[PATCH 1/3] i/o bandwidth controller documentation

[Andrea Righi]

Documentation of the block device I/O bandwidth controller: description, usage,
advantages and design.

Signed-off-by: Andrea Righi <righi.andrea@xxxxxxxxx>
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+
+               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.
+
+The goal of the I/O bandwidth controller is to improve performance
+predictability and QoS of the 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.
+
+The syntax is the following:
+# /bin/echo DEVICE:BANDWIDTH > CGROUP/blockio.bandwidth
+
+- DEVICE is the name of the device the limiting rule is applied to,
+- BANDWIDTH is the maximum I/O bandwidth on DEVICE allowed by CGROUP (we can
+  use a suffix k, K, m, M, g or G to indicate bandwidth values in KB/s, MB/s
+  or GB/s),
+- CGROUP is the name of the limited process container.
+
+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/sda1 for the cgroup "foo":
+  # /bin/echo /dev/sda1:1M > /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/sda1 (blockio.bandwidth is expressed in
+      KiB/s).
+
+* Give maximum 8MiB/s of I/O bandwidth on /dev/sdb for the cgroup "foo":
+  # /bin/echo /dev/sda5:8M > /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/sda1 and 8MiB/s on /dev/sda5.
+      NOTE: each partition needs its own limitation rule! In this case, for
+      example, there's no limitation on /dev/sda5 for cgroup "foo".
+
+* Run a benchmark doing I/O on /dev/sda1 and /dev/sda5; I/O limits and usage
+  defined for cgroup "foo" can be shown as following:
+  # cat /mnt/cgroup/foo/blockio.bandwidth
+  === device (8,1) ===
+    bandwidth limit: 1024 KiB/sec
+  current i/o usage: 819 KiB/sec
+  === device (8,5) ===
+    bandwidth limit: 1024 KiB/sec
+  current i/o usage: 3102 KiB/sec
+
+  Devices are reported using (major, minor) numbers when reading
+  blockio.bandwidth.
+
+  The corresponding device names can be retrieved in /proc/diskstats (or in
+  other places as well).
+
+  For example to find the name of the device (8,5):
+  # sed -ne 's/^ \+8 \+5 \([^ ]\+\).*/\1/p' /proc/diskstats
+  sda5
+
+  Current I/O usage can be greater than bandwidth limit, this means the i/o
+  controller is going to impose the limitation.
+
+* Extend the maximum I/O bandwidth for the cgroup "foo" to 8MiB/s:
+  # /bin/echo /dev/sda1:8M > /mnt/cgroup/foo/blockio-bandwidth
+
+* Remove limiting rule on /dev/sda1 for cgroup "foo":
+  # /bin/echo /dev/sda1:0 > /mnt/cgroup/foo/blockio-bandwidth
+
+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)
+* Provides zero overhead for non block device I/O bandwidth controller users
+
+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
+associated to 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.
-- 
1.5.4.3

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