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    SubjectRe: RFC: I/O bandwidth controller
    Hi Fernando,

    > This RFC ended up being a bit longer than I had originally intended, but
    > hopefully it will serve as the start of a fruitful discussion.

    Thanks a lot for posting the RFC.

    > *** Goals
    > 1. Cgroups-aware I/O scheduling (being able to define arbitrary
    > groupings of processes and treat each group as a single scheduling
    > entity).
    > 2. Being able to perform I/O bandwidth control independently on each
    > device.
    > 3. I/O bandwidth shaping.
    > 4. Scheduler-independent I/O bandwidth control.
    > 5. Usable with stacking devices (md, dm and other devices of that
    > ilk).
    > 6. I/O tracking (handle buffered and asynchronous I/O properly).
    > The list of goals above is not exhaustive and it is also likely to
    > contain some not-so-nice-to-have features so your feedback would be
    > appreciated.

    I'd like to add the following item to the goals.

    7. Selectable from multiple bandwidth control policy (proportion,
    maximum rate limiting, ...) like I/O scheduler.

    > *** How to move on
    > As discussed before, it probably makes sense to have both a block layer
    > I/O controller and a elevator-based one, and they could certainly
    > cohabitate. As discussed before, all of them need I/O tracking
    > capabilities so I would like to suggest the plan below to get things
    > started:
    > - Improve the I/O tracking patches (see (6) above) until they are in
    > mergeable shape.
    > - Fix CFQ and AS to use the new I/O tracking functionality to show its
    > benefits. If the performance impact is acceptable this should suffice to
    > convince the respective maintainer and get the I/O tracking patches
    > merged.
    > - Implement a block layer resource controller. dm-ioband is a working
    > solution and feature rich but its dependency on the dm infrastructure is
    > likely to find opposition (the dm layer does not handle barriers
    > properly and the maximum size of I/O requests can be limited in some
    > cases). In such a case, we could either try to build a standalone
    > resource controller based on dm-ioband (which would probably hook into
    > generic_make_request) or try to come up with something new.
    > - If the I/O tracking patches make it into the kernel we could move on
    > and try to get the Cgroup extensions to CFQ and AS mentioned before (see
    > (1), (2), and (3) above for details) merged.
    > - Delegate the task of controlling the rate at which a task can
    > generate dirty pages to the memory controller.

    I agree with your plan.
    We keep bio-cgroup improving and porting to the latest kernel.

    Ryo Tsuruta

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