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    Subject[PATCH 1/9] io-throttle documentation
    Documentation of the block device I/O controller: description, usage,
    advantages and design.

    Signed-off-by: Andrea Righi <>
    Documentation/cgroups/io-throttle.txt | 451 +++++++++++++++++++++++++++++++++
    1 files changed, 451 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/cgroups/io-throttle.txt

    diff --git a/Documentation/cgroups/io-throttle.txt b/Documentation/cgroups/io-throttle.txt
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    + Block device I/O bandwidth controller
    +This controller allows to limit the I/O bandwidth of specific block devices for
    +specific process containers (cgroups [1]) imposing additional delays on I/O
    +requests for those processes that exceed the limits defined in the control
    +group filesystem.
    +Bandwidth limiting rules offer better control over QoS with respect to priority
    +or weight-based solutions that only give information about applications'
    +relative performance requirements. Nevertheless, priority based solutions are
    +affected by performance bursts, when only low-priority requests are submitted
    +to a general purpose resource dispatcher.
    +The goal of the I/O bandwidth controller is to improve performance
    +predictability from the applications' point of view and provide performance
    +isolation of different control groups sharing the same block devices.
    +NOTE #1: If you're looking for a way to improve the overall throughput of the
    +system probably you should use a different solution.
    +NOTE #2: The current implementation does not guarantee minimum bandwidth
    +levels, the QoS is implemented only slowing down I/O "traffic" that exceeds the
    +limits specified by the user; minimum I/O rate thresholds are supposed to be
    +guaranteed if the user configures a proper I/O bandwidth partitioning of the
    +block devices shared among the different cgroups (theoretically if the sum of
    +all the single limits defined for a block device doesn't exceed the total I/O
    +bandwidth of that device).
    +A new I/O limitation rule is described using the files:
    +- blockio.bandwidth-max
    +- blockio.iops-max
    +The I/O bandwidth (blockio.bandwidth-max) can be used to limit the throughput
    +of a certain cgroup, while blockio.iops-max can be used to throttle cgroups
    +containing applications doing a sparse/seeky I/O workload. Any combination of
    +them can be used to define more complex I/O limiting rules, expressed both in
    +terms of iops/s and bandwidth.
    +The same files can be used to set multiple rules for different block devices
    +relative to the same cgroup.
    +The following syntax can be used to configure any limiting rule:
    +- DEV is the name of the device the limiting rule is applied to.
    +- LIMIT is the maximum I/O activity allowed on DEV by CGROUP; LIMIT can
    + represent a bandwidth limitation (expressed in bytes/s) when writing to
    + blockio.bandwidth-max, or a limitation to the maximum I/O operations per
    + second (expressed in iops/s) issued by CGROUP.
    + A generic I/O limiting rule for a block device DEV can be removed setting the
    + LIMIT to 0.
    +- STRATEGY is the throttling strategy used to throttle the applications' I/O
    + requests from/to device DEV. At the moment two different strategies can be
    + used [2][3]:
    + 0 = leaky bucket: the controller accepts at most B bytes (B = LIMIT * time)
    + or O operations (O = LIMIT * time); further I/O requests
    + are delayed scheduling a timeout for the tasks that made
    + those requests.
    + Different I/O flow
    + | | |
    + | v |
    + | v
    + v
    + .......
    + \ /
    + \ / leaky-bucket
    + ---
    + |||
    + vvv
    + Smoothed I/O flow
    + 1 = token bucket: LIMIT tokens are added to the bucket every seconds; the
    + bucket can hold at the most BUCKET_SIZE tokens; I/O
    + requests are accepted if there are available tokens in the
    + bucket; when a request of N bytes arrives N tokens are
    + removed from the bucket; if fewer than N tokens are
    + available the request is delayed until a sufficient amount
    + of token is available in the bucket.
    + Tokens (I/O rate)
    + o
    + o
    + o
    + ....... <--.
    + \ / | Bucket size (burst limit)
    + \ooo/ |
    + --- <--'
    + |ooo
    + Incoming --->|---> Conforming
    + I/O |oo I/O
    + requests -->|--> requests
    + |
    + ---->|
    + Leaky bucket is more precise than token bucket to respect the limits, because
    + bursty workloads are always smoothed. Token bucket, instead, allows a small
    + irregularity degree in the I/O flows (burst limit), and, for this, it is
    + better in terms of efficiency (bursty workloads are not smoothed when there
    + are sufficient tokens in the bucket).
    +- BUCKET_SIZE is used only with token bucket (STRATEGY == 1) and defines the
    + size of the bucket in bytes (blockio.bandwidth-max) or in I/O operations
    + (blockio.iops-max).
    +- CGROUP is the name of the limited process container.
    +Also the following syntaxes are allowed:
    +- remove an I/O bandwidth limiting rule
    +# /bin/echo DEV:0 > CGROUP/blockio.bandwidth-max
    +- configure a limiting rule using leaky bucket throttling (ignore bucket size):
    +# /bin/echo DEV:LIMIT:0 > CGROUP/blockio.bandwidth-max
    +- configure a limiting rule using token bucket throttling
    + (with bucket size == LIMIT):
    +# /bin/echo DEV:LIMIT:1 > CGROUP/blockio.bandwidth-max
    +2.2. Show I/O limiting rules
    +All the defined rules and statistics for a specific cgroup can be shown reading
    +the files blockio.bandwidth-max for bandwidth constraints and blockio.iops-max
    +for I/O operations per second constraints.
    +The following syntax is used:
    +$ cat CGROUP/blockio.bandwidth-max
    +- MAJOR is the major device number of DEV (defined above)
    +- MINOR is the minor device number of DEV (defined above)
    +- LIMIT, STRATEGY and BUCKET_SIZE are the same parameters defined above
    +- LEAKY_STAT is the amount of bytes (blockio.bandwidth-max) or I/O operations
    + (blockio.iops-max) currently allowed by the I/O controller (only used with
    + leaky bucket strategy - STRATEGY == 0)
    +- BUCKET_FILL represents the amount of tokens present in the bucket (only used
    + with token bucket strategy - STRATEGY == 1)
    +- TIME_DELTA can be one of the following:
    + - the amount of jiffies elapsed from the last I/O request (token bucket)
    + - the amount of jiffies during which the bytes or the number of I/O
    + operations given by LEAKY_STAT have been accumulated (leaky bucket)
    +Multiple per-block device rules are reported in multiple rows
    +(DEVi, i = 1 .. n):
    +$ cat CGROUP/blockio.bandwidth-max
    +The same fields are used to describe I/O operations/sec rules. The only
    +difference is that the cost of each I/O operation is scaled up by a factor of
    +1000. This allows to apply better fine grained sleeps and provide a more
    +precise throttling.
    +$ cat CGROUP/blockio.iops-max
    +2.3. Additional I/O statistics
    +Additional cgroup I/O throttling statistics are reported in
    +$ cat CGROUP/blockio.throttlecnt
    + - MAJOR, MINOR are respectively the major and the minor number of the device
    + the following statistics refer to
    + - BW_COUNTER gives the number of times that the cgroup bandwidth limit of
    + this particular device was exceeded
    + - BW_SLEEP is the amount of sleep time measured in clock ticks (divide
    + by sysconf(_SC_CLK_TCK)) imposed to the processes of this cgroup that
    + exceeded the bandwidth limit for this particular device
    + - IOPS_COUNTER gives the number of times that the cgroup I/O operation per
    + second limit of this particular device was exceeded
    + - IOPS_SLEEP is the amount of sleep time measured in clock ticks (divide
    + by sysconf(_SC_CLK_TCK)) imposed to the processes of this cgroup that
    + exceeded the I/O operations per second limit for this particular device
    +$ cat CGROUP/blockio.throttlecnt
    +8 0 0 0 0 0
    +^ ^ ^ ^ ^ ^
    + \ \ \ \ \ \___iops sleep (in clock ticks)
    + \ \ \ \ \____iops throttle counter
    + \ \ \ \_____bandwidth sleep (in clock ticks)
    + \ \ \______bandwidth throttle counter
    + \ \_______minor dev. number
    + \________major dev. number
    +Distinct statistics for each process are reported in
    +$ cat /proc/PID/io-throttle-stat
    +$ cat /proc/$$/io-throttle-stat
    +0 0 0 0
    +^ ^ ^ ^
    + \ \ \ \_____global iops sleep (in clock ticks)
    + \ \ \______global iops counter
    + \ \_______global bandwidth sleep (clock ticks)
    + \________global bandwidth counter
    +2.5. Generic usage examples
    +* Mount the cgroup filesystem (blockio subsystem):
    + # mkdir /mnt/cgroup
    + # mount -t cgroup -oblockio blockio /mnt/cgroup
    +* Instantiate the new cgroup "foo":
    + # mkdir /mnt/cgroup/foo
    + --> the cgroup foo has been created
    +* Add the current shell process to the cgroup "foo":
    + # /bin/echo $$ > /mnt/cgroup/foo/tasks
    + --> the current shell has been added to the cgroup "foo"
    +* Give maximum 1MiB/s of I/O bandwidth on /dev/sda for the cgroup "foo", using
    + leaky bucket throttling strategy:
    + # /bin/echo /dev/sda:$((1024 * 1024)):0:0 > \
    + > /mnt/cgroup/foo/blockio.bandwidth-max
    + # sh
    + --> the subshell 'sh' is running in cgroup "foo" and it can use a maximum I/O
    + bandwidth of 1MiB/s on /dev/sda
    +* Give maximum 8MiB/s of I/O bandwidth on /dev/sdb for the cgroup "foo", using
    + token bucket throttling strategy, bucket size = 8MiB:
    + # /bin/echo /dev/sdb:$((8 * 1024 * 1024)):1:$((8 * 1024 * 1024)) > \
    + > /mnt/cgroup/foo/blockio.bandwidth-max
    + # sh
    + --> the subshell 'sh' is running in cgroup "foo" and it can use a maximum I/O
    + bandwidth of 1MiB/s on /dev/sda (controlled by leaky bucket throttling)
    + and 8MiB/s on /dev/sdb (controlled by token bucket throttling)
    +* Run a benchmark doing I/O on /dev/sda and /dev/sdb; I/O limits and usage
    + defined for cgroup "foo" can be shown as following:
    + # cat /mnt/cgroup/foo/blockio.bandwidth-max
    + 8 16 8388608 1 0 8388608 -522560 48
    + 8 0 1048576 0 737280 0 0 216
    +* Extend the maximum I/O bandwidth for the cgroup "foo" to 16MiB/s on /dev/sda:
    + # /bin/echo /dev/sda:$((16 * 1024 * 1024)):0:0 > \
    + > /mnt/cgroup/foo/blockio.bandwidth-max
    + # cat /mnt/cgroup/foo/blockio.bandwidth-max
    + 8 16 8388608 1 0 8388608 -84432 206436
    + 8 0 16777216 0 0 0 0 15212
    +* Remove limiting rule on /dev/sdb for cgroup "foo":
    + # /bin/echo /dev/sdb:0:0:0 > /mnt/cgroup/foo/blockio.bandwidth-max
    + # cat /mnt/cgroup/foo/blockio.bandwidth-max
    + 8 0 16777216 0 0 0 0 110388
    +* Set a maximum of 100 I/O operations/sec (leaky bucket strategy) to /dev/sdc
    + for cgroup "foo":
    + # /bin/echo /dev/sdc:100:0 > /mnt/cgroup/foo/blockio.iops-max
    + # cat /mnt/cgroup/foo/blockio.iops-max
    + 8 32 100000 0 846000 0 2113
    + ^ ^
    + /________/
    + /
    + Remember: these values are scaled up by a factor of 1000 to apply a fine
    + grained throttling (i.e. LIMIT == 100000 means a maximum of 100 I/O operation
    + per second)
    +* Remove limiting rule for I/O operations from /dev/sdc for cgroup "foo":
    + # /bin/echo /dev/sdc:0 > /mnt/cgroup/foo/blockio.iops-max
    +* Allow I/O traffic shaping for block device shared among different cgroups
    +* Improve I/O performance predictability on block devices shared between
    + different cgroups
    +* Limiting rules do not depend of the particular I/O scheduler (anticipatory,
    + deadline, CFQ, noop) and/or the type of the underlying block devices
    +* The bandwidth limitations are guaranteed both for synchronous and
    + asynchronous operations, even the I/O passing through the page cache or
    + buffers and not only direct I/O (see below for details)
    +* It is possible to implement a simple user-space application to dynamically
    + adjust the I/O workload of different process containers at run-time,
    + according to the particular users' requirements and applications' performance
    + constraints
    +4. DESIGN
    +The I/O throttling is performed imposing an explicit timeout on the processes
    +that exceed the I/O limits dedicated to the cgroup they belong to. I/O
    +accounting happens per cgroup.
    +Only the actual I/O that flows in the block devices is considered. Multiple
    +re-reads of pages already present in the page cache as well as re-writes of
    +dirty pages are not considered to account and throttle the I/O activity, since
    +they don't actually generate any real I/O operation.
    +This means that a process that re-reads or re-writes multiple times the same
    +blocks of a file is affected by the I/O limitations only for the actual I/O
    +performed from/to the underlying block devices.
    +4.1. Synchronous I/O tracking and throttling
    +The io-throttle controller just works as expected for synchronous (read and
    +write) operations: the real I/O activity is reduced synchronously according to
    +the defined limitations.
    +If the operation is synchronous we automatically know that the context of the
    +request is the current task and so we can charge the cgroup the current task
    +belongs to. And throttle the current task as well, if it exceeded the cgroup
    +4.2. Buffered I/O (write-back) tracking
    +For buffered writes the scenario is a bit more complex, because the writes in
    +the page cache are processed asynchronously by kernel threads (pdflush), using
    +a write-back policy. So the real writes to the underlying block devices occur
    +in a different I/O context respect to the task that originally generated the
    +dirty pages.
    +The I/O bandwidth controller uses the following solution to resolve this
    +If the operation is a buffered write, we can charge the right cgroup looking at
    +the owner of the first page involved in the I/O operation, that gives the
    +context that generated the I/O activity at the source. This information can be
    +retrieved using the page_cgroup functionality originally provided by the cgroup
    +memory controller [4], and now provided specifically by the bio-cgroup
    +controller [5].
    +In this way we can correctly account the I/O cost to the right cgroup, but we
    +cannot throttle the current task in this stage, because, in general, it is a
    +different task (e.g., pdflush that is processing asynchronously the dirty
    +For this reason, all the write-back requests that are not directly submitted by
    +the real owner and that need to be throttled are not dispatched immediately in
    +submit_bio(). Instead, they are added into an rbtree and processed
    +asynchronously by a dedicated kernel thread: kiothrottled.
    +A deadline is associated to each throttled write-back request depending on the
    +bandwidth usage of the cgroup it belongs. When a request is inserted into the
    +rbtree kiothrottled is awakened. This thread periodically selects all the
    +requests with an expired deadline and submit the bunch of selected requests to
    +the underlying block devices using generic_make_request().
    +4.3. Usage of bio-cgroup controller
    +The controller bio-cgroup can be used to track buffered-io (in delay-write
    +condition) and for properly apply throttling. The simplest way is to mount
    +io-throttle (blockio) and bio-cgroup (bio) together to track buffered-io.
    +That's it.
    +An alternative way is making the use of bio-cgroup id. An association between a
    +given io-throttle cgroup and a given bio-cgroup can be built by writing a
    +bio-cgroup id to the file blockio.bio_id.
    +This file is exported for the purpose of associating io-throttle and bio-cgroup
    +groups. If you'd like to create an association, you must ensure the io-throttle
    +group is empty, that is, there are no tasks in this group. Otherwise,
    +association creating will fail. If an association is successfully built, task
    +moving in this group will be denied. Of course, you can remove an association,
    +just echo an negative number into blockio.bio_id.
    +In this way, we don't have to necessarily mount io-throttle and bio-cgroup
    +together. It's more gentle to the other subsystems who also want to use
    +* Create an association between an io-throttle group and a bio-cgroup group
    + with "bio" and "blockio" subsystems mounted in different mount points:
    + # mount -t cgroup -o bio bio-cgroup /mnt/bio-cgroup/
    + # cd /mnt/bio-cgroup/
    + # mkdir bio-grp
    + # cat bio-grp/
    + 1
    + # mount -t cgroup -o blockio blockio /mnt/io-throttle
    + # cd /mnt/io-throttle
    + # mkdir foo
    + # echo 1 > foo/blockio.bio_id
    +* Now move the current shell in the new io-throttle/bio-cgroup group:
    + # echo $$ > /mnt/bio-cgroup/bio-grp/tasks
    +The task will be also present in /mnt/io-throttle/foo/tasks, due to the
    +previous blockio/bio association.
    +4.4. Per-block device IO limiting rules
    +Multiple rules for different block devices are stored in a linked list, using
    +the dev_t number of each block device as key to uniquely identify each element
    +of the list. RCU synchronization is used to protect the whole list structure,
    +since the elements in the list are not supposed to change frequently (they
    +change only when a new rule is defined or an old rule is removed or updated),
    +while the reads in the list occur at each operation that generates I/O. This
    +allows to provide zero overhead for cgroups that do not use any limitation.
    +WARNING: per-block device limiting rules always refer to the dev_t device
    +number. If a block device is unplugged (i.e. a USB device) the limiting rules
    +defined for that device persist and they are still valid if a new device is
    +plugged in the system and it uses the same major and minor numbers.
    +4.5. Asynchronous I/O (AIO) handling
    +Explicit sleeps are *not* imposed on tasks doing asynchronous I/O (AIO)
    +operations; AIO throttling is performed returning -EAGAIN from sys_io_submit().
    +Userspace applications must be able to handle this error code opportunely.
    +5. TODO
    +* Support proportional I/O bandwidth for an optimal bandwidth usage. For
    + example use the kiothrottled rbtree: all the requests queued to the I/O
    + subsystem first will go into the rbtree; then based on a per-cgroup I/O
    + priority and feedback from I/O schedulers dispatch the requests to the
    + elevator. This would allow to provide both bandwidth limiting and
    + proportional bandwidth functionalities using a generic approach.
    +* Implement a fair throttling policy: distribute the time to sleep equally
    + among all the tasks of a cgroup that exceeded the I/O limits, e.g., depending
    + of the amount of I/O activity previously generated in the past by each task
    + (see task_io_accounting).
    +[1] Documentation/cgroups/cgroups.txt
    +[4] Documentation/controllers/memory.txt

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