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SubjectRe: [PATCH 1/3] i/o bandwidth controller documentation
On Fri, 20 Jun 2008 12:05:33 +0200 Andrea Righi wrote:

> 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 | 163 +++++++++++++++++++++++++++++
> 1 files changed, 163 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..e1df98a
> --- /dev/null
> +++ b/Documentation/controllers/io-throttle.txt
> @@ -0,0 +1,163 @@
> +
> + 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

I would s/if/If/

> +system probably you should use a different solution.
> +
> +NOTE #2: the current implementation does not guarantee minimum bandwidth

s/the/The/

> +levels, the QoS is implemented only slowing down i/o "traffic" that exceeds the

Please consistenly use "I/O" instead of "i/o".

Above comma makes a run-on sentence. A period or semi-colon would be better IMO.

> +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

Ugh, this makes it look like the output does "pretty printing" (formatting),
which is generally not a good idea. Let some app be responsible for that,
not the kernel. Basically this means don't use leading spaces just to make the
":"s line up in the output.


> +
> + 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

Run-on sentence. Change , to . (with This) or use ;

> + 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

associated with (?)

> +plugged in the system and it uses the same major and minor numbers.
> --

---
~Randy
Linux Plumbers Conference, 17-19 September 2008, Portland, Oregon USA
http://linuxplumbersconf.org/


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