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    SubjectRe: dm-ioband + bio-cgroup benchmarks
    Vivek Goyal wrote:
    > On Thu, Sep 18, 2008 at 05:18:50PM +0200, Andrea Righi wrote:
    >> 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
    > Andrea,
    > Ok, you are first retrieving cgroup associated page owner and then
    > retrieving repsective iothrottle state using that
    > cgroup, (cgroup_to_iothrottle). I have yet to dive deeper into cgroup


    > data structures but does it work if iothrottle and memory controller
    > are mounted on separate hierarchies?

    ehm... I've to check. I usually mount all the controllers into the same
    hierarchy. :P

    > bio-cgroup guys are also doing similar thing in the sense retrieving
    > relevant pointer through page and page_cgroup and use that to reach
    > bio_cgroup strucutre. The difference is that they don't retrieve first
    > css object of mem_cgroup instead they directly store the pointer of
    > bio_cgroup in page_cgroup (When page is being charged in memory controller).
    > While page is being charged, determine the bio_cgroup, associated with
    > the task and store this info in page->page_cgroup->bio_cgroup.
    > static inline struct bio_cgroup *bio_cgroup_from_task(struct task_struct
    > *p)
    > {
    > return container_of(task_subsys_state(p, bio_cgroup_subsys_id),
    > struct bio_cgroup, css);
    > }
    > At any later point, one can look at bio and reach respective bio_cgroup
    > by.
    > bio->page->page_cgroup->bio_cgroup.
    > Looks like now we are getting rid of page_cgroup pointer in "struct page"
    > and we shall have to change the implementation accordingly.

    Actually, only page_get_page_cgroup() implementation would change. And
    we don't have to worry about the particular implementation (hash,
    radix_tree, whatever..), in any case bio-cgroup has to simply use the
    opportune interface: page_get_page_cgroup(struct *page).


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