Messages in this thread |  | | | Date | Wed, 4 Jun 2008 10:42:05 -0700 | | From | Randy Dunlap <> | | Subject | Re: [PATCH 1/3] Add io-throttle controller documentation |
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On Sat, 24 May 2008 18:56:55 +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 | 81 +++++++++++++++++++++++++++++ > 1 files changed, 81 insertions(+), 0 deletions(-) > > diff --git a/Documentation/controllers/io-throttle.txt b/Documentation/controllers/io-throttle.txt > new file mode 100644 > index 0000000..e7ab050 > --- /dev/null > +++ b/Documentation/controllers/io-throttle.txt > @@ -0,0 +1,81 @@ > + > + Block device I/O bandwidth controller > + > +1. Description > + > +This controller allows to limit the block I/O bandwidth 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 offers a better control over QoS respect to priority
offer better control over QoS with respect to priority
> +or weighted-based solutions, that only give information about applications'
weight-based solutions that ...
> +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: if you're looking for a way to improve the overall throughput of the > +system probably you should use a different solution. > + > +2. User Interface > + > +A new I/O bandwidth limitation rule is described using the file > +blockio.bandwidth. > + > +Example: > + > +* 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 "foo" cgroup: > + # /bin/echo $$ > /mnt/cgroup/foo/tasks > + --> the current shell has been added to the cgroup "foo" > + > +* give maximum 1MiB/s of I/O bandwidth for the cgroup "foo": > + # /bin/echo 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 (blockio.bandwidth is expressed in KiB/s). > + > +3. Advantages of providing this feature > + > +* Allow QoS for block device I/O among different cgroups > +* Improve I/O performance predictability on block devices shared between > + different cgroups > +* It is independent on the particular I/O scheduler (anticipatory, deadline,
of
> + CFQ, noop) and/or 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) > + > +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.
they belong to.
> + > +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 on the dirty pages ratio > +(write throttling in memory), since the writes to the real block device 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.
--- ~Randy
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