The objective of the i/o bandwidth controller is to improve i/o performance
predictability of different cgroups sharing the same block devices.

Respect to other priority/weight-based solutions the approach used by this
controller is to explicitly choke applications' requests that directly (or
indirectly) generate i/o activity in the system.

The direct bandwidth limiting method has the advantage of improving the
performance predictability at the cost of reducing, in general, the overall
performance of the system (in terms of throughput).

Detailed informations about design, its goal and usage are described in the
documentation.

Tested against 2.6.26-rc8-mm1.

The all-in-one patch (and previous versions) can be found at:
http://download.systemimager.org/~ar...s/io-throttle/

Thanks to Li Zefan for reviewing.

Changelog: (v4 -> v5)
- rebase to 2.6.26-rc8-mm1
- handle AIO opportunely: return -EAGAIN from sys_io_submit(), instead of
making to sleep tasks doing AIO
- userspace=>kernel interface now accepts the following syntaxes:
* dev:0 <- to delete a limiting rule
* dev:bw-limit:0 <- define a leaky bucket throttling rule
* dev:bw-limit:1:bucket-size <- define a token bucket throttling rule
- use .write_string and .read_seq_string to simplify iothrottle_read() and
iothrottle_write() functions
- use a enum structure to enumerate the various throttling algorithms

TODO:
- see documentation

-Andrea
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