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    Subject[PATCH -mm 1/3] i/o bandwidth controller documentation
    Date
    Documentation of the block device I/O bandwidth controller: description, usage,
    advantages and design.

    Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
    ---
    Documentation/controllers/io-throttle.txt | 282 +++++++++++++++++++++++++++++
    1 files changed, 282 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/controllers/io-throttle.txt

    diff --git a/Documentation/controllers/io-throttle.txt b/Documentation/controllers/io-throttle.txt
    new file mode 100644
    index 0000000..ab33633
    --- /dev/null
    +++ b/Documentation/controllers/io-throttle.txt
    @@ -0,0 +1,282 @@
    +
    + Block device I/O bandwidth controller
    +
    +1. Description
    +
    +This controller allows to limit the I/O bandwidth of specific block devices for
    +specific process containers (cgroups) 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 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).
    +
    +2. User Interface
    +
    +A new I/O bandwidth limitation rule is described using the file
    +blockio.bandwidth.
    +
    +The same file can be used to set multiple rules for different block devices
    +relative to the same cgroup.
    +
    +2.1. Configure I/O limiting rules
    +
    +The syntax to configure a limiting rule is the following:
    +
    +# /bin/echo DEV:BW:STRATEGY:BUCKET_SIZE > CGROUP/blockio.bandwidth
    +
    +- DEV is the name of the device the limiting rule is applied to.
    +
    +- BW is the maximum I/O bandwidth on DEVICE allowed by CGROUP; bandwidth must
    + be expressed in bytes/s. A generic I/O bandwidth limiting rule for a block
    + device DEV can be removed setting the BW value 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:
    +
    + 0 = leaky bucket: the controller accepts at most B bytes (B = BW * 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: BW 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 bandwidth
    + 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.
    +
    +- 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
    +
    +- configure a limiting rule using leaky bucket throttling (ignore bucket size):
    +# /bin/echo DEV:BW:0 > CGROUP/blockio.bandwidth
    +
    +2.2. Show I/O limiting rules
    +
    +All the defined rules and statistics for a specific cgroup can be shown reading
    +the file blockio.bandwidth. The following syntax is used:
    +
    +$ cat CGROUP/blockio.bandwidth
    +MAJOR MINOR BW STRATEGY LEAKY_STAT BUCKET_SIZE BUCKET_FILL TIME_DELTA
    +
    +- MAJOR is the major device number of DEV (defined above)
    +
    +- MINOR is the minor device number of DEV (defined above)
    +
    +- BW, STRATEGY and BUCKET_SIZE are the same parameters defined above
    +
    +- LEAKY_STAT is the amount of bytes currently allowed by the I/O bandwidth
    + 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 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
    +MAJOR1 MINOR1 BW1 STRATEGY1 LEAKY_STAT1 BUCKET_SIZE1 BUCKET_FILL1 TIME_DELTA1
    +MAJOR1 MINOR1 BW2 STRATEGY2 LEAKY_STAT2 BUCKET_SIZE2 BUCKET_FILL2 TIME_DELTA2
    +...
    +MAJORn MINORn BWn STRATEGYn LEAKY_STATn BUCKET_SIZEn BUCKET_FILLn TIME_DELTAn
    +
    +2.3. 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:$((1024 * 1024)):0:0 > \
    + > /mnt/cgroup/foo/blockio.bandwidth
    + # sh
    + --> the subshell 'sh' is running in cgroup "foo" and it can use a maximum I/O
    + bandwidth of 1MiB/s on /dev/sda
    +
    +* Give maximum 8MiB/s of I/O bandwidth on /dev/sdb for the cgroup "foo", using
    + token bucket throttling strategy, bucket size = 8MB:
    + # /bin/echo /dev/sdb:$((8 * 1024 * 1024)):1:$((8 * 1024 * 1024)) > \
    + > /mnt/cgroup/foo/blockio.bandwidth
    + # sh
    + --> the subshell 'sh' is running in cgroup "foo" and it can use a maximum I/O
    + bandwidth of 1MiB/s on /dev/sda (controlled by leaky bucket throttling)
    + and 8MiB/s on /dev/sdb (controlled by token bucket throttling)
    +
    +* Run a benchmark doing I/O on /dev/sda and /dev/sdb; I/O limits and usage
    + defined for cgroup "foo" can be shown as following:
    + # cat /mnt/cgroup/foo/blockio.bandwidth
    + 8 16 8388608 1 0 8388608 -522560 48
    + 8 0 1048576 0 737280 0 0 216
    +
    +* Extend the maximum I/O bandwidth for the cgroup "foo" to 16MiB/s on /dev/sda:
    + # /bin/echo /dev/sda:$((16 * 1024 * 1024)):0:0 > \
    + > /mnt/cgroup/foo/blockio.bandwidth
    + # cat /mnt/cgroup/foo/blockio.bandwidth
    + 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
    + # cat /mnt/cgroup/foo/blockio.bandwidth
    + 8 0 16777216 0 0 0 0 110388
    +
    +3. Advantages of providing this feature
    +
    +* 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
    +* It is even possible to implement event-based performance throttling
    + mechanisms; for example the same user-space application could actively
    + throttle the I/O bandwidth to reduce power consumption when the battery of a
    + mobile device is running low (power throttling) or when the temperature of a
    + hardware component is too high (thermal throttling)
    +
    +4. Design
    +
    +The I/O throttling is performed imposing an explicit timeout, via
    +schedule_timeout_killable() on the processes that exceed the I/O bandwidth
    +dedicated to the cgroup they belong to. I/O accounting happens per cgroup.
    +
    +It just works as expected for read operations: the real I/O activity is reduced
    +synchronously according to the defined limitations.
    +
    +Write operations, instead, are modeled depending of the dirty pages ratio
    +(write throttling in memory), since the writes to the real block devices are
    +processed asynchronously by different kernel threads (pdflush). However, the
    +dirty pages ratio is directly proportional to the actual I/O that will be
    +performed on the real block device. So, due to the asynchronous transfers
    +through the page cache, the I/O throttling in memory can be considered a form
    +of anticipatory throttling to the underlying block devices.
    +
    +Multiple re-writes in already dirtied page cache areas are not considered for
    +accounting the I/O activity. This is valid for multiple re-reads of pages
    +already present in the page cache as well.
    +
    +This means that a process that re-writes and/or re-reads multiple times the
    +same blocks in a file (without re-creating it by truncate(), ftrunctate(),
    +creat(), etc.) is affected by the I/O limitations only for the actual I/O
    +performed to (or from) the underlying block devices.
    +
    +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.
    +
    +NOTE: 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
    +
    +* Try to reduce the cost of calling cgroup_io_throttle() on every
    + submit_bio(READ, ...); this is not too much expensive, but the call of
    + task_subsys_state() has surely a cost. A possible solution could be to
    + temporarily account I/O in the current task_struct and call
    + cgroup_io_throttle() only on each X MB of I/O. Or on each Y number of I/O
    + requests as well. Better if both X and/or Y can be tuned at runtime by a
    + userspace tool.
    +
    +* Think an alternative design for general purpose usage; special purpose usage
    + right now is restricted to improve I/O performance predictability and
    + evaluate more precise response timings for applications doing I/O. To a large
    + degree the block I/O bandwidth controller should implement a more complex
    + logic to better evaluate real I/O operations cost, depending also on the
    + particular block device profile (i.e. USB stick, optical drive, hard disk,
    + etc.). This would also allow to appropriately account I/O cost for seeky
    + workloads, respect to large stream workloads. Instead of looking at the
    + request stream and try to predict how expensive the I/O cost will be, a
    + totally different approach could be to collect request timings (start time /
    + elapsed time) and based on collected informations, try to estimate the I/O
    + cost and usage (idea proposed by Andrew Morton <akpm@linux-foundation.org>).
    --
    1.5.4.3


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