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+ 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
+====================
+- Currently only CFQ supports hierarchical groups. For throttling,
+ cgroup interface does allow creation of hierarchical cgroups and
+ internally it treats them as flat hierarchy.
+
+ If somebody created a hierarchy like as follows.
+
+ root
+ / \
+ test1 test2
+ |
+ test3
+
+ CFQ will handle the hierarchy correctly but and throttling will
+ practically treat all groups at same level. For details on CFQ
+ hierarchy support, refer to Documentation/block/cfq-iosched.txt.
+ Throttling will treat the hierarchy as if it looks like the
+ following.
+
+ pivot
+ / / \ \
+ root test1 test2 test3
+
+ Nesting cgroups, while allowed, isn't officially supported and blkio
+ genereates warning when cgroups nest. Once throttling implements
+ hierarchy support, hierarchy will be supported and the warning will
+ be removed.
+
+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.
+
+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.