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    SubjectRe: dm-ioband + bio-cgroup benchmarks
    Vivek Goyal wrote:
    > On Thu, Sep 18, 2008 at 04:37:41PM +0200, Andrea Righi wrote:
    >> Vivek Goyal wrote:
    >>> On Thu, Sep 18, 2008 at 09:04:18PM +0900, Ryo Tsuruta wrote:
    >>>> Hi All,
    >>>> I have got excellent results of dm-ioband, that controls the disk I/O
    >>>> bandwidth even when it accepts delayed write requests.
    >>>> In this time, I ran some benchmarks with a high-end storage. The
    >>>> reason was to avoid a performance bottleneck due to mechanical factors
    >>>> such as seek time.
    >>>> You can see the details of the benchmarks at:
    >>> Hi Ryo,
    >>> I had a query about dm-ioband patches. IIUC, dm-ioband patches will break
    >>> the notion of process priority in CFQ because now dm-ioband device will
    >>> hold the bio and issue these to lower layers later based on which bio's
    >>> become ready. Hence actual bio submitting context might be different and
    >>> because cfq derives the io_context from current task, it will be broken.
    >>> To mitigate that problem, we probably need to implement Fernando's
    >>> suggestion of putting io_context pointer in bio.
    >>> Have you already done something to solve this issue?
    >>> Secondly, why do we have to create an additional dm-ioband device for
    >>> every device we want to control using rules. This looks little odd
    >>> atleast to me. Can't we keep it in line with rest of the controllers
    >>> where task grouping takes place using cgroup and rules are specified in
    >>> cgroup itself (The way Andrea Righi does for io-throttling patches)?
    >>> To avoid creation of stacking another device (dm-ioband) on top of every
    >>> device we want to subject to rules, I was thinking of maintaining an
    >>> rb-tree per request queue. Requests will first go into this rb-tree upon
    >>> __make_request() and then will filter down to elevator associated with the
    >>> queue (if there is one). This will provide us the control of releasing
    >>> bio's to elevaor based on policies (proportional weight, max bandwidth
    >>> etc) and no need of stacking additional block device.
    >>> I am working on some experimental proof of concept patches. It will take
    >>> some time though.
    >>> I was thinking of following.
    >>> - Adopt the Andrea Righi's style of specifying rules for devices and
    >>> group the tasks using cgroups.
    >>> - To begin with, adopt dm-ioband's approach of proportional bandwidth
    >>> controller. It makes sense to me limit the bandwidth usage only in
    >>> case of contention. If there is really a need to limit max bandwidth,
    >>> then probably we can do something to implement additional rules or
    >>> implement some policy switcher where user can decide what kind of
    >>> policies need to be implemented.
    >>> - Get rid of dm-ioband and instead buffer requests on an rb-tree on every
    >>> request queue which is controlled by some kind of cgroup rules.
    >>> It would be good to discuss above approach now whether it makes sense or
    >>> not. I think it is kind of fusion of io-throttling and dm-ioband patches
    >>> with additional idea of doing io-control just above elevator on the request
    >>> queue using an rb-tree.
    >> Thanks Vivek. All sounds reasonable to me and I think this is be the right way
    >> to proceed.
    >> I'll try to design and implement your rb-tree per request-queue idea into my
    >> io-throttle controller, maybe we can reuse it also for a more generic solution.
    >> Feel free to send me your experimental proof of concept if you want, even if
    >> it's not yet complete, I can review it, test and contribute.
    > Currently I have taken code from bio-cgroup to implement cgroups and to
    > provide functionality to associate a bio to a cgroup. I need this to be
    > able to queue the bio's at right node in the rb-tree and then also to be
    > able to take a decision when is the right time to release few requests.
    > Right now in crude implementation, I am working on making system boot.
    > Once patches are at least in little bit working shape, I will send it to you
    > to have a look.
    > Thanks
    > Vivek

    I wonder... wouldn't be simpler to just use the memory controller
    to retrieve this information starting from struct page?

    I mean, following this path (in short, obviously using the appropriate
    interfaces for locking and referencing the different objects):

    cgrp = page->page_cgroup->mem_cgroup->css.cgroup

    Once you get the cgrp it's very easy to use the corresponding controller

    Actually, this is how I'm doing in cgroup-io-throttle to associate a bio
    to a cgroup. What other functionalities/advantages bio-cgroup provide in
    addition to that?


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