being used.
  14
  15 HOWTO
  16 =====
  17 You can do a very simple testing of running two dd threads in two different
  18 cgroups. Here is what you can do.
  19
  20 - Enable Block IO controller
  21         CONFIG_BLK_CGROUP=y
  22
  23 - Enable group scheduling in CFQ
  24         CONFIG_CFQ_GROUP_IOSCHED=y
  25
  26 - Compile and boot into kernel and mount IO controller (blkio).
  27
  28         mount -t cgroup -o blkio none /cgroup
  29
  30 - Create two cgroups
  31         mkdir -p /cgroup/test1/ /cgroup/test2
  32
  33 - Set weights of group test1 and test2
  34         echo 1000 > /cgroup/test1/blkio.weight
  35         echo 500 > /cgroup/test2/blkio.weight
  36
  37 - Create two same size files (say 512MB each) on same disk (file1, file2) and
  38   launch two dd threads in different cgroup to read those files.
  39
  40         sync
  41         echo 3 > /proc/sys/vm/drop_caches
  42
  43         dd if=/mnt/sdb/zerofile1 of=/dev/null &
  44         echo $! > /cgroup/test1/tasks
  45         cat /cgroup/test1/tasks
  46
  47         dd if=/mnt/sdb/zerofile2 of=/dev/null &
  48         echo $! > /cgroup/test2/tasks
  49         cat /cgroup/test2/tasks
  50
  51 - At macro level, first dd should finish first. To get more precise data, keep
  52   on looking at (with the help of script), at blkio.disk_time and
  53   blkio.disk_sectors files of both test1 and test2 groups. This will tell how
  54   much disk time (in milli seconds), each group got and how many secotors each
  55   group dispatched to the disk. We provide fairness in terms of disk time, so
  56   ideally io.disk_time of cgroups should be in proportion to the weight.
  57
  58 Various user visible config options
  59 ===================================
  60 CONFIG_BLK_CGROUP
  61         - Block IO controller.
  62
  63 CONFIG_DEBUG_BLK_CGROUP
  64         - Debug help. Right now some additional stats file show up in cgroup
  65           if this option is enabled.
  66
  67 CONFIG_CFQ_GROUP_IOSCHED
  68         - Enables group scheduling in CFQ. Currently only 1 level of group
  69           creation is allowed.
  70
  71 Details of cgroup files
  72 =======================
  73 - blkio.weight
  74         - Specifies per cgroup weight. This is default weight of the group
  75           on all the devices until and unless overridden by per device rule.
  76           (See blkio.weight_device).
  77           Currently allowed range of weights is from 100 to 1000.
  78
  79 - blkio.weight_device
  80         - One can specify per cgroup per device rules using this interface.
  81           These rules override the default value of group weight as specified
  82           by blkio.weight.
  83
  84           Following is the format.
  85
  86           #echo dev_maj:dev_minor weight > /path/to/cgroup/blkio.weight_device
  87           Configure weight=300 on /dev/sdb (8:16) in this cgroup
  88           # echo 8:16 300 > blkio.weight_device
  89           # cat blkio.weight_device
  90           dev     weight
  91           8:16    300
  92
  93           Configure weight=500 on /dev/sda (8:0) in this cgroup
  94           # echo 8:0 500 > blkio.weight_device
  95           # cat blkio.weight_device
  96           dev     weight
  97           8:0     500
  98           8:16    300
  99
 100           Remove specific weight for /dev/sda in this cgroup
 101           # echo 8:0 0 > blkio.weight_device
 102           # cat blkio.weight_device
 103           dev     weight
 104           8:16    300
 105
 106 - blkio.time
 107         - disk time allocated to cgroup per device in milliseconds. First
 108           two fields specify the major and minor number of the device and
 109           third field specifies the disk time allocated to group in
 110           milliseconds.
 111
 112 - blkio.sectors
 113         - number of sectors transferred to/from disk by the group. First
 114           two fields specify the major and minor number of the device and
 115           third field specifies the number of sectors transferred by the
 116           group to/from the device.
 117
 118 - blkio.io_service_bytes
 119         - Number of bytes transferred to/from the disk by the group. These
 120           are further divided by the type of operation - read or write, sync
 121           or async. First two fields specify the major and minor number of the
 122           device, third field specifies the operation type and the fourth field
 123           specifies the number of bytes.
 124
 125 - blkio.io_serviced
 126         - Number of IOs completed to/from the disk by the group. These
 127           are further divided by the type of operation - read or write, sync
 128           or async. First two fields specify the major and minor number of the
 129           device, third field specifies the operation type and the fourth field
 130           specifies the number of IOs.
 131
 132 - blkio.io_service_time
 133         - Total amount of time between request dispatch and request completion
 134           for the IOs done by this cgroup. This is in nanoseconds to make it
 135           meaningful for flash devices too. For devices with queue depth of 1,
 136           this time represents the actual service time. When queue_depth > 1,
 137           that is no longer true as requests may be served out of order. This
 138           may cause the service time for a given IO to include the service time
 139           of multiple IOs when served out of order which may result in total
 140           io_service_time > actual time elapsed. This time is further divided by
 141           the type of operation - read or write, sync or async. First two fields
 142           specify the major and minor number of the device, third field
 143           specifies the operation type and the fourth field specifies the
 144           io_service_time in ns.
 145
 146 - blkio.io_wait_time
 147         - Total amount of time the IOs for this cgroup spent waiting in the
 148           scheduler queues for service. This can be greater than the total time
 149           elapsed since it is cumulative io_wait_time for all IOs. It is not a
 150           measure of total time the cgroup spent waiting but rather a measure of
 151           the wait_time for its individual IOs. For devices with queue_depth > 1
 152           this metric does not include the time spent waiting for service once
 153           the IO is dispatched to the device but till it actually gets serviced
 154           (there might be a time lag here due to re-ordering of requests by the
 155           device). This is in nanoseconds to make it meaningful for flash
 156           devices too. This time is further divided by the type of operation -
 157           read or write, sync or async. First two fields specify the major and
 158           minor number of the device, third field specifies the operation type
 159           and the fourth field specifies the io_wait_time in ns.
 160
 161 - blkio.io_merged
 162         - Total number of bios/requests merged into requests belonging to this
 163           cgroup. This is further divided by the type of operation - read or
 164           write, sync or async.
 165
 166 - blkio.io_queued
 167         - Total number of requests queued up at any given instant for this
 168           cgroup. This is further divided by the type of operation - read or
 169           write, sync or async.
 170
 171 - blkio.avg_queue_size
 172         - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 173           The average queue size for this cgroup over the entire time of this
 174           cgroup's existence. Queue size samples are taken each time one of the
 175           queues of this cgroup gets a timeslice.
 176
 177 - blkio.group_wait_time
 178         - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 179           This is the amount of time the cgroup had to wait since it became busy
 180           (i.e., went from 0 to 1 request queued) to get a timeslice for one of
 181           its queues. This is different from the io_wait_time which is the
 182           cumulative total of the amount of time spent by each IO in that cgroup
 183           waiting in the scheduler queue. This is in nanoseconds. If this is
 184           read when the cgroup is in a waiting (for timeslice) state, the stat
 185           will only report the group_wait_time accumulated till the last time it
 186           got a timeslice and will not include the current delta.
 187
 188 - blkio.empty_time
 189         - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 190           This is the amount of time a cgroup spends without any pending
 191           requests when not being served, i.e., it does not include any time
 192           spent idling for one of the queues of the cgroup. This is in
 193           nanoseconds. If this is read when the cgroup is in an empty state,
 194           the stat will only report the empty_time accumulated till the last
 195           time it had a pending request and will not include the current delta.
 196
 197 - blkio.idle_time
 198         - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 199           This is the amount of time spent by the IO scheduler idling for a
 200           given cgroup in anticipation of a better request than the exising ones
 201           from other queues/cgroups. This is in nanoseconds. If this is read
 202           when the cgroup is in an idling state, the stat will only report the
 203           idle_time accumulated till the last idle period and will not include
 204           the current delta.
 205
 206 - blkio.dequeue
 207         - Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This
 208           gives the statistics about how many a times a group was dequeued
 209           from service tree of the device. First two fields specify the major
 210           and minor number of the device and third field specifies the number
 211           of times a group was dequeued from a particular device.
 212
 213 - blkio.reset_stats
 214         - Writing an int to this file will result in resetting all the stats
 215           for that cgroup.
 216
 217 CFQ sysfs tunable
 218 =================
 219 /sys/block/<disk>/queue/iosched/group_isolation
 220
 221 If group_isolation=1, it provides stronger isolation between groups at the
 222 expense of throughput. By default group_isolation is 0. In general that
 223 means that if group_isolation=0, expect fairness for sequential workload
 224 only. Set group_isolation=1 to see fairness for random IO workload also.
 225
 226 Generally CFQ will put random seeky workload in sync-noidle category. CFQ
 227 will disable idling on these queues and it does a collective idling on group
 228 of such queues. Generally these are slow moving queues and if there is a
 229 sync-noidle service tree in each group, that group gets exclusive access to
 230 disk for certain period. That means it will bring the throughput down if
 231 group does not have enough IO to drive deeper queue depths and utilize disk
 232 capacity to the fullest in the slice allocated to it. But the flip side is
 233 that even a random reader should get better latencies and overall throughput
 234 if there are lots of sequential readers/sync-idle workload running in the
 235 system.
 236
 237 If group_isolation=0, then CFQ automatically moves all the random seeky queues
 238 in the root group. That means there will be no service differentiation for
 239 that kind of workload. This leads to better throughput as we do collective
 240 idling on root sync-noidle tree.
 241
 242 By default one should run with group_isolation=0. If that is not sufficient
 243 and one wants stronger isolation between groups, then set group_isolation=1
 244 but this will come at cost of reduced throughput.
 245
 246 What works
 247 ==========
 248 - Currently only sync IO queues are support. All the buffered writes are
 249   still system wide and not per group. Hence we will not see service
 250   differentiation between buffered writes between groups.